[Git][freetype/freetype][1100-sdf-precision-improvement] [sdf] Impliment deviation based splitting for bezier curves.

[Anuj Verma (@anujv)]

Anuj Verma pushed to branch 1100-sdf-precision-improvement at FreeType / FreeType

Commits:

• 3d3be1df
  by Anuj Verma at 2021-10-15T08:55:41+05:30

  [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.
  

2 changed files:

• src/sdf/ftsdf.c
• src/sdf/ftsdfcommon.h

Changes:

src/sdf/ftsdf.c

---

...	...	@@ -1137,9 +1137,10 @@
1137	1137	FT_Int max_splits,
1138	1138	SDF_Edge** out )
1139	1139	{
1140		- FT_Error error = FT_Err_Ok;
1141		- FT_26D6_Vec cpos[7];
1142		- SDF_Edge* left,* right;
	1140	+ FT_Error error = FT_Err_Ok;
	1141	+ FT_26D6_Vec cpos[7];
	1142	+ SDF_Edge* left,* right;
	1143	+ const FT_26D6 threshold = ONE_PIXEL / 4;
1143	1144
1144	1145
1145	1146	if ( !memory || !out )
...	...	@@ -1148,12 +1149,25 @@
1148	1149	goto Exit;
1149	1150	}
1150	1151
1151		- /* split the conic */
	1152	+ /* split the cubic */
1152	1153	cpos[0] = control_points[0];
1153	1154	cpos[1] = control_points[1];
1154	1155	cpos[2] = control_points[2];
1155	1156	cpos[3] = control_points[3];
1156	1157
	1158	+ /* If the segment is flat enough, we won't get any benifit by */
	1159	+ /* splitting it further, so we can just stop splitting. Here, */
	1160	+ /* we check the deviation of the bezier and stop if it is */
	1161	+ /* lower than a pre-defined `threhold` value. */
	1162	+ if ( FT_ABS( 2 * cpos[0].x - 3 * cpos[1].x + cpos[3].x ) < threshold &&
	1163	+ FT_ABS( 2 * cpos[0].y - 3 * cpos[1].y + cpos[3].y ) < threshold &&
	1164	+ FT_ABS( cpos[0].x - 3 * cpos[2].x + 2 * cpos[3].x ) < threshold &&
	1165	+ FT_ABS( cpos[0].y - 3 * cpos[2].y + 2 * cpos[3].y ) < threshold )
	1166	+ {
	1167	+ split_cubic( cpos );
	1168	+ goto Append;
	1169	+ }
	1170	+
1157	1171	split_cubic( cpos );
1158	1172
1159	1173	/* If max number of splits is done */
...	...	@@ -1250,13 +1264,31 @@
1250	1264	/* Subdivide the curve and add it to the list. */
1251	1265	{
1252	1266	FT_26D6_Vec ctrls[3];
	1267	+ FT_26D6 dx, dy;
	1268	+ FT_UInt num_splits;
1253	1269
1254	1270
1255	1271	ctrls[0] = edge->start_pos;
1256	1272	ctrls[1] = edge->control_a;
1257	1273	ctrls[2] = edge->end_pos;
1258	1274
1259		- error = split_sdf_conic( memory, ctrls, 32, &new_edges );
	1275	+ dx = FT_ABS( ctrls[2].x + ctrls[0].x - 2 * ctrls[1].x );
	1276	+ dy = FT_ABS( ctrls[2].y + ctrls[0].y - 2 * ctrls[1].y );
	1277	+ if ( dx < dy )
	1278	+ dx = dy;
	1279	+
	1280	+ /* Here we calculate the number of necessary bisections. Each */
	1281	+ /* bisection reduces the deviation by exactly 4-fold, hence */
	1282	+ /* we bisect the bezier until the deviation becomes less than */
	1283	+ /* 1/8th of a pixel. For more details check `ftgrays.c`. */
	1284	+ num_splits = 1;
	1285	+ while ( dx > ONE_PIXEL / 8 )
	1286	+ {
	1287	+ dx >>= 2;
	1288	+ num_splits <<= 1;
	1289	+ }
	1290	+
	1291	+ error = split_sdf_conic( memory, ctrls, num_splits, &new_edges );
1260	1292	}
1261	1293	break;
1262	1294

src/sdf/ftsdfcommon.h

---

...	...	@@ -48,7 +48,8 @@ FT_BEGIN_HEADER
48	48	#define MIN_SPREAD 2
49	49	/* maximum spread supported by the renderer */
50	50	#define MAX_SPREAD 32
51		-
	51	+ /* pixel size in 26.6 */
	52	+#define ONE_PIXEL ( 1 << 6 )
52	53
53	54	/**************************************************************************
54	55	*

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