Re: [ft-devel] FT_MAX_CURVE_DEVIATION vs ONE_PIXEL

[Алексей Подтележников]

Hi all,

On Wed, Oct 13, 2010 at 9:41 AM, Werner LEMBERG <address@hidden> wrote:
>
>> I guess my version has PIXEL_BITS = 6. Sorry, didn't notice the
>> discrepancy.  Yes, FT_MAX_CURVE_DEVIATIO ought to be defined so that
>> it's always 1/4 of a pixel.
>
> Alexej, can you provide a patch to fix that?

The patch is below. As was intended in the original code, it aims at the max
deviation of ONE_PIXEL / 4. Remember that the original code actually used
ONE_PIXEL / 16. The speed-up is nice:

ftbench -b c -s 36 /usr/share/fonts/default/Type1/p052003l.pfb
Render                    : 29.216 us/op   (before, see 243.png)
Render                    : 25.480 us/op   (after, see 244.png)

Frankly, I think I am starting to notice the quality down a bit.
Please judge for yourself.
One quater is probably too close to the size of "subpixels", so I
would consider
ONE_PIXEL / 8 instead. Would I make it tunable? No way, I don't want people
to ever curse at freetype because someone misjudged his eyesight. It's too
subjective for one person to decide.



diff --git a/src/smooth/ftgrays.c b/src/smooth/ftgrays.c
index 17d172f..4828013 100644
--- a/src/smooth/ftgrays.c
+++ b/src/smooth/ftgrays.c
@@ -91,11 +91,6 @@
 #define FT_COMPONENT  trace_smooth


-  /* The maximum distance of a curve from the chord, in 64ths of a pixel; */
-  /* used when flattening curves.                                         */
-#define FT_MAX_CURVE_DEVIATION  16
-
-
 #ifdef _STANDALONE_


@@ -891,14 +886,14 @@ typedef ptrdiff_t  FT_PtrDist;
     if ( dx < dy )
       dx = dy;

-    if ( dx <= FT_MAX_CURVE_DEVIATION )
+    if ( dx <= ONE_PIXEL / 4 )
     {
       gray_render_line( RAS_VAR_ UPSCALE( to->x ), UPSCALE( to->y ) );
       return;
     }

     level = 1;
-    dx /= FT_MAX_CURVE_DEVIATION;
+    dx /= ONE_PIXEL / 4;
     while ( dx > 1 )
     {
       dx >>= 2;
@@ -1074,7 +1069,7 @@ typedef ptrdiff_t  FT_PtrDist;
           goto Split;

         /* Max deviation may be as much as (s/L) * 3/4 (if Hain's v = 1). */
-        s_limit = L * (TPos)( FT_MAX_CURVE_DEVIATION / 0.75 );
+        s_limit = L * (TPos)( ONE_PIXEL / 3 );

         /* s is L * the perpendicular distance from P1 to the line P0-P3. */
         dx1 = arc[1].x - arc[0].x;

243.png
Description: PNG image

244.png
Description: PNG image