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[sdf] Impliment deviation based splitting for bezier curves. 

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

* src/sdf/ftsdfcommon.h (ONE_PIXEL): Added macro for unit pixel size
  in 26.6 fixed point representation.
This commit is contained in:
Anuj Verma 2021-10-15 08:47:41 +05:30
parent 8ffa954507
commit 3d3be1dfd1
2 changed files with 39 additions and 6 deletions
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42
src/sdf/ftsdf.c
View File

@ -1137,9 +1137,10 @@
FT_Int max_splits, FT_Int max_splits,
SDF_Edge** out ) SDF_Edge** out )
{ {
FT_Error error = FT_Err_Ok; FT_Error error = FT_Err_Ok;
FT_26D6_Vec cpos[7]; FT_26D6_Vec cpos[7];
SDF_Edge* left,* right; SDF_Edge* left,* right;
const FT_26D6 threshold = ONE_PIXEL / 4;
if ( !memory || !out ) if ( !memory || !out )
@ -1148,12 +1149,25 @@
goto Exit; goto Exit;
} }
/* split the conic */ /* split the cubic */
cpos[0] = control_points[0]; cpos[0] = control_points[0];
cpos[1] = control_points[1]; cpos[1] = control_points[1];
cpos[2] = control_points[2]; cpos[2] = control_points[2];
cpos[3] = control_points[3]; cpos[3] = control_points[3];
/* If the segment is flat enough, we won't get any benifit by */
/* splitting it further, so we can just stop splitting. Here, */
/* we check the deviation of the bezier and stop if it is */
/* lower than a pre-defined `threhold` 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 ); split_cubic( cpos );
/* If max number of splits is done */ /* If max number of splits is done */
@ -1250,13 +1264,31 @@
/* Subdivide the curve and add it to the list. */ /* Subdivide the curve and add it to the list. */
{ {
FT_26D6_Vec ctrls[3]; FT_26D6_Vec ctrls[3];
FT_26D6 dx, dy;
FT_UInt num_splits;
ctrls[0] = edge->start_pos; ctrls[0] = edge->start_pos;
ctrls[1] = edge->control_a; ctrls[1] = edge->control_a;
ctrls[2] = edge->end_pos; 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;
/* Here we calculate the number of necessary bisections. Each */
/* bisection reduces the deviation by exactly 4-fold, hence */
/* we bisect the bezier until the deviation becomes less than */
/* 1/8th of a pixel. For more details check `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; break;

3
src/sdf/ftsdfcommon.h
View File

@ -48,7 +48,8 @@ FT_BEGIN_HEADER
#define MIN_SPREAD 2 #define MIN_SPREAD 2
/* maximum spread supported by the renderer */ /* maximum spread supported by the renderer */
#define MAX_SPREAD 32 #define MAX_SPREAD 32
/* pixel size in 26.6 */
#define ONE_PIXEL ( 1 << 6 )
/************************************************************************** /**************************************************************************
* *