<div dir="ltr">On Mon, Apr 11, 2016 at 2:43 PM, Bill Spitzak <span dir="ltr"><<a href="mailto:spitzak@gmail.com" target="_blank">spitzak@gmail.com</a>></span> wrote:<br> <br><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>I feel this can be fixed. It is already correct for subsample_bits==0. Since both the filter generator and filtering code would be changed in the same version, only programs that generate their own filters would actually be incompatible. And as you point out the error is very tiny for large numbers of subsamples, and Cairo is already using an excessively large number of subsamples (likely because I was trying to remove the difference between identity filters and nearest filtering, and did not realize this was the underlying problem).</div></div></blockquote><div><br></div><div class="gmail_extra">It is technically an ABI break, and cairo 1.14 did ship with a copied-and-pasted filter generator that assumes the current subpixel positioning.<br><br>But yeah, maybe we can just ignore that, if we make sure a there is a cairo 1.14.x update that uses pixman_filter_create_separate_convolution() instead of the copied-and-pasted filter generator.<br><br>Other than that, the fix should be straight-forward enough.<br><br><br></div><div class="gmail_extra">Søren<br></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Sun, Apr 10, 2016 at 10:01 PM, Søren Sandmann <span dir="ltr"><<a href="mailto:soren.sandmann@gmail.com" target="_blank">soren.sandmann@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><div class="gmail_quote"><span><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
It does look like there is something really wrong. I compared and (except for the subsample_bits==0 case) my version produces the same output as the current git head.<br>
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I think your intention is that there is a sample at offset=0 whether the filter width is even or odd. However (except when subsample_bits==0) the filter generator makes a symmetric filter for even sizes, with two equal samples around the maximum center value. If a sample was at offset==0 then it would be unique and larger than all the other samples.<br></blockquote><div><br></div></span><div>The root of this confusion is probably that when subsample_bits = k, the subpixel positions used are:<br><br></div><div> 0.5 / 2^k, 1.5 / 2^k, ..., (2^k-0.5)/2^k</div><div><br></div><div>For example, for subsample_bits = 2:<br><br> 0.125, 0.375, 0.625, 0.875<br></div><div><br></div><div>and for subsample_bits = 0:<br><br> 0.5<br></div><div><br></div><div>That is, they are regularly spaced, but centered within the pixel. When there is an even number of them, this means there will not be a filter position at 0.5, and therefore no sample at offset 0. And the only case where number of subpixel locations is odd, is when subsample_bits = 0.<br><br>I'm pretty sure that the existing code gets the filter generation right for these subpixel positions.<br><br><br>[ You can argue that it would be better to use the sampling positions<br><br> 0, 0.25, 0.5, 0.75<br></div></div><div class="gmail_quote"><br></div><div class="gmail_quote">for subsample_bits = 2, as Owen did here:<br><br> <a href="https://lists.cairographics.org/archives/cairo/2014-March/025105.html" target="_blank">https://lists.cairographics.org/archives/cairo/2014-March/025105.html</a><br><br></div><div class="gmail_quote">and I agree that that would have been better. ]<span><font color="#888888"><br> <br></font></span></div><span><font color="#888888"><div class="gmail_quote"><br></div><div class="gmail_quote">Søren<br></div></font></span></div></div>
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