[cairo] drawing on and reusing an off-screen image surface

[Matt Hoosier]

On Dec 3, 2007 1:31 AM, Peter Groves <pdgroves at gmail.com> wrote:
> ##at my application startup, draw the background and save it as
> ##a cairo_pattern
> startup() {
>         background_surface = cairo_image_surface_create(...)
>         background_ctx = cairo_create(background_surface)
>
>         #do all the cairo drawings for the background
>         my_draw_the_background_method(background_ctx)
>
>         #this global variable will hopefully be a fully rendered version of
>         #the rasterized background image
>         _background_pattern = cairo_pattern_create_for_surface(background_surface)
> }
>
> ##now, inside my main re-paint function for a gtk drawing area
> ##named 'widg', which will be called on expose_event and some other
> ##events
> repaint(widg){
>
>         #get the cairo_t for the gtk widget
>         gtk_widget_ctx = gdk_cairo_create(widg)
>
>         #tell cairo to use the cached rasterized image as the "paint"
>         cairo_set_source(gtk_widget_ctx, _background_pattern)
>
>         #(**)paint the whole gtk_widget_ctx canvas with the image.
>         cairo_paint(gtk_widget_ctx)
>
>         #do any additional drawings that occur each re-paint on top
>         my_draw_the_foreground_method(gtk_widget_ctx)
> }

You might want to consider allocating an offscreen GdkPixmap and pass
that to the gdk_cairo_create() call. Then you can repeatedly do very
cheap area copies from the GdkPixmap onto your on-screen GdkWindow.