compositor: reduce the number of triangles

The intersection of two rectangles is guaranteed to be convex. Therefore
we do not need a center vertex for the triangle fan, we can simply use
the whatever first vertex the intersection polygon has. This reduces the
number of triangles, while still painting the exact same area.

While at it, emit_vertex() nested function is factored into the
for-loop, since that is the only calling site left.

Comments are updated to reflect the changes, and some unrelated comment
fixes are in repaint_region().

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Rob Clark <rob.clark@linaro.org>

src/compositor.c

@@ -1160,10 +1160,10 @@ texture_region(struct weston_surface *es, pixman_region32_t *region,
 	rects = pixman_region32_rectangles(region, &nrects);
 	surf_rects = pixman_region32_rectangles(surf_region, &nsurf);
 
-	/* worst case we can have 10 vertices per rect (ie. clipped into
+	/* worst case we can have 8 vertices per rect (ie. clipped into
 	 * an octagon):
 	 */
-	v = wl_array_add(&ec->vertices, nrects * nsurf * 10 * 4 * sizeof *v);
+	v = wl_array_add(&ec->vertices, nrects * nsurf * 8 * 4 * sizeof *v);
 	vtxcnt = wl_array_add(&ec->vtxcnt, nrects * nsurf * sizeof *vtxcnt);
 
 	inv_width = 1.0 / es->pitch;
@@ -1173,26 +1173,14 @@ texture_region(struct weston_surface *es, pixman_region32_t *region,
 		pixman_box32_t *rect = &rects[i];
 		for (j = 0; j < nsurf; j++) {
 			pixman_box32_t *surf_rect = &surf_rects[j];
-			GLfloat cx, cy;
+			GLfloat sx, sy;
 			GLfloat ex[8], ey[8];          /* edge points in screen space */
 			int n;
 
-			void emit_vertex(GLfloat gx, GLfloat gy)
-			{
-				GLfloat sx, sy;
-				surface_from_global_float(es, gx, gy, &sx, &sy);
-				/* In groups of 4 attributes, first two are 'position', 2nd two
-				 * are 'texcoord'.
-				 */
-				*(v++) = gx;
-				*(v++) = gy;
-				*(v++) = sx * inv_width;
-				*(v++) = sy * inv_height;
-			}
-
 			/* The transformed surface, after clipping to the clip region,
 			 * can have as many as eight sides, emitted as a triangle-fan.
-			 * The first vertex is the center, followed by each corner.
+			 * The first vertex in the triangle fan can be chosen arbitrarily,
+			 * since the area is guaranteed to be convex.
 			 *
 			 * If a corner of the transformed surface falls outside of the
 			 * clip region, instead of emitting one vertex for the corner
@@ -1201,33 +1189,24 @@ texture_region(struct weston_surface *es, pixman_region32_t *region,
 			 *
 			 * To do this, we first calculate the (up to eight) points that
 			 * form the intersection of the clip rect and the transformed
-			 * surface. After that we calculate the average to determine
-			 * the center point, and emit the center and edge vertices of
-			 * the fan.
+			 * surface.
 			 */
 			n = calculate_edges(es, rect, surf_rect, ex, ey);
 			if (n < 3)
 				continue;
 
-			/* calculate/emit center point: */
-			cx = 0;
-			cy = 0;
+			/* emit edge points: */
 			for (k = 0; k < n; k++) {
-				cx += ex[k];
-				cy += ey[k];
+				surface_from_global_float(es, ex[k], ey[k], &sx, &sy);
+				/* position: */
+				*(v++) = ex[k];
+				*(v++) = ey[k];
+				/* texcoord: */
+				*(v++) = sx * inv_width;
+				*(v++) = sy * inv_height;
 			}
-			cx /= n;
-			cy /= n;
-			emit_vertex(cx, cy);
-
-			/* then emit edge points: */
-			for (k = 0; k < n; k++)
-				emit_vertex(ex[k], ey[k]);
-
-			/* and close the fan: */
-			emit_vertex(ex[0], ey[0]);
 
-			vtxcnt[nvtx++] = n + 2;
+			vtxcnt[nvtx++] = n;
 		}
 	}
 
@@ -1285,10 +1264,10 @@ repaint_region(struct weston_surface *es, pixman_region32_t *region,
 	/* The final region to be painted is the intersection of
 	 * 'region' and 'surf_region'. However, 'region' is in the global
 	 * coordinates, and 'surf_region' is in the surface-local
-	 * corodinates. texture_region() will iterate over all pairs of
+	 * coordinates. texture_region() will iterate over all pairs of
 	 * rectangles from both regions, compute the intersection
 	 * polygon for each pair, and store it as a triangle fan if
-	 * it has a non-zero area.
+	 * it has a non-zero area (at least 3 vertices, actually).
 	 */
 	nfans = texture_region(es, region, surf_region);