How does Quartz handle texture compression?

Question

I'm developing on the iPhone and the majority of our game is using OpenGL ES, but there are also menus that use CGImage and Quartz in order to be displayed. In OpenGL ES, I know that no matter what image compression goes in (JPG, PNG, etc.), the data stored in memory as a texture is an 8-bit texture, unless I use PVRTC in which case I can get it to 2 or 4 bits. We've been having memory issues due to large CGImages, so my question is... what sort of optimizations and compressions do Quartz and CGImage use? I can't find the details in Apple's docs, when really I want to know if it would make a difference to put a 256-color image in, or a JPG vs a PNG, if having the dimensions at a power of 2 help, etc. Speed is unimportant, memory is the bottleneck here.

Thanks.

Answer 1

Quartz is uncompressed. It is for quickly compositing and rendering pixel accurate content. Once your images have been drawn into a context it doesn't matter where they came from, they take whatever that context takes per pixel for however many pixels they have (generally 4 bytes per pixel in a device if I recall correctly). The one pig thing it does is that premultiplied alpha stuff to avoid blending.

Now, some views under memory pressure can evict their contents if not displayed, and reconstitute them as needed. In those cases a CGImage from a compressed source generally ends up taking less memory, but I suspect that is not relevant in the case you described.