[Libav-user] Syncing multiple sources.

[Hristo Ivanov]

Hi.

In my program I have the following filter:
"
 [in0]format=pix_fmts=yuv420p,scale=-1:540[s0];
 [in1]format=pix_fmts=yuv420p,scale=-1:540[s1];
 [s0][s1]vstack[stacked];
 [stacked]pad=1920:1080:(ow-iw)/2:0[out0]
"

My problem comes from the 'vstack' filter. The stack filters assign the
same synclevel to all of their inputs, libavfilter/vf_stack.c around line
290:
    for (i = 0; i < s->nb_inputs; i++) {
        AVFilterLink *inlink = ctx->inputs[i];

        in[i].time_base = inlink->time_base;
        in[i].sync   = 1;  // This line here is the important.
        in[i].before = EXT_STOP;
        in[i].after  = s->shortest ? EXT_STOP : EXT_INFINITY;
    }

    return ff_framesync_configure(&s->fs);

With the same synclevel on all the inputs, the framesync filter activates
on all the inputs, which has the following undesired effect:
   Being bought of the inputs(in0, in1) at 30fps, the output(out0) is at
~60fps.

The easy fix for this, was to use the fps filter and have something like
this:
"
 [in0]format=pix_fmts=yuv420p,scale=-1:540[s0];
 [in1]format=pix_fmts=yuv420p,scale=-1:540[s1];
 [s0][s1]vstack[stacked];
 [stacked]fps=fps=30[normalized];
 [normalized]pad=1920:1080:(ow-iw)/2:0[out0]
"

The problem I have with this solution is that stacking frames at 2xFps just
to discard half of them seems wasteful to me, cpu and memory wise.

Having the following filter:
"
 [in0]format=pix_fmts=yuv420p,scale=-1:540,fps=fps=30[s0];
 [in1]format=pix_fmts=yuv420p,scale=-1:540,fps=fps=30[s1];
 [s0][s1]vstack[stacked];
 [ stacked ]pad=1920:1080:(ow-iw)/2:0[out0]
"
where the framesync of the vstack filter only triggers on one of the inputs
seems to me a better solution.

Is there a way to achieve this? Can the synclevels be set by the users?

Thank.
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