From: Peter Constable <>
Date: Sun, 21 Aug 2011 19:56:44 +0000

From: [] On Behalf Of Philippe Verdy

> A GSUB operation will only be used if it is specified in the correct feature
> table. The problem here is which feature to use: "rtlm" or "ltrm" ? It's
> impossible to know because it first depend on the layout engine to KNOW
> exactly if the run of text is RTL or LTR.

The layout engine already _has_ to know the bidi level of a run regardless.

> Without a font-level support of BiDi properties of PUAs (or unassigned
> characters),

I'm trying to tell you that, wrt mirroring, that's already defined in the OpenType spec.

> the layout engine will assume the wrong guess from the "default" property
> value. And then it won't find the expected GSUB operation, because it won't
> match it in the correct feature subtable.

As I explained in an earlier message, the layout engine doesn't use the "default" property value but the resolved bidi level.

Btw, in the past few weeks, you've written several posts in which you make assertions about how rendering implementations work and, in some cases, why more is needed. And then I or others have to spend a bunch of time writing responses so that you get the correct understanding and, more importantly, so that others don't get mislead. It would be a lot easier if you just asked, "How is this done?"

Received on Sun Aug 21 2011 - 14:59:12 CDT

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