[gthumb] image viewer: keep the same pixel under the pointer after zooming

[Paolo Bacchilega <paobac src gnome org>]

commit 5e89e985025b5ef27b8e34e26a152c76f97170e0
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sun Nov 29 13:10:45 2020 +0100

    image viewer: keep the same pixel under the pointer after zooming
    
    Fixes https://gitlab.gnome.org/GNOME/gthumb/-/issues/129

 gthumb/gth-image-viewer.c | 32 +++++++++++++++++++++++---------
 1 file changed, 23 insertions(+), 9 deletions(-)
---
diff --git a/gthumb/gth-image-viewer.c b/gthumb/gth-image-viewer.c
index 812fd669..d8e8aad5 100644
--- a/gthumb/gth-image-viewer.c
+++ b/gthumb/gth-image-viewer.c
@@ -300,8 +300,8 @@ static void _set_surface (GthImageViewer  *self,
 static void
 set_zoom (GthImageViewer *self,
          gdouble         zoom_level,
-         int             center_x,
-         int             center_y)
+         int             pointer_x,
+         int             pointer_y)
 {
        g_return_if_fail (self != NULL);
 
@@ -328,26 +328,40 @@ set_zoom (GthImageViewer *self,
                }
        }
 
-       /* try to keep the center of the view visible. */
+       /* Try to keep the same pixel under the pointer after the zoom. */
 
        {
                cairo_surface_t *image;
 
                image = gth_image_viewer_get_current_image (self);
                if (image != NULL) {
+                       int    frame_border;
                        double quality_zoom;
                        double old_zoom_level;
                        int    x, y;
-                       int    frame_border;
+                       double x0, y0;
+                       double dx, dy;
                        int    new_center_x, new_center_y;
 
+                       frame_border = _gth_image_viewer_get_frame_border (self);
                        quality_zoom = (double) self->priv->original_width / cairo_image_surface_get_width 
(image);
                        old_zoom_level = self->priv->zoom_level * quality_zoom;
-                       x = (center_x + self->visible_area.x - self->image_area.x) / old_zoom_level;
-                       y = (center_y + self->visible_area.y - self->image_area.y) / old_zoom_level;
-                       frame_border = _gth_image_viewer_get_frame_border (self);
-                       new_center_x = x * zoom_level * quality_zoom + frame_border + 1;
-                       new_center_y = y * zoom_level * quality_zoom + frame_border + 1;
+
+                       /* Pixel under the pointer. */
+                       x = (pointer_x + self->visible_area.x - self->image_area.x) / old_zoom_level;
+                       y = (pointer_y + self->visible_area.y - self->image_area.y) / old_zoom_level;
+
+                       /* Pixel at the center of the visibile area. */
+                       x0 = ((self->visible_area.width / 2) + self->visible_area.x - self->image_area.x) / 
old_zoom_level;
+                       y0 = ((self->visible_area.height / 2) + self->visible_area.y - self->image_area.y) / 
old_zoom_level;
+
+                       /* Delta to keep the (x, y) pixel under the pointer.  */
+                       dx = ((x - x0) * zoom_level) - ((x - x0) * old_zoom_level);
+                       dy = ((y - y0) * zoom_level) - ((y - y0) * old_zoom_level);
+
+                       /* Center on (x0, y0) and add (dx, dy) */
+                       new_center_x = round ((x0 * zoom_level * quality_zoom) + frame_border + dx);
+                       new_center_y = round ((y0 * zoom_level * quality_zoom) + frame_border + dy);
 
                        self->visible_area.x = new_center_x - (self->visible_area.width / 2);
                        self->visible_area.y = new_center_y - (self->visible_area.height / 2);