Is it better to use a single texture or multiple textures for a YUV image

Question

This question is for OpenGL ES 2.0 (on Android) but may be more general to OpenGL.

Ultimately all performance questions are implementation-dependent, but if anyone can answer this question in general or based on their experience that would be helpful. I'm writing some test code as well.

I have a YUV (12bpp) image I'm loading into a texture and color-converting in my fragment shader. Everything works fine but I'd like to see where I can improve performance (in terms of frames per second).

Currently I'm actually loading three textures for each image - one for the Y component (of type GL_LUMINANCE), one for the U component (of type GL_LUMINANCE and of course 1/4 the size of the Y component), and one for the V component (of type GL_LUMINANCE and of course 1/4 the size of the Y component).

Assuming I can get the YUV pixels in any arrangement (e.g. the U and V in separate planes or interspersed), would it be better to consolidate the three textures into only two or only one? Obviously it's the same number of bytes to push to the GPU no matter how you do it, but maybe with fewer textures there would be less overhead. At the very least, it would use fewer texture units. My ideas:

• If the U and V pixels were interspersed with each other, I could load them in a single texture of type GL_LUMINANCE_ALPHA which has two components.
• I could load the entire YUV image as a single texture (of type GL_LUMINANCE but 3/2 the size of the image) and then in the fragment shader I could call texture2D() three times on the same texture, doing a bit of arithmetic figure out the correct co-ordinates to pass to texture2D to get the correct texture co-ordinates for the Y, U and V components.

Answer 1

I would combine the data into as few textures as possible. Fewer textures is usually a better option for a few reasons.

1. Fewer state changes to setup the draw call.
2. The fewer texture fetches in a fragment shader the better.
3. Less upload time.

Sources:

I understand some of these are focused on more specific hardware, but the principles apply to most Mobile graphics architectures.

Best Practices for Working with Texture Data

Optimize OpenGL for Tegra

Optimizing performance of a heavy fragment shader

1. "Binding to a texture takes time for OpenGL ES to process. Apps that reduce the number of changes they make to OpenGL ES state perform better. "

2. "In my experience mobile GPU performance is roughly proportional to the number of texture2D calls." "There are two texture loads, so the minimum cycle count for the texture sub-unit is two." (Tegra has a texture unit which has to run a cycle for reach texture read)

3. "making calls to the glTexSubImage and glCopyTexSubImage functions particularly expensive" - upload operations must stall the pipeline until textures are uploaded. It is faster to batch these into a single upload than block a bunch of separate times.