[cairo-bugs] [Bug 73531] New: Extreme slowness drawing dashed lines

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=73531

          Priority: medium
            Bug ID: 73531
          Assignee: chris at chris-wilson.co.uk
           Summary: Extreme slowness drawing dashed lines
        QA Contact: cairo-bugs at cairographics.org
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: terra at gnome.org
          Hardware: Other
            Status: NEW
           Version: unspecified
         Component: general
           Product: cairo

First off, apologies for a bug report with precious few details.  I am
hoping this rings a bell for some cairo developer.

The Gnumeric team has received a few reports of severe cpu usage when we
do our "walking ants" animation that marks a copied selection.  I do not
see this on my Linux machine; I might see it with our win32 binary under
wine, but that's a whole lot of extra variables.

To see the walking ants animation in action, start Gnumeric, select a
large area, and press ctrl-c.  Alternatively, watch
http://jeff.ecchi.ca/public/gnumeric-381011.webm

Here's what the animation code does:

1. We set a two-item dashed line style using cairo_set_dash.
2. We set a line width using cairo_set_line_width
3. We set one of the two ant colours.
4. We set a rectangle path using cairo_rectangle
5. We draw a line along that path using cairo_stroke_preserve.
   [That draws half the ant pattern, say the black bits.]
6. We set the other ant colour.
7. We set the stipple with an offset of one "ant" length
8. We draw a line along that path using cairo_stroke.
   [That draws half the ant pattern, say the wite bits.]

Repeat that every 150ms with the two colours swapped.

The actual code is at
    https://git.gnome.org/browse/gnumeric/tree/src/item-cursor.c
near line 450.

Is there any reason why this should bring a modern machine to its knees?



Further random info:

Both my Linux and win32 cairo are 1.12.16.

One original bug report for this:
https://bugzilla.gnome.org/show_bug.cgi?id=381011 starting at comment 12.

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