JPEG Colorspace conversion from GREY to RGB fails

[devinheitmueller]

I have a JPEG renderer which is a little finicky about what formats it supports. As a result, I need to ensure the images are in the RGB colorspace.

I have an image where the colorspace is greyscale, and I am trying the following:

convert -colorspace RGB infile.jpg outfile.jpg

However, if I run "identify -verbose" against the resulting file, the colorspace is still greyscale:

Code: Select all

  Properties:
    date:create: 2010-11-19T10:45:00-05:00
    date:modify: 2010-11-19T10:45:00-05:00
    jpeg:colorspace: 1
    jpeg:sampling-factor: 1x1
    signature: 0047730fba96801bad08be7d183e251b6477e412841c1788874a1fbe05862204
 

I can convert the file to CMYK and that works, but it fails just when converting to RGB.

Am I missing something obvious here?

Thanks,

Devin Heitmueller

Version: ImageMagick 6.6.2-6 2010-09-17 Q16 http://www.imagemagick.org

[GreenKoopa]

Maybe try using -type TrueColor instead of -colorspace.

[GreenKoopa]

Writing JPEG Images
http://www.imagemagick.org/Usage/formats/#jpg_write

Image Type when Reading and Writing
http://www.imagemagick.org/Usage/basics/#type


Note: Resaving a jpeg is usually a lossy operation. You can do an image compare to see how much (or often little) is lost. Somewhere I saw a tool that could make many conversions, probably including this one, losslessly.

[fmw42]

Your syntax is old. The current IM 6 syntax in this case should be:

convert infile.jpg -colorspace RGB outfile.jpg

see http://www.imagemagick.org/Usage/basics/#why

you may also be better off using profiles to convert colorspaces

see http://www.imagemagick.org/Usage/formats/#profiles

[devinheitmueller]

Your syntax is old. The current IM 6 syntax in this case should be:

convert infile.jpg -colorspace RGB outfile.jpg

see http://www.imagemagick.org/Usage/basics/#why

you may also be better off using profiles to convert colorspaces

see http://www.imagemagick.org/Usage/formats/#profiles

Perfectly rationale explanation, but unfortunately not the issue. It behaves the same way even with the command line you provided. Also, I have to wonder why "-colorspace RGB" would fail but "-colorspace CMYK" would succeed.

Anyway, using "-type TrueColor" did achieve the desired behavior, although it still feels like a bug (the "convert -debug All" log even said that it was outputting RGB).

Thanks for the help,

Devin

Devin

[GreenKoopa]

I second using -profile if applicable. Sometimes color management can matter little, but it makes a huge difference when I convert my grayscale ProPhoto (gamma=1.8 ) photos to sRGB (with its weird gamma curve of about 2.2).

I just started using ImageMagick two weeks ago, and largely working in grayscale. I also found this unexpected behavior, although not really a bug as it works as the design intends. My understanding is that ImageMagick understands the image as RGB, but the output coder saves the file in the most efficient format unless told otherwise. This is doubly frustrating if my image contains a color management profile because then my image is invalid (one channel of data but a three-channel profile). I would have defaulted the other way, outputting in the colorspace or profile specified unless told otherwise, but maybe there is a logic or history to ImageMagick's design beyond my experience. I also wonder if/when my time-consuming operations are working on one channel or repeating for all three for my grayscale images.