[Devel] Getting accurate advances

[David Bevan]

Hi!

In our product (Emtex VIP) we keep all character advances to the nearest 
1/14400".

I was using advance.x but this is rounded to the nearest pixel - though 
the first problem I noticed was that a character could end up with a 
smaller advance in a larger point size :( - something to do with the 2% 
correction in the FT_CONFIG_CHESTER_BLUE_SCALE code apparently.

So I tried linearHoriAdvance which was better but, despite what the 
tutorial says, still not accurate enough - because it is calculated 
using x_ppem (ftobjs.c line 554) which has been rounded to the nearest 
pixel (x_scale is similarly rounded). The cumulative error can be 
several mm with a text string of 15cm or so.

In the end I added my own fudge factor to correct the advance:

  UINT uiVIPSPerPel = VIP_BASE_UNITS / usResolutionDPI;

  DOUBLE flAdvanceAdjust = ((flSizeX * usResolutionDPI) / 72) / 
pftFace->size->metrics.x_ppem;

  lInlineIncr = ToLONG(flAdvanceAdjust * 
FromFT16Dot16(pftGlyph->linearHoriAdvance) * uiVIPSPerPel);

Perhaps the tutorial could be changed so that it doesn't claim that 
linearHoriAdvance is accurate to 1/65536th of a pixel - or, better, 
linearHoriAdvance could be calculated accurately (I can't see how to do 
this without storing an additional unrounded value in a new field).

Thanks.

David Bevan

--
 ________________________________________________________
 David Bevan, Emtex Ltd.
 Emtex House, Station Road, Kings Langley, Herts. WD4 8LH England
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