Am 12.03.2013 02:23, schrieb Marek Olšák: > On Mon, Mar 11, 2013 at 6:49 PM, Ian Romanick <i...@freedesktop.org> wrote: >> Once upon a time Matt Turner was talking about using pixman to accelerate >> operations like this in Mesa. It has a lot of highly optimized paths for >> just this sort of thing. Since it's used by other projects, it gets a lot >> more testing, etc. It may be worth looking at using that to solve this >> problem. > > I think that using pixman or any other CPU-based solution is a waste > of time (for dedicated GPUs at least). The OpenGL packing and > unpacking can be implemented entirely on the GPU using streamout and > TBOs, and we generally only want memcpy on the CPU side. That would > also allow us to finally accelerate pixel buffer objects. > > For now, the easiest and fastest solution is to do a blit, which > should cover swizzling and format conversions. We just need a lot of > texture formats or do swizzling in the fragment shader. The > destination of the blit can be a temporary texture allocated in GTT. > The author of the patch (at least I think it's him) has actually > started working on the blit-based solution for ReadPixels in st/mesa > and the time spent in ReadPixels went from 2300 ms to 9 ms (so he can > still use additional 7.6 ms for rendering and be at 60 fps).
For format conversions you're probably right, however if you only need easy swizzling (like rgba->abgr or some such) the cpu solution should be faster - because last time I checked swizzling is essentially an operation which can be done for free (completely hidden by the cost of read/write memory, you can easily do such swizzles for 4 rgba8 pixels at once with a single cpu instruction, at least if you've got ssse3 - though it would require memcpy-like optimizations for fetching and storing the values, essentially you need a memcpy implementation with inlined swizzling). That is unless you'd need some blit on the gpu side in any case, in which case the conversion there is also free. I agree though it is probably easier to do a blit, which covers more cases (both because you can also do format conversion and because you don't need separate code path depending on cpu). Roland _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
