Re: [FFmpeg-devel] [PATCH v2 10/18] avcodec/jpegxl: parse and signal correct alpha mode

[Kacper Michajlow <kasper93@gmail.com>]

On Wed, 23 Jul 2025 at 15:57, Niklas Haas <ffmpeg@haasn.xyz> wrote:
>
> From: Niklas Haas <git@haasn.dev>
>
> This header bit ("alpha_associated") was incorrectly ignored.
> ---
>  libavcodec/jpegxl_parse.c  | 7 +++++--
>  libavcodec/jpegxl_parse.h  | 1 +
>  libavcodec/jpegxl_parser.c | 5 +++++
>  3 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/jpegxl_parse.c b/libavcodec/jpegxl_parse.c
> index 022eed322d..47b831e2b4 100644
> --- a/libavcodec/jpegxl_parse.c
> +++ b/libavcodec/jpegxl_parse.c
> @@ -190,6 +190,7 @@ static void jpegxl_get_bit_depth(GetBitContext *gb, FFJXLMetadata *meta)
>  static int jpegxl_read_extra_channel_info(GetBitContext *gb, FFJXLMetadata *meta, int validate)
>  {
>      int default_alpha = get_bits1(gb);
> +    int alpha_associated = 0;
>      uint32_t type, name_len = 0;
>
>      if (!default_alpha) {
> @@ -213,7 +214,7 @@ static int jpegxl_read_extra_channel_info(GetBitContext *gb, FFJXLMetadata *meta
>      skip_bits_long(gb, name_len);
>
>      if (!default_alpha && type == JPEGXL_CT_ALPHA)
> -        skip_bits1(gb);
> +        alpha_associated = get_bits1(gb);
>
>      if (type == JPEGXL_CT_SPOT_COLOR)
>          skip_bits_long(gb, 16 * 4);
> @@ -221,8 +222,10 @@ static int jpegxl_read_extra_channel_info(GetBitContext *gb, FFJXLMetadata *meta
>      if (type == JPEGXL_CT_CFA)
>          jxl_u32(gb, 1, 0, 3, 19, 0, 2, 4, 8);
>
> -    if (meta && type == JPEGXL_CT_ALPHA)
> +    if (meta && type == JPEGXL_CT_ALPHA) {
>          meta->have_alpha = 1;
> +        meta->alpha_associated = alpha_associated;
> +    }
>
>      return 0;
>  }
> diff --git a/libavcodec/jpegxl_parse.h b/libavcodec/jpegxl_parse.h
> index 0602f4d409..bb929ccee1 100644
> --- a/libavcodec/jpegxl_parse.h
> +++ b/libavcodec/jpegxl_parse.h
> @@ -35,6 +35,7 @@ typedef struct FFJXLMetadata {
>      uint32_t coded_height;
>      int bit_depth;
>      int have_alpha;
> +    int alpha_associated;
>      /*
>       * offset, in bits, of the animation header
>       * zero if not animated
> diff --git a/libavcodec/jpegxl_parser.c b/libavcodec/jpegxl_parser.c
> index 75acc53708..c13601f3d7 100644
> --- a/libavcodec/jpegxl_parser.c
> +++ b/libavcodec/jpegxl_parser.c
> @@ -1074,6 +1074,11 @@ static void populate_fields(AVCodecParserContext *s, AVCodecContext *avctx, cons
>          else
>              s->format = meta->have_alpha ? AV_PIX_FMT_RGBAF32 : AV_PIX_FMT_RGBF32;
>      }
> +
> +    if (meta->have_alpha) {
> +        avctx->alpha_mode |= meta->alpha_associated ? AVALPHA_MODE_PREMULTIPLIED
> +                                                    : AVALPHA_MODE_STRAIGHT;

Is this OR expected here? Setting (2 | 1) seems strange, no?

- Kacper

> +    }
>  }
>
>  static int skip_icc_profile(void *avctx, JXLParseContext *ctx, GetBitContext *gb)
> --
> 2.50.1
>
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