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Re: Analysis of redisplay performance on Windows
From: |
Chong Yidong |
Subject: |
Re: Analysis of redisplay performance on Windows |
Date: |
Sat, 26 Jul 2008 23:07:26 -0400 |
Jason Rumney <address@hidden> writes:
> The redisplay performance problems on Windows seem to be mostly caused
> by the left_overwriting and right_overwriting functions. These functions
> analyse all glyphs, one by one, before and after (respectively) the
> current glyph string on the same row to see if the glyphs overlap the
> current glyph string.
>
> To determine the overlap for each glyph, it is necessary to encode it
> into a glyph code point in the font used to display it, then measure its
> text extents. Even with caching of the text extents, the Windows code
> is still especially slow here because of the encoding to glyph code points.
>
> Currently we cache glyph code points at the glyph_string level. Perhaps
> caching them at the glyph row level would help, as we could then reuse
> them after we have finished with the glyph string. Alternatively we
> could keep all the glyph strings for the row until we are finished so we
> could use glyph code points and other information from there. This might
> reduce the number of iterations we need to find the left and right
> overwriting glyphs, since we can check the glyph strings for overlaps
> first, and only check the glyphs inside glyph strings that overlap.
>
> There may also be a problem with the setting of
> row->contains_overlapping_glyphs_p on Windows. The above functions
> should only be called when that is set, but after inserting debugging
> code I can see them being called frequently even when using fonts that
> contain no overlapping glyphs (confirmed by further debugging code in
> w32font_text_metrics).
Thanks for looking into this, Jason.
But I don't understand why the problem of left_overwriting and
right_overwriting, if that's indeed the culprit, would be specific to
Windows. These functions are called in the platform-independent
redisplay code. Or is it much more expensive to look up glyph code
points in fonts on Windows, compared to GNU/Linux?
- Re: Analysis of redisplay performance on Windows,
Chong Yidong <=
- Re: Analysis of redisplay performance on Windows, Jason Rumney, 2008/07/27
- Re: Analysis of redisplay performance on Windows, Chong Yidong, 2008/07/27
- Re: Analysis of redisplay performance on Windows, Jason Rumney, 2008/07/27
- Re: Analysis of redisplay performance on Windows, Chong Yidong, 2008/07/27
- Re: Analysis of redisplay performance on Windows, Jason Rumney, 2008/07/27
- Re: Analysis of redisplay performance on Windows, Chong Yidong, 2008/07/27
- Re: Analysis of redisplay performance on Windows, Adrian Robert, 2008/07/27
- Re: Analysis of redisplay performance on Windows, Chong Yidong, 2008/07/28
- Re: Analysis of redisplay performance on Windows, Jason Rumney, 2008/07/28
- Re: Analysis of redisplay performance on Windows, Kevin Yu, 2008/07/27