<@include gpu/Config.slh@>
<$VERSION_HEADER$>
//  Generated on <$_SCRIBE_DATE$>
//
//  particle vertex shader
//
//  Copyright 2015 High Fidelity, Inc.
//
//  Distributed under the Apache License, Version 2.0.
//  See the accompanying file LICENSE or http://www.apache.org/licenses/LICENSE-2.0.html
//

<@include gpu/Transform.slh@>

<$declareStandardTransform()$>

struct Radii {
    float start;
    float middle;
    float finish;
    float spread;
};
struct Colors {
    vec4 start;
    vec4 middle;
    vec4 finish;
    vec4 spread;
};

struct ParticleUniforms {
    Radii radius;
    Colors color;
    vec4 lifespan; // x is lifespan, 3 spare floats
};

layout(std140) uniform particleBuffer {
    ParticleUniforms particle;
};

in vec3 inPosition;
in vec2 inColor; // This is actual Lifetime + Seed

out vec4 varColor;
out vec2 varTexcoord;

const int NUM_VERTICES_PER_PARTICLE = 4;
// This ordering ensures that un-rotated particles render upright in the viewer.
const vec4 UNIT_QUAD[NUM_VERTICES_PER_PARTICLE] = vec4[NUM_VERTICES_PER_PARTICLE](
    vec4(-1.0, 1.0, 0.0, 0.0),
    vec4(-1.0, -1.0, 0.0, 0.0),
    vec4(1.0, 1.0, 0.0, 0.0),
    vec4(1.0, -1.0, 0.0, 0.0)
);

float bezierInterpolate(float y1, float y2, float y3, float u) {
    // https://en.wikipedia.org/wiki/Bezier_curve
    return (1.0 - u) * (1.0 - u) * y1 + 2.0 * (1.0 - u) * u * y2 + u * u * y3;
}

float interpolate3Points(float y1, float y2, float y3, float u) {
    // Makes the interpolated values intersect the middle value.

    if ((u <= 0.5f && y1 == y2) || (u >= 0.5f && y2 == y3)) {
        // Flat line.
        return y2;
    }

    float halfSlope;
    if ((y2 >= y1 && y2 >= y3) || (y2 <= y1 && y2 <= y3)) {
        // U or inverted-U shape.
        // Make the slope at y2 = 0, which means that the control points half way between the value points have the value y2.
        halfSlope = 0.0f;

    } else {
        // L or inverted and/or mirrored L shape.
        // Make the slope at y2 be the slope between y1 and y3, up to a maximum of double the minimum of the slopes between y1
        // and y2, and y2 and y3. Use this slope to calculate the control points half way between the value points.
        // Note: The maximum ensures that the control points and therefore the interpolated values stay between y1 and y3.
        halfSlope = (y3 - y1) / 2.0f;
        float slope12 = y2 - y1;
        float slope23 = y3 - y2;
        if (abs(halfSlope) > abs(slope12)) {
            halfSlope = slope12;
        } else if (abs(halfSlope) > abs(slope23)) {
            halfSlope = slope23;
        }
    }

    float stepU = step(0.5f, u);  // 0.0 if u < 0.5, 1.0 otherwise.
    float slopeSign = 2.0f * stepU - 1.0f; // -1.0 if u < 0.5, 1.0 otherwise
    float start = (1.0f - stepU) * y1 + stepU * y2;  // y1 if u < 0.5, y2 otherwise
    float middle = y2 + slopeSign * halfSlope;
    float finish = (1.0f - stepU) * y2 + stepU * y3; // y2 if u < 0.5, y3 otherwise
    float v = 2.0f * u - step(0.5f, u);  // 0.0-0.5 -> 0.0-1.0 and 0.5-1.0 -> 0.0-1.0
    return bezierInterpolate(start, middle, finish, v);
}

vec4 interpolate3Vec4(vec4 y1, vec4 y2, vec4 y3, float u) {
    return vec4(interpolate3Points(y1.x, y2.x, y3.x, u),
                interpolate3Points(y1.y, y2.y, y3.y, u),
                interpolate3Points(y1.z, y2.z, y3.z, u),
                interpolate3Points(y1.w, y2.w, y3.w, u));
}

void main(void) {
    TransformCamera cam = getTransformCamera();
    TransformObject obj = getTransformObject();
    
    // Which icon are we dealing with ?
    int particleID = gl_VertexID / NUM_VERTICES_PER_PARTICLE;
    // Which quad vertex pos?
    int twoTriID = gl_VertexID - particleID * NUM_VERTICES_PER_PARTICLE;

    // Particle properties
    float age = inColor.x / particle.lifespan.x;
    float seed = inColor.y;

    // Pass the texcoord and the z texcoord is representing the texture icon
    // Offset for corrected vertex ordering.
    varTexcoord = vec2((UNIT_QUAD[twoTriID].xy -1.0) * vec2(0.5, -0.5)); 
    varColor = interpolate3Vec4(particle.color.start, particle.color.middle, particle.color.finish, age);

    // anchor point in eye space
    float radius = interpolate3Points(particle.radius.start, particle.radius.middle, particle.radius.finish, age);
    vec4 quadPos = radius * UNIT_QUAD[twoTriID];

    vec4 anchorPoint;
    vec4 _inPosition = vec4(inPosition, 1.0);
    <$transformModelToEyePos(cam, obj, _inPosition, anchorPoint)$>

    vec4 eyePos = anchorPoint + quadPos;
    <$transformEyeToClipPos(cam, eyePos, gl_Position)$>
}