In OpenGL what's the difference between the GL_RED and GL_R color formats?

Question

When loading in a single channel 8 byte texture my renderer works only with GL_RED. It took me a while to figure out GL_R doesn't work for me. I couldn't get an explanation of what it is.

Answer 1

The is no GL_R color format, hence one cannot describe its differences to GL_RED.

The GL_R enum is totally invalid for any use as a color format, and does not mean "red". This enum is part of the four enums GL_S, GL_T, GL_R, GL_Q, which are the symbolic names for the four-dimensional texture coordinates in the GL. Note that this enum was used to specify the old fixed-function texture coordinate generation (see for example glTexGen()), which is deprecated nowadays. As a result, the GL_R enum does not even exist in modern OpenGL at all (so you find it in the legacy gl.h, but for example not in glcorearb.h).