[FFmpeg-user] anomalous JPEG resolution

[Steven Weed]

Resolutions (and file sizes) for JPEG files from
.MOV frames vary based on extraction sequence.
Specifically, the third and fourth files are always larger
(more than 2x average size) basically independent
of the corresponding frame.

Third images typically resolve 80 lines horizontally, while others resolve
70.
For the following example,
changing "-ss 0.6" to "-ss 0.3" yields different files
but with similar relative sizes and resolutions.
PNG output shows similar resolution behavior, although file sizes vary less.

Can other command-line options more consistently extract
still images of highest resolution?

Example:
* captured short iPad 1280x72 videos of ITU Fax chart,
  which includes targets of converging black/white lines.
* transferred to OS X 10.6 iMac, converted to JPEG, e.g.

$ ffmpeg -b 2000K -ss 0.6  -vframes 10 -i IMG_0031.MOV IMG31ss.6_v%d.jpg
ffmpeg version 0.7.4, Copyright (c) 2000-2011 the FFmpeg developers
  built on Sep 15 2011 10:14:37 with gcc 4.2.1 (Apple Inc. build 5666) (dot
3)
  configuration: --prefix=/opt/local --enable-gpl --enable-postproc
--enable-swscale --enable-avfilter --enable-libmp3lame --enable-libvorbis
--enable-libtheora --enable-libdirac --enable-libschroedinger
--enable-libxvid --enable-libx264 --enable-libvpx --enable-libspeex
--mandir=/opt/local/share/man --enable-shared --enable-pthreads
--disable-indevs --cc=/usr/bin/gcc-4.2 --arch=x86_64 --enable-yasm
--enable-nonfree --enable-libfaac
  libavutil    50. 43. 0 / 50. 43. 0
  libavcodec   52.122. 0 / 52.122. 0
  libavformat  52.110. 0 / 52.110. 0
  libavdevice  52.  5. 0 / 52.  5. 0
  libavfilter   1. 80. 0 /  1. 80. 0
  libswscale    0. 14. 1 /  0. 14. 1
  libpostproc  51.  2. 0 / 51.  2. 0

Seems stream 0 codec frame rate differs from container frame rate: 1200.00
(1200/1) -> 29.97 (30000/1001)
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'IMG_0031.MOV':
  Metadata:
    major_brand     : qt
    minor_version   : 0
    compatible_brands: qt
    creation_time   : 2011-09-26 18:41:29
  Duration: 00:00:01.09, start: 0.000000, bitrate: 10384 kb/s
    Stream #0.0(und): Video: h264 (Baseline), yuv420p, 1280x720, 10516 kb/s,
29.95 fps, 29.97 tbr, 600 tbn, 1200 tbc
    Metadata:
      creation_time   : 2011-09-26 18:41:29
    Stream #0.1(und): Audio: aac, 44100 Hz, mono, s16, 59 kb/s
    Metadata:
      creation_time   : 2011-09-26 18:41:29
Incompatible pixel format 'yuv420p' for codec 'mjpeg', auto-selecting format
'yuvj420p'
[buffer @ 0x101732780] w:1280 h:720 pixfmt:yuv420p tb:1/1000000 sar:0/1
sws_param:
[buffersink @ 0x101732980] auto-inserting filter 'auto-inserted scaler 0'
between the filter 'src' and the filter 'out'
[scale @ 0x101732c80] w:1280 h:720 fmt:yuv420p -> w:1280 h:720 fmt:yuvj420p
flags:0x4
Output #0, image2, to 'IMG31ss.6_v%d.jpg':
  Metadata:
    major_brand     : qt
    minor_version   : 0
    compatible_brands: qt
    creation_time   : 2011-09-26 18:41:29
    encoder         : Lavf52.110.0
    Stream #0.0(und): Video: mjpeg, yuvj420p, 1280x720, q=2-31, 200 kb/s,
90k tbn, 29.97 tbc
    Metadata:
      creation_time   : 2011-09-26 18:41:29
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop, [?] for help
frame=   10 fps=  0 q=20.0 Lsize=      -0kB time=00:00:00.33 bitrate=
-0.5kbits/s dup=0 drop=17
video:722kB audio:0kB global headers:0kB muxing overhead -100.002975%

$ ls -lt | head -11
total 72456
-rw-r--r--  1 stevenweed  staff    72198 Sep 29 09:31 IMG31ss.6_v1.jpg
-rw-r--r--  1 stevenweed  staff    45372 Sep 29 09:31 IMG31ss.6_v10.jpg
-rw-r--r--  1 stevenweed  staff    85183 Sep 29 09:31 IMG31ss.6_v2.jpg
-rw-r--r--  1 stevenweed  staff   106895 Sep 29 09:31 IMG31ss.6_v3.jpg
-rw-r--r--  1 stevenweed  staff   107535 Sep 29 09:31 IMG31ss.6_v4.jpg
-rw-r--r--  1 stevenweed  staff    85583 Sep 29 09:31 IMG31ss.6_v5.jpg
-rw-r--r--  1 stevenweed  staff    69671 Sep 29 09:31 IMG31ss.6_v6.jpg
-rw-r--r--  1 stevenweed  staff    62969 Sep 29 09:31 IMG31ss.6_v7.jpg
-rw-r--r--  1 stevenweed  staff    54815 Sep 29 09:31 IMG31ss.6_v8.jpg
-rw-r--r--  1 stevenweed  staff    49305 Sep 29 09:31 IMG31ss.6_v9.jpg