[FFmpeg-devel] Scaling of flashsv stream

Brian Candler B.Candler
Sat Nov 7 11:57:32 CET 2009

I have been using ffmpeg to process screencasts created by vnc2flv using the
flashsv codec, and I've found a tweak that allows them to compress much
better after scaling.

My test 9.5 minute screencast has a frame size of 992x608. After a null pass
through ffmpeg, the video portion is 10543K:

  ./ffmpeg -i in.flv -vcodec flashsv -acodec copy -r 12 -g 120 out.flv

If I scale this down by a factor of 2:1, the video still accounts for 9048K,
which is hardly any smaller at all.

  ./ffmpeg -i in.flv -vcodec flashsv -acodec copy -r 12 -g 120 -s 496x304 out.flv

Poking around the codec, I discover that the scaled blocks don't zlib
compress as well, presumably because ffmpeg is doing such a good job of
anti-aliasing that a much wider colour palette is being used.

So I tried quantising the palette to 16x16x16 with the following patch, and
after this the video is only 4086K.

--- libavcodec/flashsvenc.c.orig	2009-11-06 22:13:00.000000000 +0000
+++ libavcodec/flashsvenc.c	2009-11-06 22:23:27.000000000 +0000
@@ -88,7 +88,7 @@
         npfptr = pfptr+(i*stride)+dy*3;
         for (j=0 ; j<w*3 ; j++) {
             diff |=npfptr[j]^nsptr[j];
-            dptr[j] = nsptr[j];
+            dptr[j] = ((nsptr[j] + 8)/ 17) * 17;
         dptr += w*3;

The screen quality is hardly any different - if anything, the text is
subjectively a little sharper.

Is there an existing colour quantisation facility in ffmpeg that I could
have used to do this? If so please accept my apologies, and I will move over
to ffmpeg-user :-)

Otherwise, do you think this would be worth adding? How would it be enabled?
Does it belong in the flashsv encoder, or is there somewhere more
appropriate to plug it in earlier in the chain?



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