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Re: [ft] sub pixel positioning when Auto hinter and cache are used

From: Werner LEMBERG
Subject: Re: [ft] sub pixel positioning when Auto hinter and cache are used
Date: Wed, 03 Jun 2009 09:06:45 +0200 (CEST)

> I have a question on whether it is possible to perform sub pixel
> positioning when my system is using Auto Hinting and FreeType's
> cache subsystem.  If yes, I would be really keen to know how.  I
> need to perform text rendering that is fast (thus it requires cache
> to be activated), support sub pixel positioning and all glyphs are
> hinted.

You can cache glyph outlines using FTC_ImageCache.

> There is one problem with this which I know not yet how to solve,
> which is that FTC_SBit only return the metric values in pixels
> instead of in 1/64th points.

[In the following, I use `pixmap' for anti-aliased renderings of a

Exactly.  It is not possible to cache pixmaps with non-integer
positions.  You would have to store 64 pixmaps for a single glyph

> In addition to that, when we perform Auto hinting, we can't get the
> lsb_delta and rsb_delta of a glyph when FTC_SBitCache_Lookup() is
> used.

Well, since the sbit cache don't allow subpixel positioning, lsb_delta
and rsb_delta are always zero, so there's no need to cache them.


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