Why aren't 3-channel storage types (mostly) color renderable in OpenGL ES

Question

I'm having a hard time finding out why some int and unsigned int internal storage types are "color renderable" and others aren't. A few examples

Four channels

internal storage type	color renderable
GL_RGBA8	Y
GL_RGBA8I	Y
GL_RGBA32UI	Y

Three channels

internal storage type	color renderable
GL_RGB8	Y
GL_RGB8I
GL_RGB32UI
GL_RGB565	Y

Two- and one channels

internal storage type	color renderable
GL_RG8	Y
GL_RG8UI	Y
GL_R8	Y
GL_R8UI	Y

I understand (I think) why sRGB- and floating-point internal storage types are non-color-renderable, but why are non-normalized, three-channel, int and unsigned int storage types non-color renderable? Why aren't one- and two-channel int and unsigned int storage types non-color-renderable too? I read through the applicable parts of the OpenGL ES 3 spec and all it seems to say is 'because it's not marked as "color renderable" in the table'. I think I'm missing something embarrassingly basic.

Answer 1

In general three channel storage types are disliked by hardware, as three is not a power of two and therefore texels/pixels can end up spanning cache lines.

To avoid this, in practice most 3 channel formats will end up being rounded up to a 4 channel internal format with 1 channel of padding.

Just use a 4 channel format, you're no worse off.