[cairo] Text metric adventures
keithp at keithp.com
Tue Dec 28 11:57:35 PST 2004
Around 11 o'clock on Dec 28, Owen Taylor wrote:
> What about rounding advances to integer pixel amounts when hinting? If you
> don't do that and instead round positions for each glyph individually
> you'll get different results than Xft, etc.
FreeType provides two sets of advances -- the linear advances which are
unhinted linearly scaled values and the "normal" advances which come right
out of the hinting engine. If the hinting engine snaps those to pixel
boundaries (and I think they generally do), then our advances will exactly
match those from Xft, assuming no post-scaling transform.
As the metrics I'm computing are all in 'glyph space' (currently scaled by
the font scale factors), there isn't any non-scaling transforms applied to
them. This means that when you rotate the metrics, you'll end up with
non-pixel grid aligned positions. I think we have to use these and snap
each glyph into position instead of snapping the metrics and leaving the
baseline to wander all over the screen. That Xft doesn't provide sub-pixel
metrics for rotated text is not something I think we need to replicate in
I think we also want to stick both advance values into the glyph metrics
returned to the application so that it can choose which to use. With
that, we could decide to snap any un-snapped metrics to pixel boundaries.
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