559879 - vertical glyph clipping with opacity when ClearType is enabled

Status: RESOLVED WORKSFORME

(Core :: Graphics, defect)

Description

[Jonathan Kew [:jfkthame]]

Attachment: testcase showing glyph clipping on windows 7

The attached testcase is based on the reftest bidi/386339.html, which fails when run on Windows 7 using a default configuration (with ClearType enabled).

In the last line of the testcase, where the < and > characters have RTL directionality applied (thereby breaking the text run), the glyphs are badly clipped at the bottom. Zooming in or out shows that this is size-dependent; at some sizes, there is no problem, while at others the clipping may occur at either top or bottom.

The problem does not occur in the 100% opacity sample, but I believe changing the opacity causes Cairo to use a fallback path to paint the text, and this is introducing the problematic clipping. The same happens in the reftest framework (without touching opacity) because it uses similar fallback paths to render to canvas instead of a window.

The clipping does not seem to occur if ClearType is turned off.

Comment 1

[Jonathan Kew [:jfkthame]]

Attachment: Screenshot showing the testcase as displayed on Win7

Comment 2

[(mostly gone) XtC4UaLL [:xtc4uall]]

* i see the same on Windows XP
* it only happens with Antialiasing set to "Cleartype"
* it does not happen with Antialiasing set to "Standard"
* it starts with 1.9.0 branch (against 1.8.1).

maybe this is covered by already filed ClearType bugs?

Comment 3

[Jonathan Kew [:jfkthame]]

The underlying problem is similar to bug 445087, for example, but this seems to be the clearest case of *vertical* issues like this; they've more commonly been concerned with *horizontal* antialiasing. (Bug 475968 is also a similar issue.)

I think the proper resolution of this and similar bugs will require us to do bug 542595 first.

Comment 4

[Bas Schouten (:bas.schouten)]

This is WFM in nightly and Fx7. D2D & GDI. Please re-open if anyone still sees this.