111482 - PNG images with transparency used as background degrades CSS/DHTML performance

Status: VERIFIED DUPLICATE of bug 98252

(Core :: DOM: Events, defect)

Description

[Pascal Chevrel (relman team) -> :pascalc]

I Have a local page with a menu (positionned DIV) on the left side of the
screen. When you point your mouse on links in this menu, there is a a:hover
effect and an onMouseover script displays another positionned DIV on top of the
screen.

If you put a background color, a gif, jpeg or PNG images without transparency as
a background, there is no change in performance.

If you put a PNG image with alpha channel transparency as a background, all
links in this DIV have a 3 second delay before reacting to the a:hover and
showing the hidden layer. Wintop shows +- 50% CPU usage on my machine when you
quickly move the mouse over the links with no background image. It jumps to
80/85% if there is a PNG image with alpha transparency.

My machine is a slow one (K6-2-300/256Mb ram/ xpert98 gfx card) so I don't know
if the problem is visible on a more recent hardware.
Pascal

Comment 1

[Pascal Chevrel (relman team) -> :pascalc]

I put test pages online to show the problem.

Page with PNG-alphatransparent background :
http://perso.chevrelbureau.com/bombazins/index.php3

Page without any background image :
http://perso.chevrelbureau.com/bombazins/index2.php3

Page with a PNG backgroung without transparency :
http://perso.chevrelbureau.com/bombazins/index3.php3


BTW, this bug could be related to bug 99153 (Extremely slow performance with big
png backgrounds), except that I tested with small background images (10x10px)
and big ones (200x600px) and there is no difference.

Comment 2

[Olivier Cahagne]

dup of bug 98252 ?

Comment 3

[Boris Zbarsky [:bzbarsky]]

Compositing alpha channels correctly takes work.... :)  Dup, as Olivier pointed out

*** This bug has been marked as a duplicate of 98252 ***

Comment 4

[Vladimir Ermakov]

verifying