(svn r11456) -Fix [FS#1412] (r10070): Viewport is bound to its top-left corner

src/viewport.cpp

@@ -296,10 +296,13 @@ static void SetViewportPosition(Window *w, int x, int y)
 	vp->virtual_left = x;
 	vp->virtual_top = y;
 
-	old_left = UnScaleByZoom(old_left, vp->zoom);
-	old_top = UnScaleByZoom(old_top, vp->zoom);
-	x = UnScaleByZoom(x, vp->zoom);
-	y = UnScaleByZoom(y, vp->zoom);
+	/* viewport is bound to its left top corner, so it must be rounded down (UnScaleByZoomLower)
+	 * else glitch described in FS#1412 will happen (offset by 1 pixel with zoom level > NORMAL)
+	 */
+	old_left = UnScaleByZoomLower(old_left, vp->zoom);
+	old_top = UnScaleByZoomLower(old_top, vp->zoom);
+	x = UnScaleByZoomLower(x, vp->zoom);
+	y = UnScaleByZoomLower(y, vp->zoom);
 
 	old_left -= x;
 	old_top -= y;

src/zoom.hpp

@@ -32,6 +32,13 @@ enum ZoomLevel {
 
 extern ZoomLevel _saved_scrollpos_zoom;
 
+/**
+ * Scale by zoom level, usually shift left (when zoom > ZOOM_LVL_NORMAL)
+ * When shifting right, value is rounded up
+ * @param value value to shift
+ * @param zoom  zoom level to shift to
+ * @return shifted value
+ */
 static inline int ScaleByZoom(int value, ZoomLevel zoom)
 {
 	if (zoom == ZOOM_LVL_NORMAL) return value;
@@ -39,6 +46,13 @@ static inline int ScaleByZoom(int value, ZoomLevel zoom)
 	return (zoom > ZOOM_LVL_NORMAL) ? value << izoom : (value + (1 << -izoom) - 1) >> -izoom;
 }
 
+/**
+ * Scale by zoom level, usually shift right (when zoom > ZOOM_LVL_NORMAL)
+ * When shifting right, value is rounded up
+ * @param value value to shift
+ * @param zoom  zoom level to shift to
+ * @return shifted value
+ */
 static inline int UnScaleByZoom(int value, ZoomLevel zoom)
 {
 	if (zoom == ZOOM_LVL_NORMAL) return value;
@@ -46,4 +60,30 @@ static inline int UnScaleByZoom(int value, ZoomLevel zoom)
 	return (zoom > ZOOM_LVL_NORMAL) ? (value + (1 << izoom) - 1) >> izoom : value << -izoom;
 }
 
+/**
+ * Scale by zoom level, usually shift left (when zoom > ZOOM_LVL_NORMAL)
+ * @param value value to shift
+ * @param zoom  zoom level to shift to
+ * @return shifted value
+ */
+static inline int ScaleByZoomLower(int value, ZoomLevel zoom)
+{
+	if (zoom == ZOOM_LVL_NORMAL) return value;
+	int izoom = (int)zoom - (int)ZOOM_LVL_NORMAL;
+	return (zoom > ZOOM_LVL_NORMAL) ? value << izoom : value >> -izoom;
+}
+
+/**
+ * Scale by zoom level, usually shift right (when zoom > ZOOM_LVL_NORMAL)
+ * @param value value to shift
+ * @param zoom  zoom level to shift to
+ * @return shifted value
+ */
+static inline int UnScaleByZoomLower(int value, ZoomLevel zoom)
+{
+	if (zoom == ZOOM_LVL_NORMAL) return value;
+	int izoom = (int)zoom - (int)ZOOM_LVL_NORMAL;
+	return (zoom > ZOOM_LVL_NORMAL) ? value >> izoom : value << -izoom;
+}
+
 #endif /* ZOOM_HPP */