Rounding error in scaling expression

[blaw]

Looks like the calculation in the scaling filter is not rounding properly.

This is the math I'm doing to convert all inputs to square pixels while maintaining aspect ratio.

-vf scale='iw:ow/dar',setsar=1:1

The input video is 352x480 with SAR of 20:11 and DAR of 4:3.

The output file becomes 352x263. But when I do the math it should be 352x264. And since 263 isn't divisible by 2, the output video has color artifacts at the bottom.

Here's my math (using windows calc):
ow = 352
dar = 4:3 = 1.3333333333333333333333333333333
352/1.3333333333333333333333333333333 = 264.00000000000000000000000000007

=================================================

*

• Input Video *

*
Input #0, mpeg, from '/data/transcode/input/Code Violation_ Azarenka_Clijsters in Miami.mpg':

Duration: 00:01:16.70, start: 0.213367, bitrate: 2661 kb/s

Stream #0.0[0x1e0]: Video: mpeg2video (Main), yuv420p, 352x480 [SAR 20:11 DAR 4:3], 2714 kb/s, 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
Stream #0.1[0x80]: Audio: ac3, 48000 Hz, stereo, s16, 224 kb/s

[STREAM]
index=0
codec_name=mpeg2video
codec_long_name=MPEG-2 video
codec_type=video
codec_time_base=1001/60000
codec_tag_string=[0][0][0][0]
codec_tag=0x0000
width=352
height=480
has_b_frames=1
sample_aspect_ratio=20:11
display_aspect_ratio=4:3
pix_fmt=yuv420p
level=8
id=0x1e0
r_frame_rate=30000/1001
avg_frame_rate=30000/1001
time_base=1/90000
start_time=0.213367
duration=76.576500
STREAM

*

• Output Video *

*
Input #0, matroska,webm, from '/data/transcode/input/Code Violation_ Azarenka_Clijsters in Miami.mpg.webm':

Duration: 00:00:07.57, start: 0.000000, bitrate: N/A

Stream #0.0: Video: vp8, yuv420p, 352x263, SAR 1:1 DAR 352:263, 29.97 fps, 29.97 tbr, 1k tbn, 29.97 tbc (default)
Stream #0.1: Audio: vorbis, 44100 Hz, stereo, s16 (default)

[STREAM]
index=0
codec_name=vp8
codec_long_name=On2 VP8
codec_type=video
codec_time_base=1001/30000
codec_tag_string=[0][0][0][0]
codec_tag=0x0000
width=352
height=263
has_b_frames=0
pix_fmt=yuv420p
level=-99
r_frame_rate=30000/1001
avg_frame_rate=1349849997/45039994
time_base=1/1000
start_time=0.000000
duration=N/A
STREAM

*

• ffmpeg cmd *

*
WEBM_PROFILE="-vf yadif=0:-1:1,unsharp=3:3:1.0,scale='iw:ow/dar',setsar=1:1 -f webm -threads 0 -vcodec libvpx -g 360 -keyint_min 0 -vb 800k -qmin 1 -qmax 63 -quality good -slices 1 -vp8flags altref -acodec libvorbis -ac 2 -ab 128k -ar 44100"

ffmpeg -i <filename> $WEBM_PROFILE -y <filename>.webm

ffmpeg version 0.8.2.git, Copyright (c) 2000-2011 the FFmpeg developers

built on Aug 17 2011 15:52:09 with gcc 4.4.3
configuration: --enable-gpl --enable-version3 --enable-nonfree --enable-postproc --enable-libfaac --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-x11grab --enable-libmp3lame --enable-libvpx --enable-static
libavutil 51. 12. 0 / 51. 12. 0
libavcodec 53. 11. 0 / 53. 11. 0
libavformat 53. 7. 0 / 53. 7. 0
libavdevice 53. 3. 0 / 53. 3. 0
libavfilter 2. 32. 0 / 2. 32. 0
libswscale 2. 0. 0 / 2. 0. 0
libpostproc 51. 2. 0 / 51. 2. 0

[Stefano Sabatini]

Unreproducible here with the command (you need git of the day for testing it):

$ ffplay -f lavfi  "testsrc=s=352x480:sar=20/11,scale=iw:ow/dar"
ffplay version N-32028-g34c9975, Copyright (c) 2003-2011 the FFmpeg developers
  built on Aug 21 2011 15:27:17 with gcc 4.4.5
  configuration: --prefix=/home/stefano --enable-libx264 --enable-libxvid --enable-librtmp --disable-shared --enable-static --enable-debug=3 --enable-pthreads --enable-libvorbis --enable-avfilter --enable-gpl --enable-nonfree --enable-libmp3lame --enable-libtheora --enable-gpl --enable-x11grab --enable-libschroedinger --enable-libopenjpeg --enable-frei0r --enable-libopencv --disable-optimizations --disable-mmx --enable-libfreetype
  libavutil    51. 13. 0 / 51. 13. 0
  libavcodec   53. 11. 0 / 53. 11. 0
  libavformat  53.  9. 0 / 53.  9. 0
  libavdevice  53.  3. 0 / 53.  3. 0
  libavfilter   2. 34. 1 /  2. 34. 0
  libswscale    2.  0. 0 /  2.  0. 0
  libpostproc  51.  2. 0 / 51.  2. 0
[testsrc @ 0xa2c0790] size:352x480 rate:25/1 duration:-1.000000 sar:20/11
[scale @ 0xa28efe0] w:352 h:480 fmt:rgb24 -> w:352 h:264 fmt:yuv420p flags:0x2
[lavfi @ 0xa2bf8a0] Estimating duration from bitrate, this may be inaccurate
Input #0, lavfi, from 'testsrc=s=352x480:sar=20/11,scale=iw:ow/dar':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0.0: Video: rawvideo, yuv420p, 352x264 [SAR 20:11 DAR 80:33], 25 tbr, 25 tbn, 25 tbc
   5.32 A-V:  0.000 s:0.0 aq=    0KB vq=  817KB sq=    0B f=0/0   0/0   

Please try and report, I guess it may be a platform-specific problem.
Even in this case, you can force the h value to be a multiple of the chroma vertical subsampling value, for example by setting scale=iw:floor(ow/dar/hsub)*hsub.

[Stefano Sabatini]

Please reopen if you have more information.