[FFmpeg-devel] [PATCH 2/2] avfilter: Dont partially merge lists

[Nicolas George]

Le quartidi 4 thermidor, an CCXXI, Michael Niedermayer a écrit :
> This prevents the unneeded insertion of multiple aresample filters in some cases
> 
> The format merging is moved to avoid having to call the channel layout
> merge twice (which currently does not work as dry run mode with ff_merge_channel_layouts()
> does not work

I am rather surprised this is necessary, and there is something going on I
do not understand yet. If I take the example case you sent me, it starts
like that:

    +---+                        +-------+                        +---+
    |in |----------------------->|aformat|----------------------->|out|
    |flt|                        |   *   |                        |s16|
    |6ch|                        |  2ch  |                        | * |
    +---+                        +-------+                        +---+

The merge of the first link will fail because 6ch is not compatible with
2ch, so it becomes:

    +---+       +-------+        +-------+                        +---+
    |in |------>|arsmple|------->|aformat|----------------------->|out|
    |flt|       | * | * |        |   *   |                        |s16|
    |6ch|       | * | * |        |  2ch  |                        | * |
    +---+       +-------+        +-------+                        +---+

Then the links around aresample are merged, both with success:

    +---+       +-------+        +-------+                        +---+
    |in |------>|arsmple|------->|aformat|----------------------->|out|
    |flt|       |flt| * |        |   *   |                        |s16|
    |6ch|       |6ch|2ch|        |  2ch  |                        | * |
    +---+       +-------+        +-------+                        +---+

Then the link between aformat and out get merged (contaminating the link
between aresample and aformat since aformat does not convert):

    +---+       +-------+        +-------+                        +---+
    |in |------>|arsmple|------->|aformat|----------------------->|out|
    |flt|       |flt|s16|        |  s16  |                        |s16|
    |6ch|       |6ch|2ch|        |  2ch  |                        |2ch|
    +---+       +-------+        +-------+                        +---+

And finally lavfi can pick the formats on the links:

    +---+       +-------+        +-------+                        +---+
    |in |------>|arsmple|------->|aformat|----------------------->|out|
    |flt|  flt  |flt|s16|  s16   |  s16  |          s16           |s16|
    |6ch|  6ch  |6ch|2ch|  2ch   |  2ch  |          2ch           |2ch|
    +---+       +-------+        +-------+                        +---+

Unfortunately, it does not happen that way, and I do not understand why yet.

Regards,

-- 
  Nicolas George
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