X,y coordinates in the drawing area are shifted when other widgets are shown

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1

Please see the attached screenshots to realize the problem. My code is as follows:

        //Let's translate to origin of the drawing area allocation to avoid
	//the cross lines to be rotated too
	cairo_save(cr);
	cairo_identity_matrix(cr);
	cairo_translate(cr, allocation.x, allocation.y);
	
	//This to compensate the allocation menubar height gap
	//which prevents the horizontal line to be drawn at the center
	//I wonder why the coordinates in the drawing area are shifted
	//when the blessed menubar is hidden by the TAB key, shouldn't
	//they always be 0,0 at the top left?

	int menubar_height = gtk_widget_get_allocated_height(img->menubar);
	if (img->textbox->draw_horizontal_line)
	{
		if (gtk_widget_is_visible(img->menubar))
			center_y = menubar_height + center_y;
		else
			center_y = allocation.height / 2;

	//Draw the horizontal centering line
		cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
		cairo_set_line_width(cr, 1.0);
		cairo_move_to(cr, 0, center_y - 2);
		cairo_line_to(cr, allocation.width, center_y - 2);
		cairo_stroke(cr);
		cairo_set_source_rgb(cr, 0.8, 0.7, 0.3);
		cairo_move_to(cr, 0, center_y);
		cairo_line_to(cr, allocation.width, center_y);
		cairo_stroke(cr);
		cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
		cairo_move_to(cr, 0, center_y + 2);
		cairo_line_to(cr, allocation.width, center_y + 2);
		cairo_stroke(cr);
	}
	
	// Draw the vertical centering line
	if (img->textbox->draw_vertical_line)
	{
		cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
		cairo_set_line_width(cr, 1.0);
		cairo_move_to(cr, center_x - 2, 0);
		cairo_line_to(cr, center_x - 2, allocation.height + menubar_height);
		cairo_stroke(cr);
		cairo_set_source_rgb(cr, 0.8, 0.7, 0.3);
		cairo_move_to(cr, center_x, 0);
		cairo_line_to(cr, center_x, allocation.height + menubar_height);
		cairo_stroke(cr);
		cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
		cairo_move_to(cr, center_x + 2, 0);
		cairo_line_to(cr, center_x + 2, allocation.height + menubar_height);
		cairo_stroke(cr);
	}
	cairo_restore(cr);

This is how I pack the drawing area instead:

/* Create the image area */
vbox = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
g_object_set(G_OBJECT(vbox), "valign", GTK_ALIGN_CENTER);
gtk_box_pack_start (GTK_BOX (right_horizontal_box), vbox, TRUE, TRUE, 0);

img_struct->image_area = gtk_drawing_area_new();
gtk_box_pack_start(GTK_BOX(vbox), img_struct->image_area, FALSE, FALSE, 0);
gtk_widget_set_hexpand(img_struct->image_area, FALSE);
gtk_widget_set_vexpand(img_struct->image_area, FALSE);

gtk_widget_set_halign(img_struct->image_area, GTK_ALIGN_CENTER);
gtk_widget_set_valign(img_struct->image_area, GTK_ALIGN_CENTER);
gtk_widget_set_events(img_struct->image_area, 
									  GDK_KEY_PRESS_MASK
									| GDK_BUTTON_PRESS_MASK
									| GDK_BUTTON_RELEASE_MASK
									| GDK_POINTER_MOTION_MASK);

gtk_widget_set_can_focus(img_struct->image_area, TRUE);
gtk_widget_grab_focus(img_struct->image_area);

Can someone explain to me why the 0,0 coords are shifted in the drawing area when other widgets packed together with it are visible?

without
without1282×678 17.6 KB

with_side_widget
with_side_widget1278×680 44.7 KB

2

Does anybody have a clue please?

3

In GTK3 (which is what I assume you’re using) the coordinates are always relative to the GdkWindow of the widget: Gtk – 3.0: The GTK Drawing Model

4

Does this mean I have to use the allocation on the drawing area’s Gdkwindow rather than the area itself?

5

Ok the solution was to avoid resetting the identity matrix and translating to the drawing area allocation x and y but reuse this code with the negative angle:

        cairo_save(cr);
        cairo_translate(cr, img->textbox->x + img->textbox->width / 2, img->textbox->y + img->textbox->height / 2);
        cairo_rotate(cr, -img->textbox->angle);
        cairo_translate(cr, -(img->textbox->x + img->textbox->width / 2), -(img->t