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

[Niklas Haas <ffmpeg@haasn.xyz>]

On Wed, 23 Jul 2025 18:19:03 +0200 Kacper Michajlow <kasper93@gmail.com> wrote:
> 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?

Fixed, thanks.

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