133077 - Incorrect PNG image repainting.

Status: RESOLVED DUPLICATE of bug 121230

(Core :: Graphics: ImageLib, defect)

Description

[James Ross]

From Bugzilla Helper:
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:0.9.9) Gecko/20020311
BuildID:    2002031104 (0.9.9)

On the page specified, the logo image (100x100 PNG) exihibits some very odd
redrawing issues.

Reproducible: Always
Steps to Reproduce:
1. Go to page (http://www.warwickcompsoc.co.uk/).
2. Place a window accross the logo so that it covers the top half.
3. Move mouse over remaining bottom half.

Actual Results:  The bottom half of the image redraws to show the top half.

Expected Results:  No noticible redrawing, i.e. redrawing with the bottom half
of the image.

To round this off, I think I know exactly what is causing this, at least from a
point of view of observing it carefully, and not looking at the source code.

It seems that whenever the image is redrawn, Mozilla only redraws the
invalidated region (a good thing). However, it seems that when calculating what
part of the image should go in this region, the top is not calculated (at all,
it seems), so while the invalidated region contains the correct part of the
image horizontally, the top of the invalidated region is *always* the top of the
image. which isn't always right.

You can see this a different way to the steps above by dragging a window across
the logo so that is obscures variying amounts of image, while always having the
top visible. I will attach a screen-shot of the results.

Comment 1

[James Ross]

Attachment: CompSoc logo drawn correctly

What the logo looks like when loaded the first time, or redrawn completly.

Comment 2

[James Ross]

Attachment: CompSoc logo drawn when obscured

What happens if you cover half the image and run the mouse over it (which
causes a repaint).

Comment 3

[James Ross]

Attachment: CompSoc logo drawn completetly wrong

What hapens if you drag a window over the bottom half at varying gaps from the
top.

Comment 4

[Phil Anderton]

*** This bug has been marked as a duplicate of 121230 ***