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

[bugzilla-daemon at freedesktop.org]

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

--- Comment #1 from Uli Schlachter <psychon at znc.in> ---
I don't really understand the code, but here is what I think I found.

The 150ms timeout is implemented in item_cursor_realize() in item-cursor.c. For
items with ->style == GNM_ITEM_CURSOR_ANTED it does this:

  ic->animation_timer = g_timeout_add (150, (GSourceFunc)
cb_item_cursor_animation, ic);

Looking at the callback function, we see that it just toggles ic->state and
calls goc_item_invalidate() on the item. I haven't actually found that
function's implementation, but debian code search gave me the following
(goc_item_invalidate(item) just calls goc_item_maybe_invalidate(item, FALSE);):

http://sources.debian.net/src/goffice/0.10.9-1/goffice/canvas/goc-item.c?hl=473#L451

This calculates the bounds of the items and invalidates that rectangle on the
canvas.

TL;DR: gnumeric redraws "basically all of its content" about 7 times per second
and then wonders that this is slow. (Or did I miss something?)

Re "this used to work fine for over a decade": The implementation a decade ago
was more intelligent (I guess instead of redrawing things, it just drew the ant
line ones and then used an XOR operator to "toggle the dashes/ants").

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