Re: [ft] Modifying/Enhancing the Truetype Interpreter

[Moony]

Some issues I'm running into:

Right bearing-
Some letters, for instance Arial normal "v" or "X", or bold "s" appear 
to have TT instructions that move the right bearing toward the left, a 
pixel or so (figured this out with FontForge debug).  This results in 
"s" or "v" being smashed up against the following letter at certain 
point sizes.  (see screenshots).  Which part of the ttinterp.c code 
would control this moving of the right bearing, in order that we can 
disable that part?  (It appears that the MS engine got around this 
problem by keeping instructions on the glyph in the x-direction, to fit 
into this smaller space, but IMO, it looks bad.)


Gamma / LCD filtering-
It looks as if the MS TT engine does a major gamma "correction" after 
running its LCD filter.  I've gotten results that look very similar by 
creating a custom FIR filter { 40, 75, 86, 75, 40 } and then 
gamma-correcting in ftdiff to between 1.5 and 2.2.  (See screenshots).  
I am unable to create a FIR filter that does this all in one step 
however.  I'm guessing that this is because the FIR filter splits 
intensity across pixels, but gamma applies to the whole pixel?  Are 
there any gamma correction patches out there that can be applied to 
ftlcdfil.c to render in such a way, like it does with gamma-correction 
in ftdiff?  (I know gamma correction is expensive, and ideally shouldn't 
be done at the freetype level, but it would still be nice to have the 
option, IMO.)


FDEFs-
I'm not sure how to approach searching the FDEFs for certain opcode 
patterns, in order to disable them.  Suggestions?


Vertical stem snapping-
This is a weird one.  The MS engine is clearly doing snapping of 
vertical stems to fit symmetrically across pixels at certain ppems (13 - 
15?), but other times not (17 - 20?).  Yet, the glyphs don't seem as 
constrained in the x-direction as they do with the TT-instructed b/w 
rasterizer.  I guess I'm interested in getting a good balance between 
not deforming glpyhs too much (such as in the case of s, v, or w), yet 
doing vertical stem alignment under certain circumstances, such as n, m, 
h, or u.... and also leaving enough room for certain glyphs, like v, w, 
X, etc.  Any thoughts on this?


I like the idea about adding PGM output...  seems relatively easy to 
implement.

-Moony


Freetype-rendered, using default LCD filter:
http://www.infinality.net/files/arial-default-lcd.png

Freetype-rendered, using custom FIR filter { 40, 75, 86, 75, 40 } and 
then gamma-correcting to 1.7:
http://www.infinality.net/files/arial-darkened-gamma-corrected.png

Rendered with WinXP:
http://www.infinality.net/files/arial-cleartype.png