[Pixman] [PATCH v10 13/15] pixman-filter: Negative subsample values produces scaled result

[Oded Gabbay]

On Tue, Feb 2, 2016 at 8:28 AM,  <spitzak at gmail.com> wrote:
> From: Bill Spitzak <spitzak at gmail.com>
>
> If a negative value is used for the subsampling, then -n subsamples are
> used at scale==1, and fewer are used at larger scale, more are used at
> smaller scale, so that the total number of samples is approximately
> the same. The computed value is rounded up to the next power of 2.
>
> The scale demo is modified to allow these negative numbers, and initially
> uses -12.
>
> Signed-off-by: Bill Spitzak <spitzak at gmail.com>
> ---
>  demos/scale.ui         |  5 +++--
>  pixman/pixman-filter.c | 24 ++++++++++++++++++++++++
>  2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/demos/scale.ui b/demos/scale.ui
> index b62cbfb..1e77f56 100644
> --- a/demos/scale.ui
> +++ b/demos/scale.ui
> @@ -24,12 +24,12 @@
>      <property name="page_size">10</property>
>    </object>
>    <object class="GtkAdjustment" id="subsample_adjustment">
> -    <property name="lower">0</property>
> +    <property name="lower">-256</property>
>      <property name="upper">12</property>
>      <property name="step_increment">1</property>
>      <property name="page_increment">1</property>
>      <property name="page_size">0</property>
> -    <property name="value">4</property>
> +    <property name="value">-12</property>
>    </object>
>    <object class="GtkWindow" id="main">
>      <child>
> @@ -321,6 +321,7 @@
>                        <object class="GtkSpinButton" id="subsample_spin_button">
>                          <property name="visible">True</property>
>                         <property name="adjustment">subsample_adjustment</property>
> +                       <property name="value">-12</property>
>                        </object>
>                        <packing>
>                          <property name="left_attach">1</property>
> diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
> index f28cc29..6e8024a 100644
> --- a/pixman/pixman-filter.c
> +++ b/pixman/pixman-filter.c
> @@ -342,14 +342,38 @@ filter_width(pixman_kernel_t reconstruct, pixman_kernel_t sample,
>              double* size, int* subsample_bits)
>  {
>      int width;
> +    /* IMPULSE.x does not work for size < 1.0 */
>      if (reconstruct == PIXMAN_KERNEL_IMPULSE && *size < 1.0)
>         *size = 1.0;
> +    /* Convolution adds the widths of the filters together */
>      width = ceil (filters[reconstruct].width + *size * filters[sample].width);
> +    /* If there will only be one sample, it must be 1.0 due to normalization,
> +       and subsampling is useless. */
>      if (width <= 1)
>      {
>         width = 1;
>         *subsample_bits = 0;
>      }
> +    else if (*subsample_bits < 0)
> +    {
> +       /* The intention was to do -n / size rounded up to the next power of 2,
> +          but this non-linear function seems to work better. For large size
> +          it is the width of the BOX.BOX filter. For small size it reduces
> +          samples by 2 at maximum. */
> +       double desired_samples = -*subsample_bits;
> +       if (sample == PIXMAN_KERNEL_IMPULSE)
> +           ; /* For x.IMPULSE no scaling is done */
> +       else if (*size >= 1.0)
> +           desired_samples *= 2.0 / (*size + 1.0);
> +       else
> +           desired_samples *= 2.0 / ((*size + 1.0) * *size);
> +       *subsample_bits = (int) ceil (log2(desired_samples) - .01);
> +       if (*subsample_bits < 0)
> +           *subsample_bits = 0;
> +       else if (*subsample_bits > 8)
> +           /* Assume we cannot see more than 256 different shades and limit subsampling */
> +           *subsample_bits = 8;
> +    }
>      return width;
>  }
>
> --
> 1.9.1
>
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I tried applying just the first hunk (the scale.ui change) without the
second hunk to see If I get any visual errors in scale demo, but
couldn't see anything wrong. So, I would need to see some form of
test/example that is affected by this patch, as I don't have the
knowledge to even ACK it.

Otherwise, you will need to get someone else to review this (Soren ?).

Oded