[x264-devel] FPGAs and x264

[Gabriel Bouvigne]

David Smith a écrit :
> I am currently working on a project to offload x264 functions to an 
> FPGA.  I have read several papers recently that describe the offloading 
> of x264_me_search_ref.   However, that is a large function to replicate 
> in hardware and well beyond the scope of my project.
> 
> Fo my current project I am designing a SAD/SATD 16 core processor.  Each 
> core will have 4 processing elements.  Each element can compute up to 
> either a 8x8 SAD or SATD.

Just offloading simple DSP functions to an fpga is a bad idea when the 
host is a modern cpu.

Offloading more advanced DSP functions (FIR, big FFT or MDCT) can be 
useful, but something as simple as a 16x16 SAD/SATD will likely not be 
useful at all (except for learning). Those functions are quite fast on 
modern CPUs, and offloading them to an FGPA will likely be latency-bound 
(especially if you have to go through PCI, probably less if your FPGA is 
directly on an HyperTransport link).

To do some useful offloading to an fpga, you would either have to 
offload bigger functions (like the whole deblocking, but then it's not a 
huge part of x264's cpu use), or to add a bit of conditional logic on 
the fpga side, in order to cover something bigger than just a simple DSP 
function, without the need to go back to the cpu each time.

For educational purpose, if you really want to only offload SAD you 
might want to check its benefits in the context of a full motion search. 
(but bear in mind that full search is nearly useless in real encoders)

I'm not saying that using fpga is not usefull, but only that deciding to 
use an fpga without defining/changing the encoder architecture with this 
in mind is usually not optimal.
Note: the same is also true for GPGPU

--
Gabriel