[FFmpeg-devel] flashsvenc.c - sampling block size too low

[Benjamin Larsson]

Hi.

Jason Askew wrote:
> diff of 2 pass encoding for flashsvenc.c attached
> 
> I'm still actively working on this.  A few things to know:
> 
> const int TP_BLCK_SIZE = 8;
> 
> This can go up to 16.  Things get even slower and the resulting gains
> have been minimal to nonexistence.  That may be a different story
> depending on what the screen cap is of.

I suggest that you use an option to set the max block size to search
for. A default of 8 might be reasonable.

> 
> Each possible block size is iterated through, compared, and zlib'ed.
> This is not efficient.
> 
> Current optimization I'm working on is to do the block compare one
> time only at pixel size of 16x16, and using that data over again for
> larger block sizes.

I had this thought also, it should give a considerable boost.

> 
> The big bottle neck is zlib'ing blocks just to determine their size.
> I'm investigating a theory that optimal block size can be 'chased
> down' by following the least block size through an array, limiting the
> number of zlib compressions that need to take place.

I think a 16x16 block compare cache will give enough speedup to search
all sizes without resorting to guessing the most optimal size.

> 
> Some array pointer access can still be optimized.
> 

One more thing to test is how to compress the blocks. Currently zlib is
called with compress2. Afaik this writes a complete zlib packet with
header. So for every block that is compressed we get a header+data. It
might be possible to not close the zlib packet and chain lots of packet
data after each other. And thus get better compression.

Anyway good work.

MvH
Benjamin Larsson