[gthumb] GthImageViewer: fixed centered zoom

[Paolo Bacchilega <paobac src gnome org>]

commit 8e81df7dff1bc67bc533de4a57a3622a7cb52c23
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Fri Nov 1 20:01:48 2013 +0100

    GthImageViewer: fixed centered zoom

 gthumb/gth-image-viewer.c |   29 ++++++++++++++++++++++++-----
 1 files changed, 24 insertions(+), 5 deletions(-)
---
diff --git a/gthumb/gth-image-viewer.c b/gthumb/gth-image-viewer.c
index 55572de..d0c3682 100644
--- a/gthumb/gth-image-viewer.c
+++ b/gthumb/gth-image-viewer.c
@@ -322,8 +322,6 @@ set_zoom (GthImageViewer *self,
          int             center_x,
          int             center_y)
 {
-       gdouble zoom_ratio;
-
        g_return_if_fail (self != NULL);
 
        if (gth_image_get_is_zoomable (self->priv->image)) {
@@ -350,9 +348,30 @@ set_zoom (GthImageViewer *self,
        }
 
        /* try to keep the center of the view visible. */
-       zoom_ratio = zoom_level / self->priv->zoom_level;
-       self->visible_area.x = ((self->visible_area.x + center_x) * zoom_ratio - self->visible_area.width / 
2);
-       self->visible_area.y = ((self->visible_area.y + center_y) * zoom_ratio - self->visible_area.height / 
2);
+
+       {
+               cairo_surface_t *image;
+
+               image = gth_image_viewer_get_current_image (self);
+               if (image != NULL) {
+                       double quality_zoom;
+                       double zoom;
+                       int    x, y;
+                       int    frame_border;
+                       int    new_center_x, new_center_y;
+
+                       quality_zoom = (double) self->priv->original_width / cairo_image_surface_get_width 
(image);
+                       zoom = self->priv->zoom_level * quality_zoom;
+                       x = (center_x + self->visible_area.x - self->image_area.x) / zoom;
+                       y = (center_y + self->visible_area.y - self->image_area.y) / zoom;
+                       frame_border = _gth_image_viewer_get_frame_border (self);
+                       new_center_x = x * zoom_level * quality_zoom + frame_border;
+                       new_center_y = y * zoom_level * quality_zoom + frame_border;
+
+                       self->visible_area.x = new_center_x - (self->visible_area.width / 2);
+                       self->visible_area.y = new_center_y - (self->visible_area.height / 2);
+               }
+       }
 
        self->priv->zoom_level = zoom_level;