What are the exact color spaces of DXGI_FORMATs?

Question

In the official documentation of DXGI_FORMAT, it tells us that only a format with _SRGB enumeration postfix is in sRGB color space. I thought other format without this postfix are all in the linear space. But I found a very strange behavior of format conversion function in DirectXTex library. ( You can download it from http://directxtex.codeplex.com/ )

At first, I exported a texture file as DXGI_FORMAT_R32G32B32A32_FLOAT by using NVIDIA Photoshop DDS Plugin. Then I load this file by LoadFromDDSFile() function, and convert its format to DXGI_FORMAT_R16G16B16A16_UNORM by Convert() function. (Both of these two functions are provided by DirectXTex library.)

You guess what? After the image was converted to DXGI_FORMAT_R16G16B16A16_UNORM, the brightness of all pixels were also changed, the whole image becomes brighter than before.

If I manually convert the pixel values from sRGB space to Linear space after the image was converted to DXGI_FORMAT_R16G16B16A16_UNORM format, the resultant pixel values are same as input. Therefore, I suppose that the DirectXTex library treats DXGI_FORMAT_R32G32B32A32_FLOAT as a format in linear color space, and treats DXGI_FORMAT_R16G16B16A16_UNORM as a format in sRGB color space, then it did the color space transforming from linear space to sRGB space. ( I tried to find out why the Convert() function also converts the color space, but it was implemented by WIC, and there is no source code for it. )

So, is there any bug in DirectXTex library? Or is it the real standard for DXGI_FORMATs? If there were different color spaces for some special DXGI_FORMATs, please tell me that where can I find the specification for it.

Any help will be grateful. Thanks!

Answer 1

By convention float RGB values are linear, and integer RGB values are gamma-compressed. There is no particular benefit to gamma-compressing floats since the reason for gamma is to use more bits where it is perceptually needed, and floats have sufficient (perhaps excessive) number of bits throughout and are already pseudo-log encoded (using the exponent). (source)

Note that the colorspace of integer RGB textures in DXGI which are not specifically *_SRGB is not sRGB, it is driver dependent, and usually has a fixed gamma of 0.5.

The DirectXTex library does appear to be behaving correctly. However, please note that you are also relying on the behavior of whatever software you use to both capture and display the DDS files. A better test for just DirectXTex is simply to do a round-trip conversion float->int->float in the library and compare the results numerically rather than visually.