// source: https://github.com/greggman/tdl/blob/master/tdl/particles.js#L154

uniform mat4 world;
uniform mat4 viewInverse;
uniform vec3 worldVelocity;
uniform vec3 worldAcceleration;
uniform float timeRange;
uniform float time;
uniform float timeOffset;
uniform float frameDuration;
uniform float numFrames;

// Incoming vertex attributes
attribute vec4 uvLifeTimeFrameStart;
attribute float startTime;
attribute vec4 velocityStartSize;
attribute vec4 accelerationEndSize;
attribute vec4 spinStartSpinSpeed;
attribute vec4 colorMult;

// Outgoing variables to fragment shader
varying vec2 outputTexcoord;
varying float outputPercentLife;
varying vec4 outputColorMult;

void main() {
    vec2 uv = uvLifeTimeFrameStart.xy;
    float lifeTime = uvLifeTimeFrameStart.z;
    float frameStart = uvLifeTimeFrameStart.w;
    //vec3 position = position.xyz;
    float startTime2 = startTime;
    vec3 velocity = (world * vec4(velocityStartSize.xyz,
                                 0.)).xyz + worldVelocity;
    float startSize = velocityStartSize.w;
    vec3 acceleration = (world * vec4(accelerationEndSize.xyz,
                                     0)).xyz + worldAcceleration;
    float endSize = accelerationEndSize.w;
    float spinStart = spinStartSpinSpeed.x;
    float spinSpeed = spinStartSpinSpeed.y;

    float localTime = mod((time - timeOffset - startTime), timeRange);
    float percentLife = localTime / lifeTime;

    float frame = mod(floor(localTime / frameDuration + frameStart),
                     numFrames);
    float uOffset = frame / numFrames;
    float u = uOffset + (uv.x + 0.5) * (1. / numFrames);

    outputTexcoord = vec2(u, uv.y + 0.5);
    outputColorMult = colorMult;

    vec3 basisX = viewInverse[0].xyz;
    vec3 basisZ = viewInverse[1].xyz;

    float size = mix(startSize, endSize, percentLife);
    size = (percentLife < 0. || percentLife > 1.) ? 0. : size;
    float s = sin(spinStart + spinSpeed * localTime);
    float c = cos(spinStart + spinSpeed * localTime);

    vec2 rotatedPoint = vec2(uv.x * c + uv.y * s, -uv.x * s + uv.y * c);
    vec3 localPosition = vec3(basisX * rotatedPoint.x + basisZ * rotatedPoint.y) * size +
                        velocity * localTime +
                        acceleration * localTime * localTime +
                        position;

    outputPercentLife = percentLife;
    gl_Position = projectionMatrix * viewMatrix * vec4(localPosition + world[3].xyz, 1.);

}