try our own tile snapping

README.md

@@ -263,6 +263,15 @@ Clone and run:
 
 ## TODO
 
+- add high-DPI support
+
+    for 2900% desktop scale (for example) we probably need to render our
+    256x256 tiles as 128x128 gtk pixels
+
+    however, this seems to cause filtering problems, mysteriously, see 
+    https://github.com/jcupitt/vipsdisp/tree/add-high-dpi for a quick test
+    hack
+
 - skip unknown files on next / prev?
 
     - need to add the test to next-image, not glob, since we can't test the

src/imagedisplay.c

@@ -666,9 +666,17 @@ imagedisplay_snapshot(GtkWidget *widget, GtkSnapshot *snapshot)
 	paint.size.width = imagedisplay->paint_rect.width;
 	paint.size.height = imagedisplay->paint_rect.height;
 
+	/* If there's no gtk snapping, we do our own based on the hardware pixel
+	 * size for the surface this snapshot will be rendered to.
+	 */
+	GtkNative *native = gtk_widget_get_native(widget);
+	GdkSurface *surface = gtk_native_get_surface(native);
+	double pixel_size = 1.0 / gdk_surface_get_scale_factor(surface);
+
 	if (imagedisplay->tilecache &&
 		imagedisplay->tilecache->n_levels > 0)
 		tilecache_snapshot(imagedisplay->tilecache, snapshot,
+			pixel_size,
 			imagedisplay->scale, imagedisplay->x, imagedisplay->y,
 			&paint, imagedisplay->debug);
 

src/tilecache.c

@@ -835,15 +835,41 @@ tilecache_draw_bounds(GtkSnapshot *snapshot,
 	}
 }
 
+#ifndef HAVE_GTK_SNAPSHOT_SET_SNAP
+/* Snap a graphene rect to a hardware pixel boundary on the output surface. We
+ * need to do this if the gtk snap mechachanism is missing or we'll get thin
+ * white lines on tile edges.
+ */
+static void
+tilecache_snap_rect_to_boundary(graphene_rect_t *bounds, double pixel_size)
+{
+	double left = rint(bounds->origin.x * pixel_size) / pixel_size;
+	double top = rint(bounds->origin.y * pixel_size) / pixel_size;
+	double right = 
+		rint((bounds->origin.x + bounds->size.width) * pixel_size) / pixel_size;
+	double bottom = 
+		rint((bounds->origin.y + bounds->size.height) * pixel_size) / pixel_size;
+
+	bounds->origin.x = left;
+	bounds->origin.y = top;
+	bounds->size.width = right - left;
+	bounds->size.height = bottom - top;
+}
+#endif /*!HAVE_GTK_SNAPSHOT_SET_SNAP*/
+
 /* Scale is how much the level0 image has been scaled, x/y is the position of
- * the top-left corner of the paint_rect area in the scaled image.
+ * the top-left corner of @paint in the scaled image.
+ *
+ * @pixel_scale is gdk_surface_get_scale() for the surface this snapshot will
+ * be rendered to.
  *
- * paint_rect is the pixel area in gtk coordinates that we paint in the widget.
+ * @paint is the pixel area in gtk coordinates that we paint in the widget.
  *
  * Set debug to draw tile boundaries for debugging.
  */
 void
 tilecache_snapshot(Tilecache *tilecache, GtkSnapshot *snapshot,
+	double pixel_size,
 	double scale, double x, double y, graphene_rect_t *paint, gboolean debug)
 {
 	/* In debug mode, scale and offset so we can see tile clipping.
@@ -914,6 +940,9 @@ tilecache_snapshot(Tilecache *tilecache, GtkSnapshot *snapshot,
 	/* Paint the backdrop.
 	 */
 	graphene_rect_t backdrop = *paint;
+#ifndef HAVE_GTK_SNAPSHOT_SET_SNAP
+	tilecache_snap_rect_to_boundary(&backdrop, pixel_size);
+#endif /*!HAVE_GTK_SNAPSHOT_SET_SNAP*/
 	gtk_snapshot_push_repeat(snapshot, &backdrop, NULL);
 
 	backdrop.origin.x = 0;
@@ -947,11 +976,7 @@ tilecache_snapshot(Tilecache *tilecache, GtkSnapshot *snapshot,
 			bounds.size.height = tile->bounds0.height * scale;
 
 #ifndef HAVE_GTK_SNAPSHOT_SET_SNAP
-			/* Without set snap, we have to hide tile edges by expanding the
-			 * tile.
-			 */
-			bounds.size.width += 1;
-			bounds.size.height += 1;
+			tilecache_snap_rect_to_boundary(&bounds, pixel_size);
 #endif /*!HAVE_GTK_SNAPSHOT_SET_SNAP*/
 
 			gtk_snapshot_append_scaled_texture(snapshot,

src/tilecache.h

@@ -110,6 +110,7 @@ Tilecache *tilecache_new();
 /* Render the tiles to a snapshot.
  */
 void tilecache_snapshot(Tilecache *tilecache, GtkSnapshot *snapshot,
+	double pixel_size,
 	double scale, double x, double y, graphene_rect_t *paint, gboolean debug);
 
 #endif /*__TILECACHE_H*/