421587 - Rotated text draws non-deterministically when repainted.

Status: RESOLVED WONTFIX

(Core :: SVG, defect)

Description

[Gavin Kistner]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9b3) Gecko/2008020514 Firefox/3.0b3
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9b3) Gecko/2008020514 Firefox/3.0b3

With a JS-animated object moving over text that is in a rotated group, the text repaints in slightly different ways as it is dirtied and redrawn. The result is that the text 'jiggles' on the screen when it should be static, and (when the redraw covers only portions of the glyphs) renders in broken ways.

Reproducible: Always

Steps to Reproduce:
1. Preview the attached file "JigglyText.svg".
(May require Tahoma or Arial fonts to be present.)
Actual Results:  
Text draws inconsistently as the arrow is drawn over it.

Expected Results:  
The text should render unchanged.

Seen on FF3.0b3 on both XP and OS X. Does not occur under FF2.

Note also how the visual weight of the rendered glyphs seems to change depending on the angle at which they are rendered, and the size of the canvas to which this is being drawn. (Particularly apparent at smaller image sizes.)

Comment 1

[Gavin Kistner]

Attachment: Numbers on a radial dial with a hand animated over them via JS.

Comment 2

[Jonathan Watt [:jwatt]]

Great bug reports, thanks! Unfortunately we're coming into the last week for possible changes before FF3 is pretty much completely locked down, and we have a lot of bugs we'd like to fix. Any chance you could help out here by simplifying the test to an absolutely minimal size? Just one <text> element and one <line> element and as little JavaScript as possible?

Comment 3

[Gavin Kistner]

My mistake - this particular bug only seems to apply to WinXP - rechecking on FF3.0b3 on OS X shows the fonts looking good (non-jiggly, and also consistent weight).

I shall endeavor to further pare down these test cases.

Comment 4

[Gavin Kistner]

Attachment: Rotated and unrotated text repainting with JS-animated line overtop.

Comment 5

[Gavin Kistner]

Drastically reduced test case attached. Note that the "OK" text does have transform="rotate(...)", but it is OK because the rotation is 0. Changing it to 1 degree rotation starts to show the problem, where 0.1 does not.

(I'm guessing this all has to do with some text legibility metrics which prevent the 0.1 rotation from having any effect.)

Comment 6

[Robert Longson [:longsonr]]

Duplicate of bug 311534 perhaps?

Comment 7

[Robert Longson [:longsonr]]

On further investigation not connected to bug 311534

Comment 8

[John Daggett (:jtd)]

Wow, very cool testcases.  Doesn't happen on Mac.

Comment 9

[Robert Longson [:longsonr]]

Fixed on trunk provided you have Vista or Windows 7 and direct write is enabled (gfx.font_rendering.directwrite.enabled = true)

Comment 10

[Robert Longson [:longsonr]]

Likely fixed by bug 525095 though I didn't check. That means you should see this working from Firefox 12

Comment 11

[Boris Zbarsky [:bzbarsky]]

> Likely fixed by bug 525095 

We have tests in the testsuite marked as failing due to this bug.... and they didn't start passing when bug 525095 was fixed.  Specifically, layout/reftests/svg/dynamic-text-04.svg

Comment 12

[Robert Longson [:longsonr]]

The letters in the second attachment testcase have stopped wobbling around though.

Comment 13

[Robert Longson [:longsonr]]

(In reply to Boris Zbarsky (:bz) from comment #11)
> 
> We have tests in the testsuite marked as failing due to this bug.... and
> they didn't start passing when bug 525095 was fixed.  Specifically,
> layout/reftests/svg/dynamic-text-04.svg

It did start passing, just not all the time :-(

Comment 14

[Robert Longson [:longsonr]]

Only affects obsolete unsupported platforms these days.