ganv_canvas_set_zoom

void ganv_canvas_set_zoom (GanvCanvas* canvas,
                           double zoom);
  

The anchor point for zooming, i.e. the point that stays fixed and all others zoom inwards or outwards from it, depends on whether the canvas is set to center the scrolling region or not. You can control this using the ganv_canvas_set_center_scroll_region function. If the canvas is set to center the scroll region, then the center of the canvas window is used as the anchor point for zooming. Otherwise, the upper-left corner of the canvas window is used as the anchor point.