ImageMagick v6 Examples --
Canvas Creation

Index

ImageMagick Examples Preface and Index

Solid Color Canvases (for image manipulations)

• Direct Generation
• Create Image of Same Size (set new size to match existing image)
• Overlay a Specific Color (replace all colors in image)
• Using an Extracted Color (using a color from an image)
• Other Canvas Techniques (white/black/transparent)

Gradients of Colors (canvases of smooth color changes)

• Gradient Image Generator
• Gradients with Transparency
• Gradients by Histogram Adjustment
• Distorted Gradients
• Gradients by Composition
• Gradients in other Colorspaces
• Resized Image Gradients
• Interpolated Lookup Gradients
• Roll your own Gradient
• More Complex DIY Gradients

Randomized Canvases (for randomized background images)

• Plasma Gradients
• Fractal Plasma
• Greyscale Plasma
• Seeded Plasma
• Problems using Plasma
• Random Noise Images

Tiled Canvases (canvases using a repeating images)

• Tiled Canvas (existing or generated)
• Tiling with an Image In Memory
• Offset Tiling Canvases
• Modifying Built-in Patterns
• Modifying Tile Images
• Generating Tile Images
• Random Noise Tiles
• Hexagonal Tiling
• Triple Hex Tiling

Canvases are used by ImageMagick both as a starting image for drawing on, backgrounds to overlay images with transparent areas, or even just as part of general image processing. They can be a solid color, or a range of colors, or even a tile of a smaller image. Here we look at some of the more common methods of generating a canvas image.

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Solid Color Canvases

Direct Generation

Generating a canvas of a specific color and size is very simple to do, and is used all the time... convert -size 100x100 xc:khaki canvas_khaki.gif

If you have already created a canvas, but need one in a different color you can replace that color using the "-opaque" operator. convert canvas_khaki.gif -fill tomato -opaque khaki canvas_opaque.gif

You can even grab a single pixel from an existing image, and expand it to the canvas size you want. We use "-scale" for a simple and fast resizing of the single pixel.

Here we grab a rose color from the built-in "rose:" image. convert rose: -crop 1x1+40+30 +repage -scale 100x100 canvas_pick.gif

Create Image of same size

One most basic techniques when using ImageMagick is to generate a canvas the same size as some existing image. This can be done by converting that existing image into the canvas need, but preserving the images original size.

Naturally IM provides a large number of ways to do this, usually as a side effect of other image operations. But only one method currently stands out from the rest and is obvious in its intent.

We will need a test image... Don't worry above how I actually generated this image, it is not important for the exercise. I did design it to contain a range of colors, transparencies and other features, specifically to give IM a good workout when used. If you are really interested in the commands used to generate this image you can look at the special script, "generate_test", I use to create it.

Overlay a Specific Color

The simplist way is to use "-colorize" to overlay the fill color but with a fully opaque value. However this will preserve the original images alpha channel, unless you remove it first, using "+matte". convert test.png +matte -fill Sienna -colorize 100% color_colorize.gif

This is probably the best method for images without transparency.

As of IM v6.4.2-1 you can also use the "+level-colors" to set all the colors. convert test.png +matte +level-colors Peru,Peru color_levelc.gif

A more general way people think of is to use "-draw" to reset all the colors in the current image to the current "-fill" color. convert test.png -fill Tan -draw 'color 0,0 reset' color_reset.gif

This has the advantage that any meta-data the image may contain (such as comments or profiles) is also preserved.

Other methods are more complex as it involves using special Alpha Composition to force various operators to replace the image with the desired color. This technique only works with image operators that use "-compose", to replace the existing image, with the desired color.

For example you can use the "-flatten" (See Flatten onto Background example), which creates a canvas using the "-background" color. convert test.png -background Wheat \ -compose Dst -flatten color_flatten.gif

Or you can use "-border" (See Adding a Border), using the "-bordercolor" as the color source. convert test.png -bordercolor LemonChiffon \ -compose Dst -border 0 color_border.gif

This last method has the added advantage of also letting you slight enlarge the image canvas, which makes it doubly useful.

The "-border" method of generating canvases will not work with versions of IM before version 6.1.4. Before this the background generated by the "-border" operator was not a simple solid color, but a black canvas surounded by the border color. Not very useful.

A more flexible method of canvas generation was provided with the new IM version 6 "-fx" operator. You will also need to use the "+matte" operator to turn off the input images matte channel as by default "-fx" will not handle that channel. convert test.png +matte -fx Gold color_fx_constant.gif

The "-fx" operator will even let you do a little color mathematics. For example how about a dark gold color... convert test.png +matte -fx "Gold*.7" color_fx_math.gif

All the above methods can not only fill using a fully-opaque color, but can also use semi-transparent colors. However it is a good idea to ensure the image you are using has a matte channel using the "-matte" operator before hand.

Here for example we create a canvas that a semi-transparent red. However when overlaid on the web pages 'bluish' background we get a off purple color. convert test.png -matte -fill '#FF000040' -draw 'color 0,0 reset' \ color_semitrans.png

Also for the "-fx" operator you will need to set "-channel" to use all four 'RGBA' color channels.

Using an Extracted Color

But what if you also don't know exactly what color you need. For example you want to pick a color already used in the source image.

No problem, we can use the 'pick a color from image' technique we used above to generate a tile image (Pick a single pixel using "-crop", then expanded it to a reasonable tile size using "-sample".

The trickiness with this method, is to use the special "mpr:" (Magick Program Register) to save the image in a named memory source. The "-tile" image reader can then read the image from that source. This tile can then be tiled over the canvas using "-draw" command to reset all pixels to this tile pattern.

convert test.png -crop 1x1+100+75 +repage -scale 32x32 \ -write mpr:tile +delete -tile mpr:tile \ test.png -draw 'color 0,0 reset' color_tiled.gif (More details of this can be see in Tiled Canvases section of this page)

Another way is to just create a canvas that is large enough to overlay your image completely. This however can be very very slow, especially for very large images. You also need to figure out just how big to make your overlay image, which can be a tricky matter in itself!

Lets grab the red pixel from the image this time. convert test.png -crop 1x1+50+75 +repage -scale 200x200 \ test.png +swap -compose copy -composite color_compose.gif

Rather than actually extracting the pixel from the image, you can just reference that pixel, and copy it using the new "-fx" operator.

Just for something different lets pick a light grey semi-transparent pixel from the image to copy. Of course we need to tell this operation to not only copy the RGB values but also the alpha or matte transparency of the image. convert test.png -channel RGBA -fx 'p{40,10}' color_fx_image.png

The color doesn't need to come from the same image, but from any image in the current image sequence (all images of which are replaced in the operation). Here we picked a bluish colored pixel from the built-in "rose:" image. convert test.png rose: \ -channel RGBA -fx 'v.p{12,26}' color_fx_pick.gif

Of course with the "-fx" operator you can further modify the color, using straight mathematics, merging other colors, or color sources, or just about anything you like.

This technique of using colors from a separate image, can be taken a step further to recolor images based on a range of colors supplied by the separate image (EG: an indexed color image table). See Color Lookup Tables for details.

Other Canvas Techniques

Their lots of other ways of generating canvases of very specific colors, but they are rather obtuse. As such without some heavy commenting, it may not be obvious which you are doing when you look at your IM script months or years later.

I don't recommend these techniques, but are useful to know if you are using older less flexible versions of IM.

Black Canvas

Traditionally you can create a black canvas by using "-threshold", and then turn off the matte channel. convert test.png -threshold 100% +matte black_threshold.png

Providing the "-level" operator with same argument for both 'black' and 'white' points will have the same effect. convert test.png -level 100%,100% +matte black_level.png

The "-fx" operator provides a more obvious way of creating a black canvas by clearing all the pixels to zero. However you will also need to reset the matte channel to make it fully opaque. convert test.png -fx 0 +matte black_fx.png

However the "-evaluate" version of this should be faster, particularly on larger images. convert test.png -evaluate set 0 +matte black_evaluate.png

You can also mis-use the "-gamma" operator to make an image all black. convert test.png -gamma 0 +matte black_gamma.png

A less obvious way is to 'posterize' the image will too few color levels, resulting in only one color being used, black. convert test.png -posterize 1 +matte black_posterize.png

White Canvas

The traditional way is again using "-threshold". The value however must be a negative number, just to be sure that all colors will be mapped to white, in all versions of IM. convert test.png -threshold -1 +matte white_threshold.png

Providing the "-level" operator with same argument for both 'black' and 'white' points will have the same effect. convert test.png -level -1,-1 +matte white_level.png

You can of course set the pixel values directly using the "-fx" operator. convert test.png -fx 1 +matte white_fx.png

However the "-evaluate" version of this should be faster, particularly on larger images. convert test.png -evaluate set 100% +matte white_evaluate.png

You can also mis-use the "-gamma" operator to make an image all white, by using a negative argument. convert test.png -gamma -1 +matte white_gamma.png

Or negate some other black canvas generation method. convert test.png -posterize 1 +matte -negate white_posterize.png

Transparent Canvas

Probably the most important canvas you want to generate from a existing image is a transparent canvas.

Here we make a fully-transparent black canvas using "-compose Clear" with any overlay image (a single pixel "null:" in this case). convert test.png null: -compose Clear -composite trans_compose.png

The more important transparent canvases is when you only want to clear the alpha or matte channel, but preserve the original RGB colors existing in the image.

Here we use the "-draw matte" operator to replace the matte channel value with that of the "-fill" color setting. In this case transparent. convert test.png -fill none -draw 'matte 0,0 reset' color_matte.png

We can also to this more directly with the "-fx" operator. convert test.png -channel A -fx 0 trans_fx.png

Naturally the "-evaluate" version of this should be faster, particularly on larger images. convert test.png -channel A -evaluate set 0 trans_evaluate.png

Another way to just make the image fully transparent is to use "-threshold" but again limiting its effects to just the transparency channel. convert test.png -channel A -threshold -1 trans_threshold.png

Actually in this case we are mathematically dealing with a 'matte' channel, using threshold to set it to the maximum value, rather than zero, as we did with the "-fx" operator. This is why a '-1' was used in the above, rather than 100%'. (See Channels and Masks examples page.)

The original RGB colors are still present in the last set of images above. That is the original colors of the image are still present, they have just been made transparent.

For example, here we read in one of the above images and ask IM to turn off the matte/alpha channel in the image so as to make the colors visible again.

convert trans_fx.png +matte trans_fx_matte.jpg Note however that not all image formats, and very few image operation will preserve the not fully-transparent RGB colors that are still present in the image.

As mentioned before, and worth repeating, many of the above methods rely on an image already having a matte channel. If it doesn't, add one using the "-matte" image operator. If the image did not have a matte (or alpha) channel, then this operator will also ensure the generated channel is set to fully opaque.

Miscellanious Canvas Coloring

Other than using a specific color, only the "-gamma" operator is truely flexible enough to generate a canvas of primary color. You basically use 0 to zero out a channel, and -1 to maximize a channel values.

For example here I generate a yellow canvas... convert test.png -gamma -1,-1,0 +matte yellow_gamma.png

As of IM v6.4.2 you can also use the "+level" operator to set a specific grey level for all channels. convert test.png +level 40%,40% +matte grey_level.png

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Gradients of Color

As you saw above you can create canvases of solid colors easy enough. But sometimes you want something more interesting. And ImageMagick provides a large number of special image creation operators that will let you do this.

The full list of these creation operators are listed on the ImageMagick formats.

It is difficult however to find all the good image generators available in IM as they are all mixed up with the specific image file formats. And then not all the options and capabilities are detailed. I will try to rectify that.

One of the most common image creation operators is gradient. For example... convert -size 100x100 gradient: gradient.jpg

As you can see by default "gradient:" will create an image with white at the top, and black at the bottom, and a smooth shading of grey across the height of the image.

But it does not have to be only a grey-scale gradient, you can also generate a gradient of different colors by either specifying one color, or both.

Notice that when given a single color the second color will be either 'white' or 'black', which ever produces the largest color distance from the given color. As such 'blue' produces a 'blue-white' gradient, while 'yellow' generated a 'yellow-black' gradient.

Gradients can not currently be specified at other angles or involving more than two colors. However as this ability is in integral part of SVG gradients, this situation will likely change, with a major improvement in gradient options.

"gradient:" currently does not make use of the "-colorspace" setting. They are generated only in RGB space, so multi-color 'rainbow' gradients (using HSV space) are not possible.

Some particularly nice gradients include... convert -size 10x120 gradient:snow-navy gradient_ice-sea.jpg convert -size 10x120 gradient:gold-firebrick gradient_burnished.jpg convert -size 10x120 gradient:yellow-limegreen gradient_grassland.jpg convert -size 10x120 gradient:khaki-tomato gradient_sunset.jpg convert -size 10x120 gradient:darkcyan-snow gradient_snow_scape.jpg

As of IM v6.3.1 the algorithm used to generate gradients now produce a perfect gradients, with all the pixels of each row in the image being assigned the same color. Before this version the "gradient:" operator worked by ignoring the width of the image, and just assigning the next increment of color, going row-by-row from top-left corner to the bottom-right of the image. This gradient generation method, allowed you easilly create a one pixel high horizontal gradient. That of course is no longer the case, and all examples of this usage has been removed from IM examples to avoid confusion. The result in these older versions was a vertical gradient, just as it is now, but not a perfect one. Usally this fact was only important in special case such as test images, and image distortion maps.

Gradients with Transparency

As of IM v6.2.9-8 the "gradient:" image creation operator understands the use of transparent and semi-transparent colors. convert -size 100x100 gradient:none-firebrick gradient_transparent.png

Of course as I used semi-transparent pixels in this gradient, I needed to use the PNG image format, rather than JPEG.

One word of warning about such a gradient. It is not a pure color to transparency, but a mix of that color with "black". The reason is that the color "none" is really transparent-black. The cause of this problem is the the non-linear nature of RGBA Color Spaces.

For a proper pure color gradient from opaque to transparency, you can use this method instead... convert -size 100x100 gradient:none-black \ -fill firebrick -colorize 100% gradient_pure_trans.png

On the other hand you can use this impurity to generate interesting shading effects. For example by using a transparent-red color instead of "none"... convert -size 100x100 gradient:'rgba(255,0,0,0)-navy' \ gradient_mixed_trans.png

You can enhance this weirdness by using "-flatten" to then change transparency to some other color, to make tri-color gradients. Though the middle color will only be half as intense as the transparency color used. convert -size 100x100 gradient:'rgba(255,0,0,0)-navy' \ -background yellow -flatten gradient_tricolor.jpg

Gradients by Histogram Adjustment

You can create a non-linear gradient by applying some form of histogram adjustment to a linear gradient.

For example you can use a Sigmoidal Contrast function to create a more natural looking gradient.

convert -size 100x100 gradient: -sigmoidal-contrast 6,50% \ gradient_sigmodial.jpg

This type of gradient is especially good for generating Overlapping Photos, as it removed the sharp gradient changes at the beginning of the overlapping region.

Distorted Gradients

Rotated Gradient

Now you don't have to restrict yourself to just a vertical gradient. By increasing the size of the gradient image (multiply by the square root of 2 or 1.42), then rotate it 45 degrees, and crop the image to its final size, you can make a diagonal gradient.

convert -size 142x142 gradient: -rotate -45 \ -gravity center -crop 100x100+0+0 +repage \ gradient_diagonal.jpg

As of IM v6.3.5 you have a much faster and simplier way of generating a rotated gradient by using a SRT Distortion. For example, here is a 100 pixel gradient rotated 60 degrees, in a 100x100 pixel image.

convert -size 100x100 gradient: -distort SRT 60 gradient_srt.jpg

This uses the default Virtual Pixel, Edge setting to ensure the whole image is covered by the requested gradient. You can also use the expert 'distort:viewport' setting, to map a gradient onto a larger image, such as for a use in Overlapping Photos.

All the above gradients are linear. That is the colors follow a smooth straight line across the RGB color space, between only two colors. But that is not the only sort of gradient you can generate.

Warping Gradients

The gradient can also be twisted up... convert -size 100x100 gradient: -swirl 180 gradient_swirl.jpg

You can also wrap the gradient into a arcs and circles using the General Distortion operator...

convert -size 100x100 gradient: -distort Arc '180 0 50 0' \ gradient_arc.jpg

convert -size 100x100 gradient: -distort Arc '360 0 50 0' \ gradient_circle.jpg

Or re-map the gradient into a trapeziodial shape. convert -size 100x100 gradient: -rotate -90 \ -distort Perspective '0,0 40,0 99,0 59,0 0,99 -10,99 99,99 109,99' \ gradient_trapezoid.jpg

Gradients by Composition

You can also generated modify gradients by combining them using various composition methods. For example you can use the Add (modulus) compose method to produce venetian blind types of gradients.

convert -size 100x100 gradient: \( +clone +clone \) \ -background gray50 -compose Add -flatten gradient_venetian.jpg

And even do this diagonally.

convert -size 100x100 gradient: \( gradient: -rotate -90 \) \ \( -clone 0--1 -clone 0--1 \) \ -background gray50 -compose Add -flatten gradient_vent_diag.jpg

Or by blending two plain color gradients using either Channel Coping, or Mathematical Blending composition methods, you can generate colorful 2 dimensional colormap gradients.

convert -size 100x100 gradient:yellow-blue \ \( gradient:black-lime -rotate -90 \) \ -compose CopyGreen -composite gradient_colormap.jpg

Gradients in other Colorspaces

While "gradient:" generator currently can not generate gradients in directly in some another Color Spaces, (only RGB gradients are created) you can transfer gradients into a different color space to generate interesting effects. For example a linear gradient copied into the 'Hue' of a 'HSB' image can produce a rainbow gradient.

Also see Combining Channel Images for an explaination, as well as an example of creating Color Wheel image.

Resized Gradient

One trick that was brought up on the ImageMagick Mailing List by Glenn Randers-Pehrson, was to create a very small image, two pixels across, then expand that to the image size needed using "-resize".

The "-resize" operator trys to smooth out enlarged images, to make them look better at the larger scale. It is this smoothing that we use to generate a non-linear gradient.

For example here we generate the small image using a 'portable bitmap' (or PBM format) image and feed it into IM for enlargement. echo "P1 1 2 0 1 " | \ convert - -resize 100x100\! gradient_resize.jpg

Some shells like 'csh' and variants, can not handle the '!' character in the above resize geometry setting very well -- not even in quotes. Hence the backslash '\' character may be needed. Caution is advised.

The gradient produced is not linear, with a smooth start and finish to the colors given, making those colors much more pronounced, than you would get using a normal gradient.

A simple way to generate that initial two pixel image is actually with gradient itself! This lets you specify the colors directly. Of course that will limit you to a vertical gradient, unless you rotate the result as well. convert -size 1x2 gradient:khaki-tomato \ -resize 100x100\! gradient_resize2.jpg

Of course you are not limited to just a single dimension, with this technique. Here I use a four pixel 'portible greymap' (or PGM image format) to generate a 2-dimensional gradient. echo "P2 2 2 2 2 1 1 0 " | \ convert - -resize 100x100\! gradient_resize3.jpg

As you can see this diagonal gradient is not very linear when compared to the rotated diagonal gradient above.

The Network Portible Bitmap image formats, are very versitile for generating images from scripts. They can generate bitmaps (P1), greymaps (P2), and pixmaps (P3), in both ASCII (see above) and binary (P4,P5,P6) formats. Also the quality, or color range used in each image is completely controlable, allowing you to use any number range you like to specify the images (see above). ASIDE: I was the one to make the 1995 release of NetPBM, so I have experience using this format in script image generators I have created in the past.

The "-resize" operator smooths the color between these pixels according to the current "-filter" setting. By adjusting that parameter (see Resize Filter), you can make the resize gradient more edge to edge in effect. convert -size 1x2 gradient: \ -filter Cubic -resize 100x100\! gradient_resize4.jpg

Here is rough "Rainbow Gradient" created using the 'resize' technique.

Interpolated Lookup Gradients

Another method of generating gradients is to use the special "-interpolation" setting, when using a "-fx" operator. This setting is used to determine the pixel color returned when the pixel lookup is not a integer, and thus falls between two or four different pixel values.

The default setting of 'bilinear' for example will linearly determine the color for a lookup that falls between two pixels.

Here the lookup X position 'i/(w-1)' goes from '0.0' to '1.0' over the second two pixel image. The floating point number produces a perfect linear gradient.

Interpolated lookup gradients can also be expanded to 2 dimensions, and generate square linear gradients, just as easily as purely one dimensions gradients. Here are examples of the default 'bilinear' "-interpolate" setting.

convert \( xc:red xc:blue +append \) \( xc:yellow xc:cyan +append \) \ -append -size 100x100 xc: +insert \ -fx 'v.p{i/(w-1),j/(h-1)}' gradient_bilinear.jpg

This same result can also be achieved faster using a 'Triangle' "-filter" setting with the Resized Gradient trchnique above.

The 'mesh' "-interpolate" setting however is not available as a Resize Filters. It is a special 2 dimensional interpolation that divides the intra-pixel area into two flat linear triangles, hinged along the diagonal connecting the corners with the minimal color difference.

By making the two diagonal corners the same color, you end up with two joined diagonal gradients.

convert \( xc:red xc:gold +append \) \( xc:gold xc:green +append \) \ -append -size 100x100 xc: +insert -interpolate mesh \ -fx 'v.p{i/(w-1),j/(h-1)}' gradient_mesh.gif

As the two diagonally opposite yellow corners are the same, a diagonal of yellow was used to join them. With the other colors linearly mapped to those triangles.

For more information on the "-interpolate" setting see Interpolation Setting.

Roll your own gradient

The FX DIY Operator, lets you define your own gradients or other image generation, based on the current pixel position.

As this operator requires an image to work with, you can generate your gradients or other images to match that image. That is you don't have to know the size of the image to generate a gradient for it!

For example you can easily generate a linear gradient, sized correctly for the image you may be working on.

convert rose: -channel G -fx 'i/w' -separate gradient_fx_linear.gif

When generating gray-scale gradients, you can make the -fx operator 3 times faster, simply by asking it to only generate one color channel only, such as the 'G' or green channel in the above example. This channel can then be Separated to form the required gray-scale image. This can represent a very large speed boost, especially when using a very complex "-fx" formula.

You can even generate some neat non-linear gradients.

convert rose: -channel G -fx '(i/w)^4' -separate gradient_fx_x4.gif
convert rose: -channel G -fx 'cos(pi*(i/w-.5))' \ -separate gradient_fx_cos.gif

How about a 2-dimensional circular radial gradient. convert -size 100x100 xc: -channel G \ -fx 'rr=hypot(i/w-.5, j/h-.5); 1-rr*1.42' \ -separate gradient_fx_radial.gif

The "-fx" function 'rr=hypot(xx,yy)' was added to IM v6.3.6 to speed up the very commonly used expression 'rr=sqrt(xx*xx+yy*yy)' It also meant that we no longer need to make extra assignments such as 'xx=i/w-.5' when creating a radial gradient.

Note how I use some assignment expressions to simplify the calculation of the distance from center of the image, then convert it to a gradient. This feature was added in IM v6.3.0.

The value '1.42' (or sqrt(2)) in the above controls the overall size of the gradient relative to the images dimensions. In this way the radius of the gradient is diagonal distance to the corner.

You can even remove the 'sqrt()' (built into the 'hypot()' function) from the expression to make a more interesting spherical gradient, which can be useful for 3D Shading Effects. convert -size 100x100 xc: -channel G \ -fx 'xx=i/w-.5; yy=j/h-.5; rr=xx*xx+yy*yy; 1-rr*4' \ -separate gradient_fx_spherical.gif

Using a high power function, you can give photos a fade off effect around the rectangular edges of the image. Adjust the power value '4' to control the amount of fading. convert -size 100x100 xc: -channel G \ -fx '(1-(2*i/w-1)^4)*(1-(2*j/h-1)^4)' \ -separate gradient_fx_quad2.gif

Here is a angular gradient, whcih is interesting in itself. convert -size 100x100 xc: -channel G \ -fx '.5 - atan2(j-h/2,w/2-i)/pi/2' \ -separate gradient_fx_angular.gif

Note that the 'atan2(y,x)' function returns a angle in radians from -PI to +PI (see its manpage), so its output needs to be be scaled and translated to correctly fit a 0.0 to 1.0 color range. This is why the above looks so much more complex than it really is.

This last example can be generated faster by Warping a linear gradient using the Generalized Distortion Operator. For an example see the Color Wheel Example.

More Complex DIY Gradients

Of course an FX function can generate color gradients. For example here is a gradient basied on distance ratios, using an extremely complex FX expression.

convert -size 100x100 xc: +size xc:red xc:yellow \ -fx 'ar=hypot( i/w-.8, j/h-.3 )*4; br=hypot( i/w-.3, j/h-.7 )*4; u[1]*br/(ar+br) + u[2]*ar/(ar+br)' \ gradient_dist_ratio.gif

When going from two points to three points the ratio of how much color each 'control point' provides, is a bit more complex, and uses a technique called Inverse Distance Weighted (IDW) Interpolation. You can see more details math for this in Wikipedia, IDW

Here is a inverse distance example for three points.

convert -size 100x100 xc: +size xc:red xc:yellow xc:lime \ -fx 'ar=1/max(1, hypot(i-50,j-10) ); br=1/max(1, hypot(i-10,j-70) ); cr=1/max(1, hypot(i-90,j-90) ); ( u[1]*ar + u[2]*br + u[3]*cr )/( ar+br+cr )' \ gradient_shepards.gif

And here I use a inverse distance squared which is the more normal method used for a IDW interpolation (Sherpard's Method)...

convert -size 100x100 xc: +size xc:red xc:yellow xc:lime \ -fx 'ar=1/max(1, (i-50)*(i-50)+(j-10)*(j-10) ); br=1/max(1, (i-10)*(i-10)+(j-70)*(j-70) ); cr=1/max(1, (i-90)*(i-90)+(j-90)*(j-90) ); ( u[1]*ar + u[2]*br + u[3]*cr )/( ar+br+cr )' \ gradient_shepards_2.gif

NOTE: That 'hypot()' function was not used as there is no need to use a square root on the pythagorean function. The problem with Shepard's Method is that all the 'control points' has effects over the whole image. As a result you get a sort of underlying 'average color' in between the 'control points'. This inturn produces 'peaks' of color rather than a smooth gradient that was wanted.

Here I tried to improve the results by generating the gradeint in HSL colorspace, but this time using blue instead of yellow.

convert -size 100x100 xc: +size xc:red xc:blue xc:lime -colorspace HSL \ -fx 'ar=1/max(1, (i-50)*(i-50)+(j-10)*(j-10) ); br=1/max(1, (i-10)*(i-10)+(j-70)*(j-70) ); cr=1/max(1, (i-90)*(i-90)+(j-90)*(j-90) ); ( u[1]*ar + u[2]*br + u[3]*cr )/( ar+br+cr )' \ -colorspace RGB gradient_shepards_HSL.gif

As you can see all the colors were nice an bright as we are only generating a hue gradient. However it also apprears very stange, which is caused by the 'cyclic' nature of the 'Hue' color channel. As a consequence the area between the blue and the red goes the long way round though green, rather than though a purple color, as you would expect. This has which has no easy solution.

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Randomized Canvases

Plasma Gradients

While gradients provide a smooth range of colors, another image creation operator "plasma:" provides a different sort of gradient. One that is ideally suited to generating a random backdrop of color for your images.

First of all I should point out that "plasma:" is a randomized image. As such it can and will produce a different image every time it is run. For example here we generate three separate 'standard' plasma images, and each image is different from each other, even though the same command was used to generate them.

You can also see that plasma images are also a type of randomized gradient of colors, and like "gradient:" started with white at the top and black at the bottom.

What isn't well document is that you can specify color for the plasma gradient in the exact same way as you can for linear gradients above.

You can also see that a mid-tone colors like 'tomato' and 'steelblue', tend to work better than pure colors like 'red' and 'blue'. That is because contain at least some colors from all three color channels, allowing the plasma image operator more scope in the colors generated.

By using the same color twice with plasma you can produce a background that is predominantly that color, but with a random splotches of colors close to that of the original color.

Again as you can see, mid-tone colors will generate more varieties of color in the resulting image, than a extreme color, like black, white, or yellow.

The 'grey' plasma in the above is particularly nice giving a iridescent 'mother-of-pearl' like effect, basically as grey has total freedom in the colors that the "plasma:" will generate.

Normalizing a prefect 50% grey plasma will produce a particularly uniform multi-color plasma image, over the full range of colors, including white and black. convert -size 100x100 plasma:grey50-grey50 -normalize plasma_grey_norm.jpg

Alternatively you can just spread the contrast of the colors to just make them bolder, but without going to extremes. convert -size 100x100 plasma:grey50-grey50 \ -sigmoidal-contrast 8x50% plasma_grey_contrast.jpg

Compare this image with the 'fractal plasma' images below.

Fractal Plasma

The plasma generator also has a special fractal mode, producing highly colorful effects. The color generated are enhanced to produce more exaggerated color changes.

Also like the constant color plasma's above they are more uniform across the whole image.

Plasma in general I often find a little 'noisy'. As such it will often benefit from a little smoothing using "-blur".

Here I have have smoothed out the noise from the middle plasma image above. convert plasma_fractal2.jpg -blur 0x2 plasma_smooth.jpg

You can use "-paint" to create random blotches of color. convert plasma_fractal2.jpg -blur 0x1 -paint 8 plasma_paint.jpg

Or make the colors more pronounced and circular using the "-emboss" image operator, after using "-blur" to remove the low level noise. convert plasma_fractal2.jpg -blur 0x5 -emboss 2 plasma_emboss.jpg

Reversing the "-blur" by using "-sharpen" can produce a more pastel color pattern. convert plasma_fractal2.jpg -blur 0x5 -sharpen 0x15 plasma_sharp.jpg

I actually find generating a swirled plasma gradient to be particularly nice, as a background pattern. convert -size 160x140 plasma:fractal \ -blur 0x1 -swirl 180 -shave 20x20 plasma_swirl.jpg

Greyscale Plasma

Now the plasma generator will always generate color, even on a pure black solid color. However it is often useful to generate a pure grey-scale plasma. Well there are two simple ways of doing this.

The simplest way is to take the plasma image and converted it to grey scale. convert -size 100x100 plasma:fractal -blur 0x2 \ -colorspace Gray plasma_greyscale.jpg
Another way is to copy one of the color channel over the other two, for a stronger, single layer, effect. convert -size 100x100 plasma:fractal -blur 0x2 \ -channel G -separate plasma_grey_copy.jpg
A final technique is to use "-shade" on the plasma. convert -size 100x100 plasma:fractal -blur 0x5 \ -shade 120x45 -normalize plasma_grey_shade.jpg

You'd probably think you would get a lot of light and shadow effects, but the raw plasma is so random, that "-shade" only seems to produce a more 'mottled plasma' effect.

Instead of using a fractal plasma, with its highly exaggerated color changes, you can create a grey-scale plasma using the constant color plasma method. As a side effect, this method also allows you to control the overall brightness of the grey-scale plasma image generated.

If this is not quite bold enough, use the channel copy method of grey-scaling the plasma image.

These grey-scale plasma images are very useful for further processing, allowing you to generate other image effects.

For example, look at the page on Background Images for a huge number of examples where the plasma fractal was used to produce lots of interesting effects.

Seeding or Repeating a Plasma Image

Remember "plasma:" can produce areas of near pure black or pure white, or any other color (though it isn't likely to be pure). And while it is unlikely you will get a image that is all in one color, it is also a possible outcome. So when you get a good result you may like to save it, for later re-use.

Because of this, scripts using plasma images, may like to include options to generate and re-use such randomized images. That is you may like to separate the plasma image generation from other parts that use that image, to allow re-use.

A simplier technique however is to 'seed' or initialize the IM random number generator so that 'plasma:' will generate the same 'randomized' image. That way you can tune a script or program to produce a good or interesting coloration or effect, over and over. convert -size 100x100 -seed 4321 plasma: plasma_seeded.jpg

The above image will never change, so unless I change the "-seed" number I will always have a 'red' area in the bottom-right corner.

Interestingly using the same seed with different initializing color gradients can produce a set of images which while random, are simular in their internal pattern.

As you can see the same pattern of colors is present in all the above images, though different color bases can highlight or obsure some of the underlying shared pattern.

Just one final word of warning. Other IM operators can also use the random number generator, such as the "-fx" 'rand()' function, the "-virtual-pixel" 'random' setting the "-random-threshold" dither operator, and the "-noise" operator. As such is a good idea to seed the generator immediatally before your specific use of the random number generator.

As of IM v6.3.4-3, you can also re-randomize the generator using "+seed". So placing it after your 'seeded plasma' will ensure that any later operators using random numbers will correctly generate a randomized result.

By default the seed is randomized when IM starts, so you normally do not need to randomize it yourself using "+seed" to get a random result.

Problems using Plasma

One problem you should avoid with "plasma:" images, is generating them with a high aspect ratio. It tends to distort the normal plasma color effects, pulling the colors out into needle-like streaks.

There is no simple solution to this, so unless this is what you are wanting, caution is advised.

There is also a definate top-left to bottom-right diagonal warp in the plasma image that should not exist. That is there is some sort of 'spatial bias' flaw in the algorithm.

For example as Thomas Maus <thomas.maus_AT_alumni.uni-karlsruhe.de> pointed out if you mirror and append the same plasma image, you will always see a distinct 'V' in the resulting image...

This should not happen. But the problem seems to be too deep to be able to fix without basically completely re-writing the whole plasma generator function.

Random Noise Images

As of IM v6.3.5 you can generate a purely random image from an existing image using Noise Generator, "+noise" method 'Random'. convert -size 100x100 xc: +noise Random random.png

If your IM is older than this you can still generate a pure random noise image using the slower DIY FX Operator, "-fx". convert -size 100x100 xc: -fx 'rand()' random_fx.png

Now these purely random images are themselves not very useful. But as a source image for various image transformations, they will allow you to generate a wide variety of different images.

For example by Blurring the image and Color Adjusting the result, you can create a mottled pattern of random color.

Note however without the Virtual Pixel Setting the "-blur" operator will have strong edge effects, which are best avoided.

As a bonus by changing the "-virtual-pixel" setting to 'tile', the randomised image remains tilable, with the colors wrapping across the image boundaries. This tiling ability is something that currently not possible with a random Plasma Images and is a inherent result of pure random images being so random to start with.

Now each of the three Color Channels of the image can be thought of as separate random gray-scale image, and these channels can be merged together in various ways.

For example you generate a mask of random dots by first Thresholding a color channel ('G' or the green channel), and separating it out as a gray-scale image.

convert random.png -channel G -threshold 5% -separate \ +channel -negate random_mask.png

As each color is a linearly random value, the threshold percentage used in the above directly defines the density of pixels selected.

You can go further and use one color color channel to select random values from another color channel channel (the 'R' or red channel), by using various Image Composition methods.

Further processing, particularly on the black background version, will let you enlarge the dots based on their gray-scale intensity, or generate streaks, and or star flares, from those dots. See Star Generators for examples of such processing.

Like Seeded Plasma Images you can also use the "-seed" setting to pre-initialise the random number generator. This allows you to generate the same random image(s) repeatably for a particular machine, just as you can for plasma images.

For more examples of using random images, see Background Images, or have a look at generating randomised canvases see Random Spots of Solid Color.

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Tiled Canvases

Tile images can be very large or very small, are designed to fit together side-by-side and vertically to cover large areas of space.

Thanks to the World Wide Web, there as been an explosion of tile images available for use (finding what you want is another matter). Below are a set of tiled images which I copied from Anthony's Icon Library for use through out these example pages.

bg.gif

tile_aqua.jpg

tile_water.jpg

tile_rings.jpg

tile_disks.jpg

tile_weave.gif

Currently there are quite a number of way in which you can tile an image, over a large area.

You can "-tile" any image so as to completely replace the original background image (using the "Copy" compose operator). (For more details see Tile Compositing).

composite -tile tile_weave.gif -size 60x60 xc:none tile_copy.gif

Another way is to read in the tile image using "tile:", and tile it to a specific size. convert -size 60x60 tile:tile_weave.gif tile_size.gif

You can use this to generate a tiled image much larger than you need, then use "-composite" to overlay it over the original image. If the tile image is partially transparent then a 'Over' "-compose" method will need to be specified. It is a very slow method of tiling, particularly for large images, and you have the problem of determining just how big an image you need to create for the overlay. convert test.png -size 200x200 tile:tile_disks.jpg \ -composite tile_over.gif

A more advanced method is to use "-fx" to tile over an image. This however makes use of a special "-virtual-pixel" setting to define what pixel color to use when a operator references a pixel outside the boundaries of the actual image. See Virtual Pixels for details. convert test.png tile_aqua.jpg -channel RGBA \ -virtual-pixel tile -fx v tile_fx.gif

This however is even slower than the previous method. But you can also use "-distort" as a faster method of tiling, with unusual warping options. See General Distortion Operator.

Finally by specify a tile as 'tile fill pattern' for the "-draw" operator, you can draw the tile image over another image, to create any shape or figure you like.

This is because the "-tile" setting will override any the "-fill" color setting used by draw. See MVG Drawing Settings.

convert -size 60x60 xc: -tile tile_weave.gif \ -draw "circle 30,30 2,30" tile_draw.gif

This only works for "-draw", and operators like "-annotate" that also make use of "-draw" to perform their function. It will not work for image operators that use "-fill" color directly, like "label:", "caption:", and "text:".

However "-draw" has built-in to it some special color primitives, such as completely resetting all the pixels in the image to the fill color, or tile pattern (if set). convert test.png -tile tile_water.jpg -draw "color 0,0 reset" \ tile_test.gif

This is actually exactly the same method as used by some Solid Color Canvases methods using a Specific Color, Only here we used "-tile" instead of a "-fill" color.

Tiling with an Image already In Memory

Tiling an image you have in memory (created or modified) is not straight forward. Although a few methods are known.

Using a "Memory Program Register"

The most straight forward method is to save the the image into a special 'In Memory' file format "mpr:", or named 'memory program register'.

From this register you can then either use a "-tile" setting, or use the special "tile:" image file reader, both of which can only be set from a 'saved' image file format.

For example using "tile:" to create a tiled image of a specific size...

convert tree.gif \ -write mpr:tile +delete \ -size 100x100 tile:mpr:tile \ tile_mpr.gif

Or use it to set the "-tile" fill pattern for the various "-draw" operations... convert tree.gif -write mpr:tile +delete \ granite: -tile mpr:tile -draw 'circle 64,64 10,50' \ tile_mpr_fill.gif

The name given after "mpr:" can be anything you like, it is only a label on where the image was saved in memory. It can even be a plain number, color, or source filename.

Clone/Append In-Memory Tile

If you are not worried about the exact size of the tiled image, you can just append the image together multiple times.

For example here we tile the image in a 3x3 array.

convert tree.gif \ \( +clone +clone \) +append \ \( +clone +clone \) -append \ tile_clone.gif

This method of tiling has the advantage of allowing you to flip-tile (mirror tile) the image.

convert tree.gif \ \( +clone -flop +clone \) +append \ \( +clone -flip +clone \) -append \ tile_clone_flip.gif

In general this method is only practical when you have some idea of how big the image being tiled is. Also as clones are actually very fast and efficent it is a fairly simple and fast tiling method, especially if you use the results to further tile the larger image.

Virtual Pixel Tiling

In this method we use the "-fx" operator to read a tile image that is 'virtually tiled'. See Edge Effects and Virtual Pixels for more details.

This however will junk the tile image and all the other images in memory, unless you use parenthesis.

convert rose: tree.gif \ -channel RGBA -virtual-pixel tile -fx v \ tile_fx_2.gif

Note however that using "-fx" to tile images will be a lot slower compared to the previous methods, though it is probably a little simpler.

You can also use "-distort" to tile images. The size of the final image however needs to be specified using a special "viewport" setting.

convert tree.gif -set option:distort:viewport 100x100+0+0 \ -virtual-pixel tile -distort SRT 0 \ tile_distort.gif

By using the General Distortion Operator in this way, you also have the added bonus of distorting the tiled image in some very complex ways. For example...

convert tree.gif -set option:distort:viewport 100x100-50-50 \ -virtual-pixel tile -distort Arc '45 0 50' \ tile_distort_polar.gif

You can also use the 'mirror' setting to get the same 'flip-flop' effect of the cloning technique.

Offset Tiling Canvases

Sometimes you need a little more control over the exact positioning of a background texture, either for aligning a tile pattern with some other image, or to avoid a bad correlation with some other part of the final image. For many of the standard tiling methods this can be achieved using the "-tile-offset" setting.

For example. Here I roll the tile image being used to directly create a tiled canvas image using "tile:" or "pattern:".

Tile Offset setting was broken before IM version 6.3.9-9 in that the 'X' offset was being used for both 'X' and 'Y' offset values (the given 'Y' value was ignored). This means that while the above examples would have worked (both X and Y offsets are the same) you may not get the expected results when the two values differ.

This also works for the "montage" background "-texture" setting. montage tree.gif -geometry +24+24 \ -tile-offset +30+30 -texture rose: offset_texture.gif

You can also use the setting by defining it before the "-tile" or "-fill" setting. For example... convert -tile-offset +30+30 -tile rose: \ -size 80x80 xc: -draw 'color 30,20 reset' offset_tile_fill.gif

However there is one major problem with offset tiling.

The problem is that due to the use of Legacy Command Line Style, the above will fail when using builtin "pattern:" tiles. For example here I tried the same thing as the above using a 'checker