fix errors reported by pixman for meter redraws (and add more comments to help the...

diff --git a/libs/canvas/meter.cc b/libs/canvas/meter.cc
index dc22a930e8a044c01c8dcb74fb918954f23094e3..a87bfe7d04d25b83bf7dc26d292fff1d8a5fef6f 100644 (file)
--- a/libs/canvas/meter.cc
+++ b/libs/canvas/meter.cc
@@ -140,9 +140,6 @@ Meter::init (int clr0, int clr1, int clr2, int clr3,
        _stp[2] = stp2;
        _stp[3] = stp3;
 
-       pixrect.x = 1;
-       pixrect.y = 1;
-
        if (!len) {
                len = 250;
        }
@@ -159,8 +156,16 @@ Meter::init (int clr0, int clr1, int clr2, int clr3,
                bgpattern = horizontal_background (pixwidth + 2, pixheight + 2, _bgc, false);
        }
 
-       pixrect.width = pixwidth;
        pixrect.height = pixheight;
+       pixrect.x = 1;
+
+       if (orientation == Vertical) {
+               pixrect.width = pixwidth;
+               pixrect.y = pixheight; /* bottom of meter, so essentially "show zero" */
+       } else {
+               pixrect.width = 0; /* right of meter, so essentially "show zero" */
+               pixrect.y = 1;
+       }
 }
 
 void
@@ -519,10 +524,17 @@ Meter::vertical_expose (ArdourCanvas::Rect const & area, Cairo::RefPtr<Cairo::Co
 
        top_of_meter = (gint) floor (pixheight * current_level);
 
-       /* reset the height & origin of the rect that needs to show the pixbuf
+       /* reset the height & origin of the rect that needs to show the meter pattern
         */
-
        pixrect.height = top_of_meter;
+
+       /* X/cairo coordinates; y grows down so y origin of pixrect (pattern
+          fill area) is the TOP of the pattern area, which we compute like this:
+
+          - start at 1
+           - go to bottom of meter (pixheight)
+           - back up by current meter height (top_of_meter)
+       */
        pixrect.y = 1 + pixheight - top_of_meter;
 
        background.x = 1;
@@ -728,19 +740,47 @@ Meter::queue_vertical_redraw (float old_level)
 {
        Cairo::RectangleInt rect;
 
-       gint new_top = (gint) floor (pixheight * current_level);
+       gint new_height = (gint) floor (pixheight * current_level);
+
+       /* this is the nominal area that needs to be filled by the meter
+        * pattern
+        */
 
        rect.x = 1;
        rect.width = pixwidth;
-       rect.height = new_top;
-       rect.y = 1 + pixheight - new_top;
+
+       /* compute new top of meter (rect.y) by starting at one (border
+        * offset), go down the full height of the meter (X/Cairo coordinates
+        * grow down) to get to the bottom coordinate, then back up by the
+        * height of the patterned area.
+        */
+       rect.y = 1 + pixheight - new_height;
+       /* remember: height extends DOWN thanks to X/Cairo */
+       rect.height = new_height;
+
+       /* now lets optimize redrawing by figuring out which part needs to be
+          actually redrawn (i.e. re-use the last drawn state).
+       */
 
        if (current_level > old_level) {
-               /* colored/pixbuf got larger, just draw the new section */
-               /* rect.y stays where it is because of X coordinates */
-               /* height of invalidated area is between new.y (smaller) and old.y
-                  (larger).
-                  X coordinates just make my brain hurt.
+               /* filled area got taller, just draw the new section */
+
+               /* rect.y (new y origin) is smaller or equal to pixrect.y (old
+                * y origin) because the top of the meter is higher (X/Cairo:
+                * coordinates grow down). 
+                *
+                * Leave rect.y alone, and recompute the height to be just the
+                * difference between the new bottom and the top of the previous
+                * pattern area.
+                *
+                * The old pattern area extended DOWN from pixrect.y to
+                * pixrect.y + pixrect.height.
+                *
+                * The new pattern area extends DOWN from rect.y to 
+                * rect.y + rect.height
+                *
+                * The area needing to be drawn is the difference between the
+                * old top (pixrect.y) and the new top (rect.y)
                */
                rect.height = pixrect.y - rect.y;
        } else {