[FFmpeg-devel] [PATCH 2/4] h264dec: be more explicit in handling container cropping

[Michael Niedermayer]

On Mon, May 08, 2017 at 03:46:23PM -0300, James Almer wrote:
> From: Anton Khirnov <anton at khirnov.net>
> 
> The current condition can trigger in cases where it shouldn't, with
> unexpected results.
> Make sure that:
> - container cropping is really based on the original dimensions from the
>   caller
> - those dimenions are discarded on size change
> 
> The code is still quite hacky and eventually should be deprecated and
> removed, with the decision about which cropping is used delegated to the
> caller.
> ---
> This merges commit 4fded0480f20f4d7ca5e776a85574de34dfead14 from libav
> 
>  libavcodec/h264_slice.c | 20 +++++++++++++-------
>  libavcodec/h264dec.c    |  3 +++
>  libavcodec/h264dec.h    |  5 +++++
>  3 files changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
> index acf6a73f60..a7916e09ce 100644
> --- a/libavcodec/h264_slice.c
> +++ b/libavcodec/h264_slice.c
> @@ -378,6 +378,8 @@ int ff_h264_update_thread_context(AVCodecContext *dst,
>      h->avctx->coded_width   = h1->avctx->coded_width;
>      h->avctx->width         = h1->avctx->width;
>      h->avctx->height        = h1->avctx->height;
> +    h->width_from_caller    = h1->width_from_caller;
> +    h->height_from_caller   = h1->height_from_caller;
>      h->coded_picture_number = h1->coded_picture_number;
>      h->first_field          = h1->first_field;
>      h->picture_structure    = h1->picture_structure;

> @@ -874,13 +876,17 @@ static int init_dimensions(H264Context *h)
>      av_assert0(sps->crop_top + sps->crop_bottom < (unsigned)h->height);
>  
>      /* handle container cropping */
> -    if (FFALIGN(h->avctx->width,  16) == FFALIGN(width,  16) &&
> -        FFALIGN(h->avctx->height, 16) == FFALIGN(height, 16) &&
> -        h->avctx->width  <= width &&
> -        h->avctx->height <= height
> -    ) {
> -        width  = h->avctx->width;
> -        height = h->avctx->height;
> +    if (h->width_from_caller > 0 && h->height_from_caller > 0     &&
> +        !sps->crop_top && !sps->crop_left                         &&
> +        FFALIGN(h->width_from_caller,  16) == FFALIGN(width,  16) &&
> +        FFALIGN(h->height_from_caller, 16) == FFALIGN(height, 16) &&
> +        h->width_from_caller  <= width &&
> +        h->height_from_caller <= height) {
> +        width  = h->width_from_caller;
> +        height = h->height_from_caller;
> +    } else {
> +        h->width_from_caller  = 0;
> +        h->height_from_caller = 0;
>      }

With this, seeking in a file could affect if croping is used
would something break if croping was unaffected by what was priorly
decoded ?

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Complexity theory is the science of finding the exact solution to an
approximation. Benchmarking OTOH is finding an approximation of the exact
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