Can I have an example?
Here is the code for the rotate counterclockwise transition:
argument0 is the image of the previous room, on a surface. Basically, you can draw it using draw_surface or draw_surface_ext. You can also manipulate the surface, etc. For more info, see "Surfaces" in the GM Manual.
argument1 is the image of the next room, on a surface
argument2 is the width of the images.
argument3 is the height of the images.
argument4 is how far done the transition is. 0 is when it just starts, 0.5 is halfway through, and 1 is completely done.
With this information, the script must draw the transition. It is like a draw event, called every step until the transition is over. To find out how far done the transition is, use argument4.
Now, to use that script up there that draws the image as a transition, put that code up there into a script named "scr_rcc". Then, somewhere else, use the code
Then when you want to do the room transition: