Screen Tearing / Wavy Effect in GameMaker Studio 2 (using Surfaces)

After playing Environmental Station Alpha, I decided I wanted to implement the screen tearing effect Hempuli is using in his game. I didn’t know how he achieved it so I had to start from scratch and think about different approaches.

Knowing nothing about shaders, I was left with surfaces. I jotted down some code and immediately hit a wall; after asking around in the yoyogames forums, reading other’s comments, I could finally come up with a pretty decent solution.

Actual Code

I wrote the code so it could be easily hackable. This goes into the create event of your game controller object.
// handy shorter names
dw = display_get_width()
dh = display_get_height()

tearings_surface  	= surface_create(dw, dh)    // We'll draw on this surface
tearings_y          = 0
band_num            = 16                        // How many bands you want on screen
band_height         = dh / band_num
tearings_x_offset   = 32                        // How much you want to displace the bands horizontally
tearing_speed       = 4                       	// Change this to speed up/slow down the tearings
I place the following code inside a draw_post event of my controller.
// Create surface if it doesn't exits
if !surface_exists(tearings_surface)
	tearings_surface = surface_create(display_get_width(), display_get_height())
// Let's set the target to our surface
draw_clear_alpha(c_black, 0)

// We draw parts of our application surface on tearings surface
for (var current_band = 0; current_band < band_num * 2; current_band++)
	draw_surface_part(application_surface, 0, band_height * current_band - tearings_y, dw, band_height, sin( (degtorad(360) / band_num ) * current_band) * tearings_x_offset , band_height * current_band - tearings_y)

// Always reset the target surface

// Draw the actual surface
draw_surface_stretched(tearings_surface, -tearings_x_offset, 0, dw + tearings_x_offset * 2, dh)

// Move the Tearings
tearings_y = (tearings_y + tearing_speed) % (band_height * band_num)

That's it.

And this is how I make that effect. I will implement a similar version for vertical tearings (for underwater levels? maybe?). Hope you find it useful.

A note about surfaces: remember to free the surfaces if you don’t need them anymore, otherwise it will lead to memory leaks.

What are your thoughts on this?