SDL2 / SDL Image strange PNG behavior with RGB values

Question

I found an interesting thing that I'm not able to debug but I'd like to solve because it's crucial for the development of a small tool I need.

Basically all reduces to the fact that I load into an SDL_Surface a PNG loaded with IMG_Load("filename.png"). The skeleton of the code is

surface = IMG_Load("filename.png");

Then this tool should copy this surface pixel by pixel to another by applying a color map (which is just an unordered_map<u32,u32>). This basically works, but many pixel have a slight change in their RGB values so this substitution fails.

For example, while a pixel 255,255,255 is correctly stored as 0xFFFFFFFF in the surface, another pixel, let's say 81,60,48 becomes 82,62,51. I thought about gamma correction but the image itself is not drawn since the color switch is applied before two surfaces, and the latter gets drawn to a texture just afterwards.

Any clues? I'm not interested in finding another solution to my problem since I require to be able to do precise pixel color switching with a fixed map, I'd just like to understand why this is happening (since PNG should just contain exact values, being lossless) and solve it.

Does your monitor have some kind of color calibration applied to it? — Mark Ransom, Jul 28 '14 at 00:55
@Mark Ransom: No, I'm running on OSX and it has its default calibration. Indeed I tried to obtain the default gamma ramp through `SDL_GetWindowGammaRamp` but the values seemed just linear. I like to emphasize the fact that I'm working on raw data, displaying it is just a later stage. — Jack, Jul 28 '14 at 00:56

score 3 · Accepted Answer · answered Jul 28 '14 at 01:38

Ok, I found the cause and fixed it. The problem resides in how the PNG is loaded through the OS X frameworks. According to this thread, this is related to the embedded calibration which is somehow taked into account while loading the image. The proposed fix is the following:

CGFloat whitePoint[3] = { 1, 1, 1 };
CGFloat blackPoint[3] = { 0, 0, 0 };
CGFloat gamma[3] = { 2.2, 2.2, 2.2 };
CGFloat matrix[9] = {
    1, 1, 1,
    1, 1, 1,
    1, 1, 1
};
CGColorSpaceRef color_space =
    CGColorSpaceCreateCalibratedRGB(
                                whitePoint, blackPoint, gamma, matrix
                                );

By patching ImageIO.m of SDL_Image and rebuilding it from sources the problem resolved itself. I wonder why it hasn't been included in SDL_Image (or rather, just a version with custom values).

SDL2 / SDL Image strange PNG behavior with RGB values

1 Answers1