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#2953 Washed out colors on Mac OS X display

obsolete: 8.5.10
open
Jeffrey Hobbs
29. Colormaps and Visuals (6)
5
2012-03-10
2012-03-10
Jason Oster
No

While building a Tk app in Python on Mac OS X 10.7.3, I noticed the colors appear consistently "washed out" as if there is some sort of bad gamma correction going on under the hood. As a test, I write a very simple app that draws 1-pixel-high rectangles to a canvas with varying levels of intensity on each of the RGB channels, creating a black-to-white gradient.

Second, I created a black-to-white gradient in GIMP to use as a baseline. When placed side-by-side, it's pretty clear that Tk is adjusting colors incorrectly.

I've tried the same test on Linux, and it appears to work correctly, displaying the same exact gradient as generated by GIMP. I have also tried updating ActiveTcl and Python on OS X, but the problem persists. I'm now using python.org Python 2.7.2, and ActiveTcl 8.5.11.1.

Attached is a PNG that illustrated the difference between the Tk gradient and the GIMP gradient, as well as the Python source I used to generate the Tk gradient.

Discussion

  • Jason Oster

    Jason Oster - 2012-03-10

    Tk gradient vs GIMP gradient

     
    Thumbnail
    grays.png

  • Jason Oster

    Jason Oster - 2012-03-10

    Python source that generated the left side of grays.png

     
    grays.py

  • Jason Oster

    Jason Oster - 2012-03-10

    Some more info:

    1. Displaying the GIMP gradient as a GIF in a Tk window shows the same "washed out" effect, where dark colors are too bright.

    2. Using the pixel API to put and get back pixel colors in a photo always returns the expected RGB values. Whats displayed on screen is always unexpected (and verified with a screenshot)

    3. The adjustment is non-linear, but the gamma correction algorithms I'm familiar with do not correctly counteract what's displayed. In the end, I decided to use the screenshot from the Tk gradient to build a LUT that can workaround the bug. It's not entirely accurate (the round-trip causes artifacts in the RGB intensities) but it's good enough for my project.

    What I'm actually working on is a very simple way to theme my GUI, and in developing it, I'm trying to match a theme that comes with Sublime Text 2. You can imagine my surprise when my dark gray background was 4% brighter than in Sublime Text 2. (Expected #211E1E, got #2C2828) Definitely off enough to see the difference by eye.