I'm currently writing a very simple parallax shader. Here's the code:

```
shader_type canvas_item;
uniform vec2 centre = vec2(640,360);
uniform vec3 facteur = vec3(0.0);
uniform bool sol = false;
void vertex() {
vec2 position = vec2(0.0);
if (sol) {position = (WORLD_MATRIX*vec4(VERTEX, 0.0,1.0)).rg}
else {position = (WORLD_MATRIX*vec4(1.0)).rg}
vec2 posrelative = (position-centre) / (centre*2.0);
float perspective = max(1.0 + posrelative.y * facteur.z, 0.0);
vec2 decalage = vec2(centre * facteur.xy * posrelative * 0.5) * perspective;
VERTEX = VERTEX * perspective + decalage;
}
```

Its purpose is to add some depth to 2D games where the usual ParallaxLayer nodes aren't useful (eg SMK / MKSC styled 2D race game). Here's how it works (+ translation):

Centre = Center : Position in the screen where all nodes get their original scale back. In a 2D race game it would be the position of the player car.

Facteur = Factor : How much the parallax affects the nodes' position (x,y = parallax layer motion scale) and scale (z).

Sol = Floor : How the shader scales the vertexes. If your sprite is a landscape element (a tree, a plant), scaling the sprite to is sufficient create a 3D effect, but if you are working with the road sprite or anything on floor, it should be "flattened" or it'll look weird.

I feel like this shader could be improved in many ways, but the first thing I'll like to know is if it's possible to get the screen size from the vertex function; or else, you'll have to change the "centre" uniform each time the screen is resized (which can be troublesome, even if you use the same material for all your sprites).

If it's possible, then the "centre" uniform could either become useless, either become relative: for example centre = vec2(0.5,0.5) or centre = vec2(0.5,0.8) considering the player's kart might not be in the true center of the screen.

If it's not possible it would be nice if anyone could explain me what exactly the constants WORLD*MATRIX, PROJECTION*MATRIX etc do represent. The line to get the absolute position of the sprite in the screen was just pasted from the docs, I still don't get a 4x4 matrix actually is. (3x3 is fine, but 4x4?)

Feel free to use / edit this btw, I think this basic shader could become useful in many situations.