composite: Perform a quick is-clipped for glyphs

Computing the exact bbox of the glyphs and whether they are overlapped
is expensive. However, we can often check whether they are visible just
by looking at the maximal extents of the fonts along with the bbox of
the positions; much cheaper.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 13 insertions, 14 deletions

diff --git a/src/cairo-scaled-font.c b/src/cairo-scaled-font.c
index ee79e2afc..efab44f3b 100644
--- a/src/cairo-scaled-font.c
+++ b/src/cairo-scaled-font.c
@@ -2193,11 +2193,14 @@ _cairo_scaled_font_glyph_approximate_extents (cairo_scaled_font_t	 *scaled_font,
 					      int                      num_glyphs,
 					      cairo_rectangle_int_t   *extents)
 {
-    double x0 = HUGE_VAL, x1 = -HUGE_VAL;
-    double y0 = HUGE_VAL, y1 = -HUGE_VAL;
+    double x0, x1, y0, y1, pad;
     int i;
 
-    for (i = 0; i < num_glyphs; i++) {
+    assert (num_glyphs);
+
+    x0 = x1 = glyphs[0].x;
+    y0 = y1 = glyphs[0].y;
+    for (i = 1; i < num_glyphs; i++) {
 	double g;
 
 	g = glyphs[i].x;
@@ -2209,18 +2212,14 @@ _cairo_scaled_font_glyph_approximate_extents (cairo_scaled_font_t	 *scaled_font,
 	if (g > y1) y1 = g;
     }
 
-    if (x0 <= x1 && y0 <= y1) {
-	extents->x = floor (x0 - scaled_font->extents.max_x_advance);
-	extents->width = ceil (x1 + scaled_font->extents.max_x_advance);
-	extents->width -= extents->x;
+    pad = MAX(scaled_font->fs_extents.max_x_advance,
+	      scaled_font->fs_extents.height);
+    pad *= scaled_font->max_scale;
 
-	extents->y = floor (y0 - scaled_font->extents.ascent);
-	extents->height = ceil (y1 + scaled_font->extents.descent);
-	extents->height -= extents->y;
-    } else {
-	extents->x = extents->y = 0;
-	extents->width = extents->height = 0;
-    }
+    extents->x = floor (x0 - pad);
+    extents->width = ceil (x1 + pad) - extents->x;
+    extents->y = floor (y0 - pad);
+    extents->height = ceil (y1 + pad) - extents->y;
 }
 
 cairo_status_t