Designing Games with Game Maker - YoYo Games

In certain situations you might want to paint not directly on the screen but on a canvas that
can then later be used to paint things on the screen. Such a canvas is called a surface. For
example, you want to let the user draw on the screen. The paint should not be drawn on the
screen (because it will be removed each next step), but instead you want to draw it on a
separate surface that is copied onto the screen in each step. Or you want to use a texture that
changes over time.
Surfaces make this all possible. They are actually rather simple to use. You first create a
surface. Next you indicate that further drawing should happen on this surface. From that
moment on all drawing functions operate on the surface. Once you are done you reset the
drawing target and further drawing happens on the screen again. You can draw the surface on
the screen in many different ways or use it as a texture. There are though a few things you
must be aware of. See the remarks at the end.
The following functions exist to deal with surfaces
surface_create(w,h) Creates a surface of the indicated width and height.
Returns the id of the surface, which must be used in all further calls. Note that
the surface will not be cleared. This is the responsibility of the user. (Set it as
a target and call the appropriate clear function.)
surface_free(id) Frees the memory used by the surface.
surface_exists(id) Returns whether the surface with the indicated id
exists.
surface_get_width(id) Returns the width of the surface.
surface_get_height(id) Returns the height of the surface.
surface_get_texture(id) Returns the texture corresponding to the
surface. This can be used to draw textured objects with the image of the
surface.
surface_set_target(id) Sets the indicated surface as the drawing target.
All subsequent drawing happens on this surface. It resets the projection to
simply cover the surface.
surface_reset_target() Resets the drawing target to the normal screen.
surface_getpixel(id,x,y) Returns the color of the pixel corresponding to