[Pixman] [PATCH 05/15] pixman-filter: Correct Simpsons integration

[Oded Gabbay]

On Sat, Dec 12, 2015 at 8:06 PM,  <spitzak at gmail.com> wrote:
> From: Bill Spitzak <spitzak at gmail.com>
>
> Simpsons uses cubic curve fitting, with 3 samples defining each cubic. This
> makes the weights of the samples be in a pattern of 1,4,2,4,2...4,1, and then
> dividing the result by 3.
>
> The previous code was using weights of 1,2,6,6...6,2,1 which produced about 2x
> the correct value, as it was still dividing by 3. The filter normalization
> removed this error. Also this is effectively a linear interpolation except for
> the ends.
> ---
>  pixman/pixman-filter.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
> index 15f9069..7c1da0d 100644
> --- a/pixman/pixman-filter.c
> +++ b/pixman/pixman-filter.c
> @@ -204,11 +204,14 @@ integral (pixman_kernel_t reconstruct, double x1,
>         {
>             double a1 = x1 + h * i;
>             double a2 = x2 + h * i;
> +           s += 4 * SAMPLE(a1, a2);
> +       }
>
> -           s += 2 * SAMPLE (a1, a2);
> -
> -           if (i >= 2 && i < N_SEGMENTS - 1)
> -               s += 4 * SAMPLE (a1, a2);
> +       for (i = 2; i < N_SEGMENTS; i += 2)
> +       {
> +           double a1 = x1 + h * i;
> +           double a2 = x2 + h * i;
> +           s += 2 * SAMPLE(a1, a2);
>         }
>
>         s += SAMPLE (x1 + width, x2 + width);
> --
> 1.9.1
>
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You say:

"The filter normalization removed this error. Also this is effectively
a linear interpolation except for the ends."

So if the error was removed, why is this change needed ? I can see it
is more accurate (similar to the Simpson equation), but it also causes
the code to run over the loop twice.

Do you have some example we can see the difference ?


    Oded