Improve performance of path clipping in rasterizer

This patch short cuts a common loop in the gray rasterizer where it
would iterate over cells it considers invalid.

This greatly speeds up clipping paths on large paint devices, but
probably has little benefit for the FreeType origin of our rasterizer.

Task-number: QTBUG-40559
Change-Id: I30845de0b82e53b619853c6c91903793b4267615
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

1 files changed, 43 insertions, 17 deletions

diff --git a/src/gui/painting/qgrayraster.c b/src/gui/painting/qgrayraster.c
index 614d95ace1..52b2371c3f 100644
--- a/src/gui/painting/qgrayraster.c
+++ b/src/gui/painting/qgrayraster.c
@@ -480,10 +480,10 @@
 
       ras.area  = 0;
       ras.cover = 0;
+      ras.ex    = ex;
+      ras.ey    = ey;
     }
 
-    ras.ex      = ex;
-    ras.ey      = ey;
     ras.invalid = ( (unsigned)ey >= (unsigned)ras.count_ey ||
                               ex >= ras.count_ex           );
   }
@@ -670,38 +670,64 @@
     }
 
     /* vertical line - avoid calling gray_render_scanline */
-    incr = 1;
-
     if ( dx == 0 )
     {
       TCoord  ex     = TRUNC( ras.x );
       TCoord  two_fx = (TCoord)( ( ras.x - SUBPIXELS( ex ) ) << 1 );
-      TPos    area;
+      TPos    area, max_ey1;
 
 
       first = ONE_PIXEL;
       if ( dy < 0 )
-      {
         first = 0;
-        incr  = -1;
-      }
 
       delta      = (int)( first - fy1 );
       ras.area  += (TArea)two_fx * delta;
       ras.cover += delta;
-      ey1       += incr;
-
-      gray_set_cell( &ras, ex, ey1 );
 
       delta = (int)( first + first - ONE_PIXEL );
       area  = (TArea)two_fx * delta;
-      while ( ey1 != ey2 )
-      {
-        ras.area  += area;
-        ras.cover += delta;
-        ey1       += incr;
+      max_ey1 = ras.count_ey + ras.min_ey;
+      if (dy < 0) {
+        if (ey1 > max_ey1) {
+          ey1 = (max_ey1 > ey2) ? max_ey1 : ey2;
+          gray_set_cell( &ras, ex, ey1 );
+        } else {
+          ey1--;
+          gray_set_cell( &ras, ex, ey1 );
+        }
+        while ( ey1 > ey2 && ey1 >= ras.min_ey)
+        {
+          ras.area  += area;
+          ras.cover += delta;
+          ey1--;
+
+          gray_set_cell( &ras, ex, ey1 );
+        }
+        if (ey1 != ey2) {
+          ey1 = ey2;
+          gray_set_cell( &ras, ex, ey1 );
+        }
+      } else {
+        if (ey1 < ras.min_ey) {
+          ey1 = (ras.min_ey < ey2) ? ras.min_ey : ey2;
+          gray_set_cell( &ras, ex, ey1 );
+        } else {
+          ey1++;
+          gray_set_cell( &ras, ex, ey1 );
+        }
+        while ( ey1 < ey2 && ey1 < max_ey1)
+        {
+          ras.area  += area;
+          ras.cover += delta;
+          ey1++;
 
-        gray_set_cell( &ras, ex, ey1 );
+          gray_set_cell( &ras, ex, ey1 );
+        }
+        if (ey1 != ey2) {
+          ey1 = ey2;
+          gray_set_cell( &ras, ex, ey1 );
+        }
       }
 
       delta      = (int)( fy2 - ONE_PIXEL + first );