[Pixman] [PATCH v14 10/22] pixman-filter: Correct integration with impulse filters

Søren Sandmann soren.sandmann at gmail.com
Tue Mar 22 02:41:11 UTC 2016

On Mon, Mar 21, 2016 at 12:29 PM, Bill Spitzak <spitzak at gmail.com> wrote:

> On Sun, Mar 20, 2016 at 11:36 PM, Søren Sandmann <soren.sandmann at gmail.com
> > wrote:
>> On Sun, Mar 6, 2016 at 8:06 PM, <spitzak at gmail.com> wrote:
>>> From: Bill Spitzak <spitzak at gmail.com>
>>> The IMPULSE special-cases did not sample the center of the of the
>>> region. This
>>> caused it to sample the filters outside their range, and produce
>>> assymetric
>>> filters and other errors. Fixing this required changing the arguments to
>>> integral() so the correct point could be determined.
>> I don't understand what is wrong and why this patch fixes it. Which
>> region precisely did not have its center sampled? When IMPULSE filters are
>> involved the width of the integral is 0 so there isn't really any "region"
>> to sample.
>> Can you give a concrete example where the previous code produced
>> asymmetric filters? Also, what "other errors" was produced? I think these
>> examples should be added to the commit log.
> It sampled the *other* filter (the one that is not impulse) at the left
> edge of the region being passed, rather than at the location of the center
> of the impulse filter. This was detected by putting asserts in the filter
> functions to see if they were being called outside their width.

I tried adding such asserts and I couldn't make them trigger with the scale
demo. It would be helpful if you could give a specific pair of filters and
scale factor where a filter is sampled outside its width.

And it really doesn't make sense to talk about the "region being passed"
when one of the filters is IMPULSE. In that case, the width parameter is
always 0 so there is no "region".

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