I repeated the tests using the color spaces and intents that I actually use, and everything was fine. Panic over. I'll put the odd cases down to a lack of understanding of Lab and the absolute intent on my part.

All the best,
James.

Interesting. I removed the images from flickr as I didn't want to leave an incorrect impression of QImage's capabilities in a public place when in all likelihood the problem is user error. I'm guessing my original post will show broken images at some point (doesn't seem to have yet), so I'll attach the main image of interest in case anyone else wants to play with Lab or AC intent. (I had to resize it to meet the max size limit)

Terry,

Many thanks for looking into this for me. I agree, your results look fine. Comparing your output with mine, it looks like you used perceptual rendering intent. I suspect that my problems are due to a lack of understanding of the Lab format and the use of the AC intent, but since I don't normally use either it's really a non-issue for me. I'll do another set of tests with the file formats and intent that I actually use in practice, and hopefully that will confirm that all is well!

Thank you again,
James.

Hi,

I've been investigating some color management issues that I had assumed were with my printer, but it looks like part of the problem is with QImage. I know that color management is easy to mess up, so bear with my and I'll try and demonstrate that I've been reasonably thorough before suggesting a QImage problem.

I wanted some standard test targets to experiment with, so after some searching I downloaded the following synthetic test charts from http://www.babelcolor.com/main_level/ColorChecker.htm

ColorChecker_Lab_from_Avg.tif
ColorChecker_ProPhoto_from_Avg_16bit.tif
ColorChecker_AdobeRGB_from_Avg_16bit.tif

These are slightly odd in that they are not tagged with their respective color profiles, so on loading them into photoshop they each look visibly different. I assigned the appropriate color space to each of the rgb files (all three then looked the same on screen) and saved them. I then printed all three via QImage onto a variety of different papers. In QImage the color spaces are correctly identified.
What was immediately obvious was that the Lab version was very different to the other two versions. I also found that switching between perceptual and absolute colorimetric caused the proPhoto version to take on a marked warm shift on the grey scale. In previous versions the AdobeRGB version took on a cool shift, but this seems to have gone after updating to 2010.100

I realised I can show this by printing to a jpeg, here's an example with the three versions of the chart, using AC rendering intent:


Perceptual looks better, but the Lab is still very different:


I then tried doing the same thing printing from Lightroom, and all three versions rendered very similarly (on the print, I don't think I could tell them apart). Lightroom can also print to file, so here is the lightroom version:


These test ouputs are on flickr if you want to look in more detail:
http://www.flickr.com/photos/74697159@N00/sets/72157622518862305/


I can only claim a partial understanding of the issues of color spaces, but it looks to me like there is something odd going on here!

Regards,
James.