[FFmpeg-devel] [PATCH v3 1/3] avcodec/mjpegdec: fix non-subsampled RGB JPEGs

[Caleb Etemesi]

if  the problem you are trying to solve is upsampling, the agreed samples
are usually 422,444 and 420 (Y,Cb,Cr order) but there can be files with
sampling factors like 242, since the jfif spec isn't to restrictive on what
sampling factors are allowed,libjpeg(and it's cousin libjpeg-turbo) support
such factors, so ideally the way to treat such things is on a channel to
channel basis wnd figure out which ones need to be upsampled(and check if
there is code relying on the Y channel not being upsampled)

Note that I haven't looked into the code, just met with a similar problem
sometime ago hence may be wrong.

On Thu, 20 Apr 2023, 00:15 Leo Izen, <leo.izen at gmail.com> wrote:

> On 4/19/23 16:37, Michael Niedermayer wrote:
> > On Wed, Apr 19, 2023 at 03:23:41PM -0400, Leo Izen wrote:
> >> On 4/19/23 14:58, Michael Niedermayer wrote:
> >>> On Wed, Apr 19, 2023 at 02:11:24PM -0400, Leo Izen wrote:
> >>>> The change introduced in b18a9c29713abc3a1b081de3f320ab53a47120c6
> >>>> created a regression for non-subsampled progressive RGB jpegs. This
> >>>> should fix that.
> >>>> ---
> >>>>    libavcodec/mjpegdec.c | 3 ++-
> >>>>    1 file changed, 2 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/libavcodec/mjpegdec.c b/libavcodec/mjpegdec.c
> >>>> index 01537d4774..1e3ddb72fb 100644
> >>>> --- a/libavcodec/mjpegdec.c
> >>>> +++ b/libavcodec/mjpegdec.c
> >>>> @@ -1698,7 +1698,8 @@ int ff_mjpeg_decode_sos(MJpegDecodeContext *s,
> const uint8_t *mb_bitmask,
> >>>>            s->h_scount[i]  = s->h_count[index];
> >>>>            s->v_scount[i]  = s->v_count[index];
> >>>> -        if(nb_components == 3 && s->nb_components == 3 &&
> s->avctx->pix_fmt == AV_PIX_FMT_GBRP)
> >>>> +        if((nb_components == 3 || nb_components == 1) &&
> s->nb_components == 3
> >>>> +                && s->avctx->pix_fmt == AV_PIX_FMT_GBRP &&
> !s->progressive)
> >>>>                index = (index+2)%3;
> >>>
> >>> Why is progressive/!progressive special cased in all the new RGB code ?
> >>>
> >>
> >> With progressive, I decode RGB in RGB-order, and then pivot it into
> >> GBR-order, whereas baseline is just decoded directly into GBR-order. If
> you
> >> decode progressive directly in GBR-order the buffers will be the wrong
> size
> >> and it will overrun the subsampled buffer when filling it with a
> >> non-subsampled one. See the allocation block on line 766 of mjpegdec.c.
> This
> >> depends on h_count and v_count, which cannot be changed or pivoted as
> if you
> >> do so, progressive JPEGs will fail to decode at all (invalid VLC
> entries,
> >> etc.)
> >>
> >> Ideally, you'd just alloc them the right size, but s->component_index[i]
> >> won't refer to the right index for many progressive files, depending on
> >> whether the SOS marker has 1 or 3 components. If you have SOS markers
> with
> >> one component it will not properly pivot the colors.
> >>
> >> Initially, I didn't have the checks and just always decoded in RGB
> order and
> >> then pivoted, but that broke some baseline files like the ones in Trac
> >> #4045. I used some casework so I could handle all files I tested with
> this.
> >>
> >> If anyone has any suggestions on how to make the casework more elegant
> I'm
> >> all ears but this is the solution I found to work with every sample I
> >> tested.
> >
> > First i would document which array is in PIXFMT vs. JPEG order
> > when anything is in 2 different orders at different points or for
> > different cases thats probably not a good idea.
> > But even if such bad cases exist, it should be documented
> >
> > progressive is complicated because one could argue that it needs
> > to be possible to both add pieces into the image and also to
> > make these pieces immedeatly available to the user so some
> > application could present to the user the image as it is
> > "progressing". Ok we maybe dont care for that feature but its
> > still not a bad way to look at the problem.
> > I presume all jpeg streams can be decoded without too much problems
> > if everything is in jpeg order.
> > at the same time to present it we need planes to be scaled and
> > ordered into a standard RGB/GBR/YUV form.
> > I think these 2 worlds JPEG vs presentation should be more clearly
> > seperated,
> >
> > am i seeing the issue correctly or am i missing the problems here ?
> >
> > thx
>
> I could try to see if I can decode *every* image in JPEG order and then
> pivot the planes from RGB to GBR order at the end, but it might take me
> a bit more time to figure it out. I'll take a look at it this week. It
> would be a more elegant solution and wouldn't require us to document
> which planes go where in which places of the code.
>
> - Leo Izen
>
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