[FFmpeg-user] anomalous JPEG resolution

[Lou]

On Thu, 29 Sep 2011 10:31:57 -0400
Steven Weed <blekenbleu at gmail.com> wrote:

> 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)

Can you duplicate this if you move -ss after "-i IMG_0031.MOV"? Moving
-ss as an output option changes its behavior:

-ss after -i: Decode until -ss value, and then start encoding.

-ss before -i: Seek before decoding. Can be significantly faster than
the other method, but has the potential of not being frame accurate
(depending on your input format).

...at least that's how I interpret it.

Consider moving your other options as well because placing options
before -i is generally meant to apply to the input only.