1764961 - stroking a rect with a non-default lineCap is blurry

Status: NEW

(Core :: Graphics: Canvas2D, defect, P3)

Description

[Julien Wajsberg [:julienw]]

Attachment: canvas1.html

STR:

1. Open the attachment.
2. Look at the left edge of the 2 rectangles at the bottom. It's blurry but it should be sharp.

My understanding is that the stroke should sit astride the edge of the rect, and that shouldn't depend on the lineCap value.

Chrome looks as expected.

Note: the code in the attachment takes care that the pixels in the canvas are always aligned with device pixels. If you're using a retina display, that could make the issue difficult to see. You can use your OS' zoom capacity to look closer. I'll attach some screenshots of what I see on my system.

Comment 1

[Julien Wajsberg [:julienw]]

Attachment: zoomed in result in Firefox

Comment 2

[Julien Wajsberg [:julienw]]

Attachment: zoomed in result in Chrome

Comment 3

[Julien Wajsberg [:julienw]]

Everything happens as if the stroke starts at the edge of the rectangle instead of half a pixel before.
What's strange is that the other edges look good.

Comment 4

[Julien Wajsberg [:julienw]]

Attachment: testcase without fill + strokeRect

There's no issue with strokeRect, cf this other testcase.

Comment 5

[Julien Wajsberg [:julienw]]

Just like bug 1759155 comment 17, on my system (Linux), I get this pushlog https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=6202ade0e6d688ffb67932398e56cfc6fa04ceb3&tochange=b2dbee5ca727e87bdaeab9ab60fb83df2a9846a2
which is probably bug 1244754.

Comment 6

[Julien Wajsberg [:julienw]]

Attachment: same testcase without resize observer

I removed the resize observer so that we can use mozregression on older builds.

Comment 7

[BugBot [:suhaib / :marco/ :calixte]]

Set release status flags based on info from the regressing bug 1244754

Comment 8

[BugBot [:suhaib / :marco/ :calixte]]

:lsalzman, since you are the author of the regressor, bug 1244754, could you take a look?
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Comment 9

[Lee Salzman [:lsalzman]]

See bug 1759155 comment 19 for a discussion of the cause of this.

Comment 10

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:lsalzman, could you have a look please?

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