select filter does not correctly set prev_pts

[Andrew]

When using the select filter, the filter's internal pts variable is always equal to prev_pts. This is presumably not the intended (or a useful) behavior.

One would expect that prev_pts should always be equal to the pts of the last frame that was processed (but not necessarily chosen) by the select filter.

For instance, if I filter a video for frames where the pts is equal to prev_pts:

ffmpeg -i  big_buck_bunny_480p_surround-fix.avi -vf select='eq(prev_pts\,pts)',showinfo -frames:v 50 out.avi 
ffmpeg version N-49610-gc2dd5a1 Copyright (c) 2000-2013 the FFmpeg developers
  built on Feb  5 2013 13:26:02 with gcc 4.7.2 (GCC)

I get back the original video, rather than nothing at all (which is what you would expect).

Poking around in the source (f_select.c), I see that the values for PTS and PREV_PTS are both set in (basically) the same place. Presumably, PREV_PTS should only be set *after* the filter has decided whether to select or discard the current frame?

 273     select->var_values[VAR_PTS] = TS2D(ref->pts);
 274     select->var_values[VAR_T  ] = TS2D(ref->pts) * av_q2d(inlink->time_base);
 275     select->var_values[VAR_POS] = ref->pos == -1 ? NAN : ref->pos;
 276     select->var_values[VAR_PREV_PTS] = TS2D(ref ->pts);

​http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavfilter/f_select.c;h=4881a85ece8717d49d592a21699cfa1f2ded2b4e;hb=HEAD#l273

Note that I'm running ffmpeg on Windows, although this does not appear to be a platform-specific bug.

[Stefano Sabatini]

Should be fixed in:

commit c0a4871c1687279c5fbe6d5d7f475a393c645b05
Author: Stefano Sabatini <stefasab@gmail.com>
Date:   Thu Feb 7 21:16:11 2013 +0100

    lavfi/select: correctly set prev_t and prev_pts
    
    In particular, fix trac ticket #2248.

Bug was present since first filter commit, thanks for reporting.