Optimize gradient drawing when only part of the gradient is visible.

src/element.toit

@@ -549,7 +549,7 @@ abstract class CustomElement extends ClippingDiv_:
       change-tracker.child-invalidated x y w h
 
   draw canvas/Canvas -> none:
-    if not (x and y): return
+    if not (x and y and w and h): return
     analysis := canvas.bounds-analysis x y w h
     if analysis == Canvas.DISJOINT: return
     autoclipped := analysis == Canvas.CANVAS-IN-AREA or analysis == Canvas.COINCIDENT

src/gradient.toit

@@ -243,14 +243,16 @@ class GradientRendering_:
       // Use fixed point with 16 bits after the decimal point.
       step := ((texture-length_ - h) << 16) / w
       offset := 0
-      for i := 0; i < w; i += repeats:
-        lines := min repeats (w - i)
-        skew-adjusted-y := update-r-g-b-block.call y2 h lines orientation offset
-        if canvas.gray-scale:
-          canvas.pixmap     (i + x2) skew-adjusted-y --pixels=b        --source-width=source-width --orientation=orientation
-        else:
-          canvas.rgb-pixmap (i + x2) skew-adjusted-y --r=r --g=g --b=b --source-width=source-width --orientation=orientation
-        offset += step
+      canvas.visible-x-range x2 w: | x3 r3 |
+        offset += step * (x3 - x2)
+        for i := x3; i < r3; i += repeats:
+          lines := min repeats (r3 - i)
+          skew-adjusted-y := update-r-g-b-block.call y2 h lines orientation offset
+          if canvas.gray-scale:
+            canvas.pixmap     i skew-adjusted-y --pixels=b        --source-width=source-width --orientation=orientation
+          else:
+            canvas.rgb-pixmap i skew-adjusted-y --r=r --g=g --b=b --source-width=source-width --orientation=orientation
+          offset += step
     else:
       // The gradient goes broadly horizontally, and we draw in horizontal
       // strips.
@@ -260,15 +262,20 @@ class GradientRendering_:
       loop-body := : | i lines |
         skew-adjusted-x := update-r-g-b-block.call x w lines ORIENTATION-0 offset
         if canvas.gray-scale:
-          canvas.pixmap     skew-adjusted-x (i + y) --pixels=b        --source-width=source-width
+          canvas.pixmap     skew-adjusted-x i --pixels=b        --source-width=source-width
         else:
-          canvas.rgb-pixmap skew-adjusted-x (i + y) --r=r --g=g --b=b --source-width=source-width
+          canvas.rgb-pixmap skew-adjusted-x i --r=r --g=g --b=b --source-width=source-width
         offset += step
       if up:
-        for i := 0; i < h; i += repeats: loop-body.call i (min repeats (h - i))
+        canvas.visible-y-range y (y + h): | y3 b3 |
+          offset += step * (y3 - y)
+          for i := y3; i < b3; i += repeats: loop-body.call i (min repeats (b3 - i))
       else:
         assert: repeats == 1
-        for i := h - 1; i >= 0; i--: loop-body.call i 1
+        b2 := y + h - 1
+        canvas.visible-y-range y b2: | y3 b3 |
+          offset += step * (b2 - b3)
+          for i := b3; i >= y3; i--: loop-body.call i 1
 
   /// Returns a list of quadruples of the form starting-percent ending-percent start-color end-color.
   static extract-ranges_ specifiers/List -> List:

src/pixel-display.toit

@@ -855,6 +855,32 @@ abstract class Canvas:
       if x2 <= 0 and y2 <= 0 and right >= width_ and bottom >= height_: return CANVAS-IN-AREA
     return OVERLAP
 
+  /**
+  Given an x position and a width in the coordinate system of the
+    canvas, calculates the leftmost and rightmost pixels that are visible, so
+    that superflous drawing operations can be optimized away
+  The block is called with the left and right pixel positions.
+    The function may conservatively add a pixel on one side.
+  */
+  visible-x-range x/int w/int [block] -> none:
+    transform.invert.xywh 0 0 (width_ + 1) (height_ + 1): | x2 _ w2 _ |
+      block.call
+        max x x2
+        min (x + w) (x2 + w2)
+
+  /**
+  Given a y position and a height in the coordinate system of the
+    canvas, calculates the top and bottom pixels that are visible, so
+    that superflous drawing operations can be optimized away
+  The block is called with the top and bottom pixel positions.
+    The function may conservatively add a pixel on one side.
+  */
+  visible-y-range y/int h/int [block] -> none:
+    transform.invert.xywh 0 0 (width_ + 1) (height_ + 1): | _ y2 _ h2 |
+      block.call
+        max y y2
+        min (y + h) (y2 + h2)
+
   /**
   Constant to indicate that all pixels are transparent.
   For use with $composit.