[x264-devel] An Improbable Bug (or whatever)

[Tuukka Toivonen]

I made this change into x264, which just removed the comment. It was a bit 
surprise to see that it changes the encoding result, increasing it by one 
byte.

Index: encoder/encoder.c
===================================================================
--- encoder/encoder.c	(revision 239)
+++ encoder/encoder.c	(working copy)
@@ -1178,7 +1178,6 @@
      /* build ref list 0/1 */
      x264_reference_build_list( h, h->fdec->i_poc, i_slice_type );

-    /* Init the rate control */
      x264_ratecontrol_start( h, i_slice_type, h->fenc->i_qpplus1 );
      i_global_qp = x264_ratecontrol_qp( h );

(also http://www.ee.oulu.fi/%7Etuukkat/tmp/x264-weirdness.patch)

Any explanation? It happens even with optimization disabled.
The sequence doesn't seem to matter, size is increase by one in both 
sequences that I tried (foreman & stefan). Should I be worried about
this? All sequences still play fine with Mplayer.

Here's a list of what I did:

svn co svn://svn.videolan.org/x264/trunk x264
./configure
nano -w config.mak
-> removed "-O4 -ffast-math -DHAVE_MMXEXT -DHAVE_SSE2 -s -fomit-frame-pointer"
# Whether I change the options or not, doesn't seem to matter
( patch -s -p0 <x264-weirdness.patch )  # either patched or did not
make
SEQ=stefan
./x264 -m 5 -v -q 32 --scenecut 0 --no-chroma-me --frames 100 -o out.264 \
/video/std-video/$SEQ-352x288.yuv 352x288 >out.log 2>&1
mv out.log /tmp/out-(no)patch.log
mv out.264 /tmp/out-(no)patch.264

tuukkat at s-inf-pc92 0 /tmp/w/x264> d /tmp/out-*.264
-rw-r--r--  1 tuukkat tuukkat 255574 May 30 20:09 /tmp/out-nopatch.264
-rw-r--r--  1 tuukkat tuukkat 255575 May 30 20:12 /tmp/out-patch.264

Available also:
http://www.ee.oulu.fi/%7Etuukkat/tmp/out-nopatch.264
http://www.ee.oulu.fi/%7Etuukkat/tmp/out-nopatch.log
http://www.ee.oulu.fi/%7Etuukkat/tmp/out-patch.264
http://www.ee.oulu.fi/%7Etuukkat/tmp/out-patch.log

> gcc -v
Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.5/specs
Configured with: ../src/configure -v 
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr 
--mandir=/usr/share/man --infodir=/usr/share/info 
--with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared 
--enable-__cxa_atexit --with-system-zlib --enable-nls 
--without-included-gettext --enable-clocale=gnu --enable-debug 
--enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc i486-linux
Thread model: posix
gcc version 3.3.5 (Debian 1:3.3.5-12)

> uname -a
Linux s-inf-pc92 2.4.29 #1 Thu Feb 24 17:03:56 EET 2005 i686 GNU/Linux

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