// Forked from "Floyd–Steinberg dithering" by imakerobots



// raytraced sphere part from https://turtletoy.net/turtle/11075dfee0
const canvas_size = 95;
const brown_rot = 360;
const brown_for_min = 1;
const brown_for_max = 10;

const light_position = [-2,3,-4];
const ro = [0,0,-3.5];
const sphere_pos = [-.2,0,0];

function get_image_intensity(x,y) {
    x /= canvas_size;
    y /= canvas_size;
    
    const rd = vec_normalize([x,-y,2]);
    let normal;
    let light = 0;
    let hit;
    let plane_hit = false;
    
    let dist = intersect_sphere(ro, rd, sphere_pos, 1);
    if (dist > 0) {
        hit = vec_add(ro, vec_mul(rd, dist));
        normal = vec_normalize(hit);
    } else {
        dist = 10000;
    }
    if (rd[1] < 0) {
        const plane_dist = -1/rd[1];
       if (plane_dist < dist) {
            dist = plane_dist;
            plane_hit = true;
            hit = vec_add(ro, vec_mul(rd, dist));
            normal = [0,1,0];
        }
    } 
    
    if (dist > 0 && dist < 100) {
        let vec_to_light = vec_sub(hit, light_position);
        const light_dist_sqr = vec_dot(vec_to_light, vec_to_light);
        
        vec_to_light = vec_mul(vec_to_light, -1/Math.sqrt(light_dist_sqr));
        
        let light = vec_dot(normal, vec_to_light);
        light *= 20 / light_dist_sqr;  // changed the intensity a bit
        
        // shadow ?
        if (plane_hit && intersect_sphere(hit, vec_to_light, sphere_pos, 1) > 0) {
            light = 0.01;  // added some ambient light.  make 0 for none.
        }
        
        return Math.sqrt(Math.min(1, Math.max(0,light)));
    } else {
        return 0;
    }
}


// math functions
function vec_normalize(a) {
    const l = Math.sqrt(vec_dot(a,a));
    return [a[0]/l,a[1]/l,a[2]/l];
}

function vec_add(a, b) {
    return [a[0]+b[0], a[1]+b[1], a[2]+b[2]]
}

function vec_mul(a, b) {
    return [a[0]*b, a[1]*b, a[2]*b]
}

function vec_sub(a, b) {
    return [a[0]-b[0], a[1]-b[1], a[2]-b[2]]
}

function vec_dot(a, b) {
    return a[0]*b[0]+a[1]*b[1]+a[2]*b[2];
}

function intersect_sphere(ro, rd, center, radius) {
    const oc = vec_sub(ro, center);
	const b = vec_dot( oc, rd );
	const c = vec_dot( oc, oc ) - radius * radius;
	const h = b*b - c;
	if( h<0 ) return -1;
	return -b - Math.sqrt( h );
}










// dithering part from https://turtletoy.net/turtle/d47e2bad0c

Canvas.setpenopacity(1);

// Global code will be evaluated once.
const turtle = new Turtle();
const level_of_detail=2;  // min=1.5, max=20, step=0.5

turtle.penup();
turtle.goto(-100,-100);
const stepSize = level_of_detail/10.0;
const stepsPerLine = Math.floor(200.0/stepSize);
stepsRemaining=stepsPerLine*stepsPerLine;
dx=1;

error1 = new Array(stepsPerLine);
error2 = new Array(stepsPerLine);
for(i=0;i<error1.length;++i) {
    error1[i] = error2[i] = 0;
}


function newLine() {
    for(var i=0;i<stepsPerLine;++i) {
        error1[i] = error2[i];
        error2[i] = 0;
    }
}

function get_pixel(x,y) {
    return get_image_intensity( x,y );
}

function find_closest_palette_color(oldpixel) {
    return oldpixel>0.5?1.0:0.0;
}

function walk(i) {
    var x=turtle.x()/stepSize+stepsPerLine/2;
    var y=turtle.y()/stepSize+stepsPerLine/2;
    if(x<0) x=0;
    if(x>=stepsPerLine) x=stepsPerLine-1;
    x=Math.floor(x);
    
    var oldpixel = error1[x] + get_pixel(turtle.x(),turtle.y());
    var newpixel = find_closest_palette_color(oldpixel);
    var quant_error = oldpixel - newpixel;
    var xP1 = x+dx;
    var xM1 = x-dx;
    if(xP1>=0 && xP1<stepsPerLine) error1[xP1] += quant_error * 7.0/16.0;
    if(xM1>=0 && xM1<stepsPerLine) error2[xM1] += quant_error * 3.0/16.0;
                                   error2[x  ] += quant_error * 5.0/16.0;
    if(xP1>=0 && xP1<stepsPerLine) error2[xP1] += quant_error * 1.0/16.0;
    
    if(newpixel>0.5) {
        turtle.penup();
    } else {
        turtle.pendown();
    }
    
    turtle.forward(stepSize);
    if(turtle.x()>=100) {
        turtle.right(90);
        turtle.forward(stepSize);
        turtle.right(90);
        dx=-dx;
        newLine();
    } else if(turtle.x()<=-100) {
        turtle.left(90);
        turtle.forward(stepSize);
        turtle.left(90);
        dx=-dx;
        newLine();
    }
    
    return turtle.y()<100 && turtle.y()>=-100;//stepsRemaining-- > 0;
}