billboarding in GLSL

billboard.glsl

attribute vec3 position;
uniform mat4 modelViewMatrix;
uniform mat4 modelMatrix;
uniform mat4 projectionMatrix;

uniform float scale;
uniform float size;
uniform float aspect;
varying vec2 vUv;

void main() {
  vec2 scale = vec2(
    length(modelViewMatrix[0]) / aspect,
    length(modelViewMatrix[1])
  );
  
  vec4 billboard = (modelViewMatrix * vec4(vec3(0.0), 1.0));
  vec4 newPosition = projectionMatrix
    * billboard
    + vec4(scale * position.xy, 0.0, 0.0);
  
  gl_Position = newPosition;
  vUv = vec2(position.x + 0.5, position.y + 0.5);
}