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Quick rules of thumb, since I can never remember which is which:

  • RGB = linear space
  • sRGB = gamma space
  • most image files store sRGB
  • sRGB is unfit for blending

The corollary is:

  • Image files loaded as textures should be converted from sRGB to RGB (but some decoders do this for you!)
  • Since alpha is linear, alpha premultiplication should happen in linear space, i.e. after converting to RGB [1]
  • The shader responsible for putting the final pixels on the screen should have a gamma correction step doing:
const float gamma = 2.2;

out vec4 color;

void main()
    // ...

    color = pow(color, 1. / gamma);

Note, however, that OpenGL may also perform this step for you on any on-screen framebuffers (i.e. the default framebuffer) if you do glEnable(GL_FRAMEBUFFER_SRGB).

See also