Rendering bitmaps

[Mark Leisher <mleisher math nmsu edu>]

It's a lazy Friday, so I'm going to be lazy and ask if anyone knows a 
better way to render a 1bpp bitmap normally and inverted.

The code below works, but when rendering the inverted bitmap (background 
and foreground colors swapped) edge pixels of the bitmap are filled in 
when they shouldn't be. That may be a known "feature" of cairo that I 
just haven't read about yet.

This used to be trivial with the XOR operator in the GdkGC. Does it have 
to be so hard in Cairo?

FYI, the 1bpp bitmap was created using 
cairo_image_surface_create_for_data().

-------------------------------------------------

/*
 * The 'data' parameter is a GtkDrawingArea widget.
 */

static cairo_surface_t *s;

static void
invert_bitmap(GtkWidget *w, gpointer data)
{
    cairo_t *cr = 
gdk_cairo_create(gtk_widget_get_window(GTK_WIDGET(data)));
    GtkStyleContext *ctx = gtk_widget_get_style_context(GTK_WIDGET(data));
    GdkRGBA c;

    if (invert) {
        /*
         * Display inverted bitmap: background of rectangle in 
foreground color
         * and bitmap in background color.
         */

gtk_style_context_get_background_color(ctx,GTK_STATE_FLAG_NORMAL,&c);
        cairo_set_operator(cr,CAIRO_OPERATOR_DIFFERENCE);
        cairo_set_source_surface(cr,s,2.0,1.0);
        cairo_set_source_rgb(cr,c.red,c.green,c.blue);
        cairo_rectangle(cr, 1, 1, 13, 22);
        cairo_fill(cr);
    } else {
        /*
         * Show normal bitmap while clearing the background behind it.
         */
        cairo_set_operator(cr,CAIRO_OPERATOR_DIFFERENCE);
        cairo_mask_surface(cr,s,2.0,1.0);
        gtk_render_background(ctx,cr,1,1,13,22);
    }

    cairo_destroy(cr);
    invert = !invert;
}

--
Mark Leisher