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-rw-r--r--src/viewport.cpp11
-rw-r--r--src/zoom.hpp40
2 files changed, 47 insertions, 4 deletions
diff --git a/src/viewport.cpp b/src/viewport.cpp
index 307b90598..74c774ff2 100644
--- a/src/viewport.cpp
+++ b/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;
diff --git a/src/zoom.hpp b/src/zoom.hpp
index e0e7cc7a0..f924b362f 100644
--- a/src/zoom.hpp
+++ b/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 */