[Mjpeg-developer] Patch for mjpeg_play/lavtools/jpeg2yuv.c to speed-up generation of stills

[Pieter du P. <mjp...@so...>]

Hi,

I attached a patch for mjpeg_play/lavtools/jpeg2yuv.c in order to
speed it up when an identical filename has to be processed in
succession. This is usually the case when you'd like to produce some
video of 'stills'.

eg. Generating 10 seconds of still video from x.jpg, in 6 different
ways:

jpeg2yuv -v3 -R0 -n1 -Ip -f25 -l250 -j x.jpg > x.yuv
jpeg2yuv -v3 -R0 -n250 -Ip -f25 -l1 -j x.jpg > x.yuv
yes x.jpg | head -3 | jpeg2yuv -v3 -R0 -n1 -Ip -f25 -l250 > x.yuv
yes x.jpg | head -3 | jpeg2yuv -v3 -R0 -n250 -Ip -f25 -l1 > x.yuv
echo x.jpg | jpeg2yuv -v3 -R0 -n1 -Ip -f25 -l250 > x.yuv
echo x.jpg | jpeg2yuv -v3 -R0 -n250 -Ip -f25 -l1 > x.yuv

Apart from the last 3 examples, the current code for jpeg2yuv opens,
reads, closes and analyses the jpg file 250 times. This has a serious
speed impact. The suggested patch simply enables the code to remember
the last used filename. If the current filename in the iteration is
the same as the last used one, it skips opening, reading, closing and
analysing the jpg file.

It would be nice to have my patch included, as:
- it does not change any functionality of jpeg2yuv.
- it does not change jpeg2yuv's command line interface.
- it massively speeds up the generation of stills, when using jpg2yuv
  as shown in my first 3 examples. As most users will opt for the
  first or second example, this patch will help them.

Regards
--
Pieter