Scaling up my game 4x without resizing a single sprite (GM:S 1.4)

This post is outdated
This post strictly refers to GM:S 1.4 and not GameMaker Studio 2. I also do not recommend doing this upscaling; using surfaces, you might be able to keep very low resolution asset, while still being able to draw smooth movement with subpixel precision.

For more info: Guide to develop low resolution 2D platformers with smooth movement and pixel perfect collisions in GameMaker Studio 2 (with slopes)

So I was developing a low res game (480 x 270) in GameMaker: Studio 1.4. The moment comes when I need to test it on mobile. I create my APK, load it up on the phone and there it is… a horrible mess of graphic artifacts!

The camera movement was all jittery, the zoom effect made all the tiles look funny and overall it just felt wrong.

The problem was obviously the very low resolution of the game. The camera (the view), along with every other object in the game, can move only by whole “game pixels”. That means that if you divide the actual display resolution by the game resolution (480 horizontal pixels in my case) you get the minimum “screen pixels” that any object can be moved/re-drawn. No in-between values are possible.

On a 1920 pixels wide screen, that’s 4 “screen pixels” per “game pixel”. So the camera (along with every other objects) can move only by 4 screen pixels at once and no less. This means jittery movement.

Since I wanted the camera to be able to move at least 1/4th of a game pixel, I had to upscale the whole game by a factor of 4… I had to go Full HD to keep my low res game enjoyable.

That meant resizing 175 sprites, move their origins, check their collision masks, resize the tilesets and backgrounds, adjust the speeds, re-make the whole levels and probably fix other bugs… unless…

Unless I used this code in the room start event of my controller.

global.game_scale = 4
// Increase the room size
room_width  = room_width  * global.game_scale
room_height = room_height * global.game_scale
// Object resize
with(all)
{
    x = x * global.game_scale
    y = y * global.game_scale
    image_xscale = image_xscale * global.game_scale
    image_yscale = image_yscale * global.game_scale
}
// Tile resize
var num = tile_get_count();
for (var i = 0; i < num; i++;)
{
    var tid = tile_get_id(i)
    var tx  = tile_get_x(tid)
    var txs = tile_get_xscale(tid)
    var ty  = tile_get_y(tid)
    var tys = tile_get_yscale(tid)
    tile_set_scale(tid, global.game_scale, global.game_scale)
    tile_set_position(tid, tx * global.game_scale, ty * global.game_scale)
}

I also increased the view size by 4 (and resized the application surface accordingly) so the objects in the game looked exactly like before (size-wise). Now everything is 4 times bigger.

View and Application Surface
My application surface (and the view) have a 1080 pixels vertical resolution. The horizontal resolution is dynamically calculated based on the monitor aspect ratio. The application surface is then manually drawn stretched to the display in the post-draw event (i.e. this game looks just as good on a 4:3 monitor than it does on a 16:9 one).

I still have to resize any in-game created instance and modify the speeds of some of my objects… but at least I didn’t have to resize 175 sprites. Now the camera moves and zooms smoothly. Everything can move 1/4th of a game pixel without looking jittery.