[FFmpeg-devel] [PATCH 1/2] libavcodec/jpeg2000dec: Enhance pix fmt selection

[Gautam Ramakrishnan]

On Sun, Jul 5, 2020 at 12:00 AM Michael Niedermayer
<michael at niedermayer.cc> wrote:
>
> On Sat, Jul 04, 2020 at 11:01:14PM +0530, Gautam Ramakrishnan wrote:
> > On Sat, Jul 4, 2020 at 12:02 AM Michael Niedermayer
> > <michael at niedermayer.cc> wrote:
> > >
> > > On Fri, Jul 03, 2020 at 01:05:12AM +0530, Gautam Ramakrishnan wrote:
> > > > On Thu, Jul 2, 2020 at 8:30 PM Michael Niedermayer
> > > > <michael at niedermayer.cc> wrote:
> > > > >
> > > > > On Thu, Jul 02, 2020 at 12:04:22AM +0530, gautamramk at gmail.com wrote:
> > > > > > From: Gautam Ramakrishnan <gautamramk at gmail.com>
> > > > > >
> > > > > > This patch assigns default pix format values when
> > > > > > a match does not take place.
> > > > > > ---
> > > > > >  libavcodec/jpeg2000dec.c | 9 +++++++++
> > > > > >  1 file changed, 9 insertions(+)
> > > > > >
> > > > > > diff --git a/libavcodec/jpeg2000dec.c b/libavcodec/jpeg2000dec.c
> > > > > > index 3f4a9ef96c..86f9170723 100644
> > > > > > --- a/libavcodec/jpeg2000dec.c
> > > > > > +++ b/libavcodec/jpeg2000dec.c
> > > > > > @@ -436,6 +436,15 @@ static int get_siz(Jpeg2000DecoderContext *s)
> > > > > >                  s->cdef[3] = 3;
> > > > > >                  i = 0;
> > > > > >              }
> > > > > > +        } else if (ncomponents == 3 && s->precision == 8) {
> > > > > > +            s->avctx->pix_fmt = AV_PIX_FMT_RGB24;
> > > > > > +            i = 0;
> > > > > > +        } else if (ncomponents == 2 && s->precision == 8) {
> > > > > > +            s->avctx->pix_fmt = AV_PIX_FMT_YA8;
> > > > > > +            i = 0;
> > > > >
> > > > > I dont think 2 and 3 component formats can ignore the subsampling
> > > > > of the planes.
> > > > > They have to match even if they are not 1
> > > > > if they mismatch the differing plane would need to be rescaled
> > > > >
> > > > > Thats unless iam missing something, which is possible
> > > > I do not have a very good understanding of colour formats. But,
> > > > I checked out p1_07.j2k, which is a 2 component image. The subsampling
> > > > of the 2 layers are different. As a result, both components have different
> > > > dimensions. As a reference, I decoded the pgx files and saw the output.
> > > > It does match with what would happen with this patch applied. However,
> > > > that is probably because the samples are designed just to test the working
> > > > of the decoder and not to actually account for intricacies of the pixel formats.
> > > > As a compromise, do you think if would make sense if this patch can be applied
> > > > if the subsampling of all the planes are the same, but not the same as
> > > > that of rgb24
> > > > or ya8. Atleast, in that case, we will not have differing planes.
> > > > In case of YCC formats, it makes sense that every subsampling rate must match.
> > > > However, for rgb, from my understanding, it could also mean that the
> > > > final image is
> > > > just scaled according to the sample separation rate?
> > >
> > > for RGB the image leaving the decoder each of the R, G and B planes must
> > > have the same size.
> > > for YA the luma and alpha planes similarly must have the same size.
> > >
> > > if all planes are considered not subsampled or they are all considered
> > > subsampled by the same amount is the same from the point of view of
> > > the pixel format.
> > It is clear. I shall add a check that ensures that the subsampling of all the
> > planes are the same.
>
> > > Also the decoder could rescale a single subsampled plane if its needed.
> > > Our jpeg decoder does that for some of these bizare cases
> > I did not understand which cases are referred here. This is important because
> > this is very relevant to the p1_07.j2k reference image.
>
> I refered to something ive seen in plain jpeg
> files which contain some YCbCr format but the Cb and Cr planes use each
> different subsampling, to support this one plane is rescaled in the decoder.
>
In that case, I'll refer to the jpeg decoder and see if a similar
exception could be made
for the jpeg2000 decoder to support p1_07.j2k.
> Iam suspecting something similar could be occuring in jpeg2000 but i did not
> check
>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> Let us carefully observe those good qualities wherein our enemies excel us
> and endeavor to excel them, by avoiding what is faulty, and imitating what
> is excellent in them. -- Plutarch
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Gautam |