[FFmpeg-devel] [PATCH 0/5] replace scale2ref by scale=rw:rh

[Timo Rothenpieler]

On 24/04/2024 13:18, Gyan Doshi wrote:
> 
> 
> On 2024-04-24 04:21 pm, Niklas Haas wrote:
>> As discussed in my previous series for fixing scale2ref[1], this filter
>> is fundamentally broken, and the only real fix would be to switch to
>> activate(), or ideally FFFrameSync.
>>
>> [1] https://ffmpeg.org//pipermail/ffmpeg-devel/2024-March/323382.html
>>
>> The main thing making this difficult is the fact that scale2ref also
>> wants to output ref frames to its secondary output, which FFFrameSync
>> does not support, and which is ultimately at least part of the root
>> cause of trac #10795.
>>
>> Since this is in principle completely unnecessary (users can just
>> 'split' the ref input and have it be consumed by vf_scale), and to make
>> the design of this filter a bit more robust and maintainable, switch to
>> an approach where vf_scale itself gains the ability to reference
>> a secondary input stream, using the "ref_*" series of variables.
>>
>> This makes the current [i][ri]scale2ref[o][ro] equivalent to the only
>> slightly more verbose [ri]split[t][ro]; [i][t]scale=rw:rh[o]. (And
>> conversely, it is no longer necessary to use nullsink to consume an
>> unused [ro])
> 
> In principle, a good idea, but how does this impact memory use and speed 
> in the not-so-uncommon scenario where multiple overlay targets are 
> scaled 2 ref and then overlaid
> e.g.
> 
> in current flow:
> 
> [a][base]scale2ref[a][ref];
> [b][ref]scale2ref[b][ref[;
> [c][ref]scale2ref[c][ref[;
> [d][ref]scale2ref[d][ref[;
> [ref][a]overlay[ref];
> [ref][b]overlay[ref];
> [ref][c]overlay[ref];
> [ref][d]overlay[ref];
> 
> in new flow:
> 
> [base]split=5[base][refa][refb][refc][refd];
> [a][refa]scale[a];
> [b][refb]scale[b];
> [c][refc]scale[c];
> [d][refd]scale[d];
> [base][a]overlay[outa];
> [outa][b]overlay[outb];
> [outb][c]overlay[outc];
> [outc][d]overlay[out];

Given that scale never modifies the reference, it just ups the refcount 
of those frames in the split filter, and will thus not change the 
memory-use whatsoever.