499628 - Potential canvas bug with strokeText() and clip()

Status: RESOLVED FIXED

(Core :: Graphics: Canvas2D, defect)

Description

[Dion Almaer]

Attachment: stroke and clip test case

While implementing a canvas animation we started to see that the clipping region was being drawn when using strokeText.

The attached test case shows just this. At first you will just see the text, and quickly after you will see the clipping region box drawn. It happens as soon as a second call to re-paint the area happens.

This doesn't happen if you change the stroke to a fill. E.g.

change: 
ctx.strokeStyle = "white";
ctx.strokeText(message, x, y);

to:
ctx.fillStyle = "red";
ctx.fillText(message, x, y);

Comment 1

[Dion Almaer]

A simple work around was adding a ctx.beginPath(); before the strokeStyle call (thanks Vlad).

Still could be a bug.... WebKit doesn't require this for example, but maybe it is their bug :)

Comment 2

[zedjmat]

Problems with Firefox 3.5+, 4b.
(OK with Chrome.)

After a strokeText() the stokeStyle used to render the text will be also used by the last displayed shape built with a beginPath.

1/ ctx.strokeStyle = "white";
2/ ctx.strokeText ("test",x,y);
3/ ctx.strokeStyle = "green";
4/ ctx.beginPath() ... ctx.arc (...) .... ctx.endPath(); ctx.stroke(); // circle 1 => ok green
5/ ctx.strokeStyle = "blue";
6/ ctx.beginPath() ... ctx.arc (...) .... ctx.endPath(); ctx.stroke(); // circle 2 => nok white

I can add a more complete example if needed.
A first workaround is to display an empty circle (size 0) at the end of the rendering or to display the text only at the end. It's not clean at all.

Regards

Matthias

Comment 3

[zedjmat]

In fact the problem occurs even if strokeText() is called at the end but the sequence
beginPath -> stokeText -> endPath works.

Affects firefox 4.0b6 (Snow Leopard)

Comment 5

[Daniel Baulig]

> A simple work around was adding a ctx.beginPath(); before the
> strokeStyle call (thanks Vlad).

> Still could be a bug.... WebKit doesn't require this for example, but maybe it
> is their bug :)</blockquote>

It is a bug. See the Canvas spec at
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#text-0
, it says:
> Text is painted without affecting the current path, and is subject
> to shadow effects, global alpha, the clipping region, and global composition
> operators.

So calling beginPath, etc shoudl not affect text operations at all.

Comment 6

[Daniel Baulig]

Just adding that this bug was also encountered on Windows 7 and Windows XP and is not limited to Mac OS X. I also believe it's importance should be escalated to normal, because it will break lots of applications using Canvas, possibly making them unusable. If applications using Canvas where more common already I'd even think about escalating it further, but for now I believe it should be on normal.

Comment 7

[Boris Zbarsky [:bzbarsky]]

Fixed by checkin for bug 478445.

Comment 8

[Dave Townsend [:mossop]]

Backed out the patch for being in the range of bug 615736