293226 - Antialiasing uses wrong colors with strokeRect() on Canvas element

Status: RESOLVED DUPLICATE of bug 293353

(Core :: Graphics: Canvas2D, defect)

Description

[Jens Bannmann]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.3) Gecko/20040910
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.8b2) Gecko/20050506 Firefox/1.0+

When drawing stroked rectangles with non-even values for .lineWidth, the display
antialiasing uses wrong colors for white and gray backgrounds.

No idea whether this is canvas-specific or a general bug in the graphics backend.

Reproducible: Always

Steps to Reproduce:
1. Load attached testcase
2. Switch to different background colors using the provided links

Actual Results:  
For gray and white backgrounds, antialiasing is performed using some dark cyan
colors around rgb(135,191,199). For red, green and blue backgrounds, things look
okay.

Expected Results:  
It should have used some clean gray colors for antialiasing.

Comment 1

[Jens Bannmann]

Attachment: testcase with stroked rectangles, lineWidth 1 to 5

Comment 2

[Jens Bannmann]

Attachment: screenshot with white background, zoomed to 300%

Comment 3

[Jens Bannmann]

Attachment: screenshot with gray background, zoomed to 300%

Comment 4

[Jens Bannmann]

Attachment: screenshot with red background, zoomed to 300%

Comment 5

[Jens Bannmann]

Additional observation: at the lower left and lower right corners, a bright
green pixel appears inside the rectangle - on all backgrounds, see the screenshots.

Comment 6

[Jens Bannmann]

Attachment: updated test case, uses styling

Since Gecko/20050507, the canvas no longer seems to be transparent by default,
so it doesn't pick up the body's bgcolor any longer (bug 293259). This is an
updated patch that directly styles the canvas itself (which won't work in older
builds due to bug bug 291262).

Comment 7

[Jens Bannmann]

Attachment: updated screenshot with white background, zoomed to 300%

Comment 8

[Jens Bannmann]

Attachment: updated screenshot with gray background, zoomed to 300%

Comment 9

[Jens Bannmann]

With bug 293219 (RGB order mixup) being fixed now, the dark cyan color is
replaced with some light brown/dark orange color around  rgb(199,191,135).

Related question: is the fact that a 1-pixel black rectangle is not drawn as
straight black line but some antialiased, two-pixel line the intended behaviour,
or a bug that should be filed?

Comment 10

[Sjoerd Visscher]

> is the fact that a 1-pixel black rectangle is not drawn as
straight black line but some antialiased, two-pixel line the intended behaviour

Yes, I think so. The coordinates are pixel boundaries, i.e. (0,0) is the top
left corner of the top left pixel, not the center of the top left pixel. So a 1
pixel line on a pixel boundary means 2 half pixel width lines on both side of
the boundary, which is rendered as 2 50% transparent 1 pixel width lines.

Comment 11

[Peter van der Woude [:Peter6]]

see bug 293353#c11
VC6. (beast) is bad, VC7.1 (vlad's) WFM

Comment 12

[(not currently active) Ted Mielczarek]

I know, I know, this bug has a lower bug #.  But the other one is set blocking,
blah blah blah.

*** This bug has been marked as a duplicate of 293353 ***