Imagination PowerVR SDK Blog

Is there any way to compress `rbga` channel to single channel



For example,the texture is in rgba mode but what I need just alpha channel.
So I want to remove the data in rbg channels and just store the alpha data.
And then I can save the texture size to 1/3.

Is there any way to achieve this goal?



Yes, it is possible. If you are using PVRTexTool just re-compress the texture with an alpha-only format (Name: A 8). Alternatively you can do it programmatically, removing the three channels from the original data (converting from 32bit to 8bit). Load it to OpenGL ES with the GL_ALPHA flag.




Am I right?
./PVRTexToolCLI -f a8 -i example.png -o output.pvr


Yes, that is correct.The format a8 will carry only the alpha channel in your png.




If you can modify your shaders, you can achieve even higher space savings if you

  1. Create an RGBA texture where each pixel is defined as [RGBA]=[A’, 0, 0, 255], where A’ is the original per-pixel alpha value
  2. Compress the texture as PVRTC with 4bpp (or even 2bpp if the quality ends up being good enough)
  3. Getting the fragment shader to transfer the R channel of the sampled result to the alpha channel.


  • Actually, it doesn’t matter if you use [A’, 0, 0, 255], [A’, A’, 0, 255] or [0, A’, 0, 255], as long as it’s consistent . It’s just that the R and G channels in PVRTC have slightly more accuracy than B.
  • It is essential that the A be set to 255 as “opaque” data will compress slightly better (as it gives more precision to the RG&B channels)
  • It is important to keep B constant (and one of a small set of particular constants) - here I’ve chosen 0, but 255 would also do equally well.