Re: [Vnc2swf-users] Shrinking a vnc2flv file

[Brian C. <B.C...@po...>]

On Thu, Nov 05, 2009 at 09:54:11AM +0000, Brian Candler wrote:
> I then tried flvrec -K 150 (and no -r flag), and the growth was 147KB
> instead of 200KB. So the key frames have some effect it seems.

flvrec -B 64 improves it from 200KB to 108KB per 30 seconds.

Looking at the source code, every unchanged block in every frame requires
something to be written to the output:

                if (x,y) in changes:
                    # changed block
                    block = zlib.compress(self.screen.get(x,y))
                    data += pack('>H', len(block)) + block
                else:
                    # unchanged block
                    data += pack('>H', 0)

'>H' is unsigned short (2 bytes). What a shame the zlib compression happens
separately per block, instead of once over the whole frame!

Anyway, I was thinking that:
- a screen of 992x608 is exactly 31x19 blocks (with default blocksize 32)
- each frame consists of 31x19x2 = 1178 zero bytes
- so 12 fps would be 414KB per 30 seconds
That's about twice what I'm seeing, so clearly I've misunderstood something.

All those zeros suggests good gzip compression. My original 19MB flv gzips
to 9MB, which isn't brilliant but is OK when you consider it contains audio
too.

I don't know if ffmpeg has a way to set the blocksize and key frame rate
parameters when outputting, but if so I could probably shrink this file
quite a lot.

Unfortunately, ffmpeg appears to be missing the unchanged-block optimisation
for flash screen video. If I record a 20-second static screen with vnc2flv,
it takes 142K. If I then copy it using

    ffmpeg -i static-20secs.flv -r 12 -vcodec flashsv static-full.flv

it increases to 497K. If I try to halve the image size at the same time
using

    ffmpeg -i static-20secs.flv -s 496x304 -r 12 -vcodec flashsv static-half.flv

then it increases to 900K!

Regards,

Brian.