[Pixman] [PATCH 12/15] pixman-filter: Turn off subsampling when not necessary

[Bill Spitzak]

On Tue, Dec 22, 2015 at 4:44 AM, Oded Gabbay <oded.gabbay at gmail.com> wrote:

> On Sat, Dec 12, 2015 at 8:06 PM,  <spitzak at gmail.com> wrote:
> > From: Bill Spitzak <spitzak at gmail.com>
> >
> > If sample is IMPULSE and reconstruct is BOX or IMPULSE the sub-pixel
> > position of the sample is not relevant, so only one subsample is needed.
> Why ?
> I mean why it is not relevant ? and why only one subsample is needed ?
>

Because all the filters for all the subsample positions are the same (a
single 1).

Actually though, the code is indicating this is happening by returning a
width of zero. But I think that is not necessary: if the filter width is 1,
and the filters are normalized so they sum to 1, then all the filters must
be equal and be a single 1. So I think the filter_width function can return
1 and the caller treats all values <= 1 as an indication that subsampling
is not needed.

Oded
> > ---
> >  pixman/pixman-filter.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
> > index 64981cd..7e10108 100644
> > --- a/pixman/pixman-filter.c
> > +++ b/pixman/pixman-filter.c
> > @@ -230,6 +230,8 @@ filter_width (pixman_kernel_t reconstruct,
> >               pixman_kernel_t sample,
> >               double scale)
> >  {
> > +    if (reconstruct == PIXMAN_KERNEL_BOX && sample ==
> PIXMAN_KERNEL_IMPULSE)
> > +       return 0;
> >      return ceil (scale * filters[sample].width +
> filters[reconstruct].width);
> >  }
> >
> > @@ -323,13 +325,13 @@ pixman_filter_create_separable_convolution (int
>          *n_values,
> >      int subsample_x, subsample_y;
> >      int width, height;
> >
> > -    subsample_x = (1 << subsample_bits_x);
> > -    subsample_y = (1 << subsample_bits_y);
> > -
> >      width = filter_width (reconstruct_x, sample_x, sx);
> > -    if (width < 1) width = 1;
> > +    if (width < 1) { width = 1; subsample_bits_x = 0; }
> >      height = filter_width (reconstruct_y, sample_y, sy);
> > -    if (height < 1) height = 1;
> > +    if (height < 1) { height = 1; subsample_bits_y = 0; }
> > +
> > +    subsample_x = (1 << subsample_bits_x);
> > +    subsample_y = (1 << subsample_bits_y);
> >
> >      *n_values = 4 + width * subsample_x + height * subsample_y;
> >
> > --
> > 1.9.1
> >
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>
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