Doc comment

src/pixel-display.toit

@@ -855,12 +855,26 @@ 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 _ |
+    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