shithub: freetype+ttf2subf

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[sdf] Implement deviation-based splitting for Bezier curves.

* src/sdf/ftsdf.c (split_sdf_cubic, split_sdf_shape): Add checks to figure
out the deviation of Bezier curves and stop splitting if the curve is flat
enough.

* src/sdf/ftsdfcommon.h (ONE_PIXEL): New macro.

git/fs: mount .git/fs: mount/attach disallowed

--- a/src/sdf/ftsdf.c

+++ b/src/sdf/ftsdf.c

@@ -764,9 +764,9 @@

                 const FT_26D6_Vec*  to,

                 void*               user )

   {

-    SDF_Shape*    shape    = ( SDF_Shape* )user;

-    SDF_Edge*     edge     = NULL;

-    SDF_Contour*  contour  = NULL;

+    SDF_Shape*    shape   = ( SDF_Shape* )user;

+    SDF_Edge*     edge    = NULL;

+    SDF_Contour*  contour = NULL;

     FT_Error   error  = FT_Err_Ok;

     FT_Memory  memory = shape->memory;

@@ -1137,23 +1137,38 @@

                    FT_Int        max_splits,

                    SDF_Edge**    out )

   {

-    FT_Error     error = FT_Err_Ok;

-    FT_26D6_Vec  cpos[7];

-    SDF_Edge*    left,*  right;

+    FT_Error       error = FT_Err_Ok;

+    FT_26D6_Vec    cpos[7];

+    SDF_Edge*      left, *right;

+    const FT_26D6  threshold = ONE_PIXEL / 4;

-    if ( !memory || !out  )

+    if ( !memory || !out )

     {

       error = FT_THROW( Invalid_Argument );

       goto Exit;

     }

-    /* split the conic */

+    /* split the cubic */

     cpos[0] = control_points[0];

     cpos[1] = control_points[1];

     cpos[2] = control_points[2];

     cpos[3] = control_points[3];

+    /* If the segment is flat enough we won't get any benefit by */

+    /* splitting it further, so we can just stop splitting.      */

+    /*                                                           */

+    /* Check the deviation of the Bezier curve and stop if it is */

+    /* smaller than the pre-defined `threshold` value.           */

+    if ( FT_ABS( 2 * cpos[0].x - 3 * cpos[1].x + cpos[3].x ) < threshold &&

+         FT_ABS( 2 * cpos[0].y - 3 * cpos[1].y + cpos[3].y ) < threshold &&

+         FT_ABS( cpos[0].x - 3 * cpos[2].x + 2 * cpos[3].x ) < threshold &&

+         FT_ABS( cpos[0].y - 3 * cpos[2].y + 2 * cpos[3].y ) < threshold )

+    {

+      split_cubic( cpos );

+      goto Append;

+    }

+

     split_cubic( cpos );

     /* If max number of splits is done */

@@ -1250,6 +1265,8 @@

           /* Subdivide the curve and add it to the list. */

           {

             FT_26D6_Vec  ctrls[3];

+            FT_26D6      dx, dy;

+            FT_UInt      num_splits;

             ctrls[0] = edge->start_pos;

@@ -1256,7 +1273,24 @@

             ctrls[1] = edge->control_a;

             ctrls[2] = edge->end_pos;

-            error = split_sdf_conic( memory, ctrls, 32, &new_edges );

+            dx = FT_ABS( ctrls[2].x + ctrls[0].x - 2 * ctrls[1].x );

+            dy = FT_ABS( ctrls[2].y + ctrls[0].y - 2 * ctrls[1].y );

+            if ( dx < dy )

+              dx = dy;

+

+            /* Calculate the number of necessary bisections.  Each      */

+            /* bisection causes a four-fold reduction of the deviation, */

+            /* hence we bisect the Bezier curve until the deviation     */

+            /* becomes less than 1/8th of a pixel.  For more details    */

+            /* check file `ftgrays.c`.                                  */

+            num_splits = 1;

+            while ( dx > ONE_PIXEL / 8 )

+            {

+              dx         >>= 2;

+              num_splits <<= 1;

+            }

+

+            error = split_sdf_conic( memory, ctrls, num_splits, &new_edges );

           }

           break;

--- a/src/sdf/ftsdfcommon.h

+++ b/src/sdf/ftsdfcommon.h

@@ -48,6 +48,8 @@

 #define MIN_SPREAD      2

   /* maximum spread supported by the renderer */

 #define MAX_SPREAD      32

+  /* pixel size in 26.6 */

+#define ONE_PIXEL       ( 1 << 6 )

   /**************************************************************************