On 05/08/10 02:09 AM, Werner LEMBERG wrote:
Might there be a key place that does some sort of equivalent of a
"PutPixel(i, j, value)" that I could watch while debugging my
Cairo-based case? Or does the algorithm not lend itself to that? I
figure someone must be turning on that offending pixel, and
detecting that case might illuminate the problem.
The two central routines are Vertical_Sweep_Span and
Horizontal_Sweep_Span in combination with the drop-out control
functions Vertical_Sweep_Drop and Horizontal_Sweep_Drop. The generic
calls to them happen in the function Draw_Sweep. The first pass
handles the vertical sweep, the second pass (if asked for) does a
horizontal sweep which basically only checks for drop-outs.
For each scanline, the start and end points of spans are calculated,
sorted, and passed to the *_Sweep_Span routines. They blacken all
pixels fully covered by the spans – this is trivial – and then decide
how to render the partially covered ones and what to do with gaps –
this is tricky. I've tried to improve the drop-out handling in 2.3.10
(the documentation in the TrueType specification is very bad
unfortunately); I've probably worsened your particular case.
Ok, thanks for the overview. I've been stepping through this stuff,
adding print statements (See attached), etc, but have a couple of
questions/comments...
- Note that I cut the "A" part out of my font, so I'm only rendering
the accent part at the top. It seems to be 4 scanlines, matching
some of the attached output.
- Even though I'm getting different rasterization results between
2.3.9 and 2.3.11, I can't seem to see anything different being done.
The output I attached is the same for both versions, leading me to
suspect I'm not looking at the relevant code.
- I found the spot(s) in Vertical_Sweep_Span() where some pixels
are being set (See attached), but I can't seem to find where the
rest of the pixels are being set. I had expected the 'while' loop
at the bottom to be doing it, setting 0xFF into *target, but that
loop is never entered...
- Vertical_Sweep_Drop() is never called.
- Horizontal_Sweep_Span() is called, but doesn't do anything (The
'if' is never entered).
- Horizontal_Sweep_Drop() doesn't do anything when called (Bails
early due to leftmost/rightmost stub tests)
- Can you clarify what you define a 'dropout' to be, so I'm not
making any incorrect assumptions?
I know all this is kindof vague, but I've still had no luck
isolating a reproducible test case, so this is the best I can
offer at the moment... By any chance, does the output maybe
suggest what is going on, or where I should be looking closer?
Again, many thanks for any help/suggestions!
Ian
Draw_Sweep
Vertical_Sweep_Init
Vertical_Sweep_Span: y=1, x1=227,x2=560 , e1=4,e2=8
VSS: Changing target[0] from 0 to 15
VSS: Changing target[1] from 0 to 128
Vertical_Sweep_Step
Vertical_Sweep_Span: y=2, x1=163,x2=496 , e1=3,e2=7
VSS: Changing *target from 0 to 31
Vertical_Sweep_Step
Vertical_Sweep_Span: y=3, x1=99,x2=432 , e1=2,e2=6
VSS: Changing *target from 0 to 62
Vertical_Sweep_Step
Vertical_Sweep_Span: y=4, x1=35,x2=368 , e1=1,e2=5
VSS: Changing *target from 0 to 124
Vertical_Sweep_Step
Draw_Sweep
Horizontal_Sweep_Init
Horizontal_Sweep_Drop: y=0, x1=289,x2=291 , e1=320,e2=256
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=1, x1=225,x2=291
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=2, x1=161,x2=291
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=3, x1=97,x2=291
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=4, x1=34,x2=291
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=5, x1=6,x2=291
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=6, x1=6,x2=241
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=7, x1=6,x2=178
Horizontal_Sweep_Step
Horizontal_Sweep_Span: y=8, x1=6,x2=114
Horizontal_Sweep_Step
Horizontal_Sweep_Drop: y=9, x1=6,x2=50 , e1=64,e2=0
Horizontal_Sweep_Step
_______________________________________________
Freetype mailing list
[email protected]
http://lists.nongnu.org/mailman/listinfo/freetype
