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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.
This commit is contained in:
Anuj Verma 2022-03-04 16:53:27 +05:30 committed by Werner Lemberg
parent 5499d7bf81
commit 2600ef637e
2 changed files with 45 additions and 9 deletions
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52
src/sdf/ftsdf.c
View File

@ -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 */
@ -1250,13 +1265,32 @@
/* 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;
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;

2
src/sdf/ftsdfcommon.h
View File

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