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NordicBeaver

Posted on

# Making a rain animation with WebGL shaders in Three.js.

I made a small shader that resembles rain on a window. You can try it here.

Check out the full code here:

The most interesting part is the fragment shader, where the magic happens. I tried to add some comments, so it's easier to read.

``````in vec2 uvInterpolator;
uniform float u_time;
uniform sampler2D u_texture;

// Generate a random float from a single input and seed.
float Random11(float inputValue, float seed) {
return fract(sin(inputValue * 345.456) * seed);
}

// Generate a random float from a 2d input and seed.
float Random21(vec2 inputValue, float seed) {
return fract(sin(dot(inputValue, vec2(123.456, 43.12))) * seed);
}

// Generate drops as distortions, that can be applied to UV coordinates
vec2 Drops(vec2 uv, float seed) {
// Randmply move everything
float shiftY = Random11(0.5, seed);
uv.y += shiftY;

// Split UV spac into cells. Each cell will contain a drop.
float cellsResolution = 10.0;
uv *= cellsResolution;

// Move each row randomly.
float rowIndex = floor(uv.y);
float shiftX = Random11(rowIndex, seed);
uv.x += shiftX;

vec2 cellIndex = floor(uv);
vec2 cellUv = fract(uv);

vec2 cellCenter = vec2(0.5);
float distanceFromCenter = distance(cellUv, cellCenter);

// We don't want to show every drop. So randomly remove some of them.
float isDropShown = step(0.8, Random21(cellIndex, seed + 14244.324));

// Decrease each drop intensity with time. Then make it appear again.
float dropIntensity = 1.0 - fract(u_time * 0.1 + Random21(cellIndex, seed + 32132.432) * 2.0) * 2.0;
dropIntensity = sign(dropIntensity) * abs(dropIntensity * dropIntensity * dropIntensity * dropIntensity);
dropIntensity = clamp(dropIntensity, 0.0, 1.0);

// We only need results from inside a specefec radius of a drop.
float isInsideDrop = 1.0 - step(0.1, distanceFromCenter);

vec2 vecToCenter = normalize(cellCenter - cellUv);

// Drop value is a vector to the center that increases with distance form it.
vec2 dropValue = vecToCenter * distanceFromCenter * distanceFromCenter * 40.0;

vec2 drop = dropValue * isInsideDrop * isDropShown * dropIntensity;
return drop;
}

void main() {
vec2 uv = uvInterpolator;

// Run the Drop function 10 times to create seemingly random pattern.
vec2 drops = vec2(0.0);
for(int i = 0; i < 10; i++) {
drops += Drops(uv, 42424.43 + float(i) * 12313.432);
}

// Distort UV.
uv += drops;

// Sample the texture after distorting the UV space.
vec4 color = texture2D(u_texture, uv);

gl_FragColor = color;
}
``````

Basically, all I do is generate drops at random positions. For every pixel around the drop I then calculate how much I need to distort the background. And then I apply the distortion to the UV space and sample the texture with the updated coordinates.

It's way easier to explain visually, so I made a video of the whole process of making this.