karma_procedurals/grasspatch.vfl

/*%vcc -l grasspatch.otl -O vop % */

cvex
main(
    // Inputs given by the vex curves procedurals
    int curv_idx = 0;
    int num_curves = 0;

    // User driven parameters
    int seed = 0x13A499;
    float radius = 0.2;
    float heightscale = 1.0;

    // Required curve points output
    export vector points[] = {};
    // Optional display color attribute to color the grass
    export vector displayColor[] = {};
    )
{
    // Choose the number of points used, from 7 to 12
    int num_points = 7 + curv_idx % 5;

    // Sample a random postion to root the grass in
    vector2 uv = random_brj(0x4AF03^seed, curv_idx);
    // Remap it to a gaussian distribution
    vector remap = filter_remap(uv, "gauss", radius);
    vector2 base = set(remap.x, remap.y);

    // Sample a random height value
    float w = random_brj(0x72BC9^seed, curv_idx);
    float height = 0.1 * w;
    height += heightscale * radius * (2 + 2*remap.z);

    // Sample a random postion for where the peak of the grass will be
    vector2 peak = random_brj(0x390F0^seed, curv_idx);
    peak.x = (base.x * fit01(peak.x, 1, 2.5)) + sign(base.x)*0.1;
    peak.y = (base.y * fit01(peak.y, 1, 2.5)) + sign(base.y)*0.1;

    // Get the direction from the base to the peak
    vector dir = set(peak.x - base.x, 0, peak.y - base.y);
    float dist = length(dir);
    dir = normalize(dir);

    // Get the step distance, by setting to 10 some
    // curves won't reach a peak while others can overshoot
    float step = dist / 10;
    // Some math to get the coefficients of the quadratic y = ax - bx^2
    float a = 2 * height / dist;
    float b = height / (dist * dist);
    float x = 0;

    // Iterate over each point and set the position and height
    for (int i = 0; i < num_points; i++)
    {
        x += step;
        points[i] = set(base.x, 0, base.y);
        points[i] += dir * x;
        points[i].y =  a*x - b*x*x;
        // Set the colour of the point, darker at the base
        displayColor[i] = set(0.1);
        displayColor[i].g = fit(float(i), 0, 12.0, 0.2, 0.85);
    }
}