Once upon a time, when > was faster than < … Wait, what?

Question

I am reading an awesome OpenGL tutorial. It\'s really great, trust me. The topic I am currently at is Z-buffer. Aside from explaining what\'s it all about, the author mentions t

Answer 1

Optimization like that will hurt performance on many embedded graphics solutions because it will make framebuffer resolve less efficient. Clearing a buffer is a clear signal to the driver that it does not need to store and restore the buffer when binning.

Little background information: a tiling/binning rasterizer processes the screen in number of very small tiles which fit into the on-chip memory. This reduces writes and reads to external memory which reduces traffic on memory bus. When a frame is complete (swap is called, or FIFOs are flushed because they are full, framebuffer bindings change, etc) the framebuffer must be resolved; this means every bin is processed in turn.

The driver must assume that the previous contents must be preserved. The preservation means that the bin has to be written out to the external memory and later restored from external memory when the bin is processed again. The clear operation tells the driver that the contents of the bin are well defined: the clear color. This is a situation which is trivial to optimize. There are also extensions to "discard" the buffer contents.