Yeah, that quote is completely confusing.

GL has

• GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS - for vertex shaders
• GL_MAX_TEXTURE_IMAGE_UNITS - for pixel shaders
• Other similarly named for tesselation, geometry shaders and compute
• GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS

To work properly:

1. Your vertex shader must contain less than GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS textures
2. Your pixel shader must contain less than GL_MAX_TEXTURE_IMAGE_UNITS textures
3. The number of vs_textures + ps_textures must be less than GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS. If a VS and PS use the same texture, it counts as 2 (1 for the VS, 1 for the PS)

So if GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS = 16; GL_MAX_TEXTURE_IMAGE_UNITS = 16; but GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS = 25 and you use 14 textures in the VS, and 14 textures in the PS, the shader will fail. It passes the first and second tests (14 < 16) but not the third one (14 + 14 = 28 > 25).

Though in practice most modern cards will do GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS + ...all other stages... + GL_MAX_TEXTURE_IMAGE_UNITS = GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS;

These stupid rules come from 2003 when GPUs had separate texture units for vertex and pixel shaders; where the VS had 4 TU chips and the PS separately had 16 TU chips. But in today GPUs everything just really gets processed by the same piece of hardware.

Thank you, that's very helpful.

One follow up question, does it matter which particular texture slots I use? For example, suppose GL_MAX_TEXTURE_IMAGE_UNITS = 2, would I be limited to the first two texture slots, or could I use say GL_TEXTURE5 and GL_TEXTURE6 so long as I'm not using more than two at once?

If you use texture0 and texture6, it counts as 2 textures.

Though for best performance avoid leaving too many gaps (specially large gaps).

Oh yeah, Osbios makes a good point. You can't use slot 7 if GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS = 2