Colors in range [0, 255] doesn't correspond to colors in range [0, 1]

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I am trying to implement in my shader a way of reading normals from a normal map. However, I found a problem when reading colors that prevents it.

I thought that one color such as (0, 0, 255) (blue) was equivalent to (0, 0, 1) in the shader. However, recently I found out that, for instance, if I pass a texture with the color (128, 128, 255), it is not equivalent to ~(0.5, 0.5, 1) in the shader.

In a fragment shader I write the following code:

    vec3 col = texture(texSampler[0], vec2(1, 1)).rgb;   // texture with color (128, 128, 255)
    
    if(inFragPos.x > 0)
        outColor = vec4(0.5, 0.5, 1, 1);    // I get (188, 188, 255)
    else
        outColor = vec4(col, 1);            // I get (128, 128, 255)

In x<0 I get the color (128, 128, 255), which is expected. But in x>0 I get the color (188, 188, 255), which I didn't expect. I expected both colors to be the same. What do I not know? What am I missing?

vulkan glsl
2 Answers

But in x>0 I get the color (188, 188, 255), which I didn't expect.

Did you render these values to a swapchain image, by chance?

If so, swapchain images are almost always in the sRGB colorspace. Which means that all floats written to them will be expected to be in a linear colorspace and therefore will be converted into sRGB.

If the source image was also in the sRGB colorspace, reading from it will reverse the transformation into a linear RGB colorspace. But since these are inverse transformations, the overall output you get will be the same as the input.

If you want to treat data in a texture as data rather than as colors, you must not use image formats that use the sRGB colorspace. And swapchain images are almost always sRGB, so you'll have to use a user-created image for such outputs.

Also, 128 will never yield exactly 0.5. 128/255 is slightly larger than 0.5.

user_7340690

After some research, I could solve it, so I will explain the solution. Nicol Bolas' answer shed some light on the problem too (thank you!).

In the old days, images were in (linear) RGB. Today, images are expected to be in (non-linear) sRGB. The sRGB color space gives more resolution to darker colors and less to lighter colors, because human eye distinguishes darker colors better.

Internet images (including normal maps) are almost always in sRGB by convention. When I analyze the colors of an image with Paint, I get the sRGB colors. When I pass that image as a texture to the shader, it is automatically converted to RGB (if you told Vulkan to do so), because the RGB color space is more appropriate for making operations with colors. Then, when the shader outputs the result, it automatically converts it back to sRGB.

My mistake was to consider the color information I got from the source image (using Paint) to be RGB, while it was really sRGB. When the color was converted to RGB in the shader, I was confused because I expected the same color I got in Paint. Since I want to use the texture as data rather than as color, I see 3 ways to solve this:

  • Save normals in a RGB image (tell Vulkan about this) (most correct option).
  • Transform the image to sRGB in the shader (my solution). Since the data was saved in an image as sRGB colors, it should be read in the shader as sRGB in order to get the correct data.

Now, talking about Vulkan, we have to specify the color space for the surface format and the swap chain (for instance: VK_COLOR_SPACE_SRGB_NONLINEAR_KHR). This way, the swapchain\display interprets the values when the image is presented. Also, we have to specify the color space of the Vulkan images we create.

References

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