Draw gradient bevel around polygon

Question

Basically I need to create a falloff texture for given polygon. For instance this is the image I have

What I need to create is this, but with bevel gradient from white to black, consider the green part as gradient.

I've got the coordinates of all the vertices and the thickness of the bevel. I'm rendering using HTML5 2d canvas. Basically the most obvious solution would be to calculate every pixel's distance to the polygon and if it's within the thickness parameter, calculate the color and color the pixel. But that's heavy calculations and would be slow, even for smallest possible texture for my needs. So are there any tricks I can do with canvas to achieve this?

Answer 1

Just draw the polygon's outline at different stroke widths changing the colour for each step down in width.

The snippet shows one way of doing it. Draws 2 polygons with line joins "miter" and "round"

 "use strict";

const canvas = document.createElement("canvas"); 
canvas.height = innerHeight;
canvas.width = innerWidth;
canvas.style.position = "absolute"; 
canvas.style.top = canvas.style.left = "0px";
const ctx = canvas.getContext("2d");
document.body.appendChild(canvas);

// poly to draw
var poly = [0.1,0.2,0.4,0.5,0.2,0.8];
var poly1 = [0.6,0.1,0.9,0.5,0.8,0.9];

// convert rgb style colour to array
function rgb2Array(rgb){
    var arr1 = rgb.split("(")[1].split(")")[0].split(",");
    var arr = [];
    while(arr1.length > 0){
        arr.push(Number(arr1.shift()));
    }
    return arr;
}
// convert array to rgb colour
function array2rgb(arr){
    return "rgb("+Math.floor(arr[0])+","+Math.floor(arr[1])+","+Math.floor(arr[2])+")"
}

// lerps array from to. Amount is from 0 @ from 1 @ to. res = is the resulting array
function lerpArr(from,to,amount,res){
    var i = 0;
    if(res === undefined){
        res = [];
    }
    while(i < from.length){
        res[i] = (to[i]-from[i]) * amount + from[i];    
        i++;
    }
    return res;
}

// draw gradient outline
// poly is the polygon verts
// width is the outline width
// fillStyle is the polygon fill style
// rgb1 is the outer colour
// rgb2 is the inner colour of the outline gradient
function drawGradientOutline(poly,width,fillStyle,rgb1,rgb2){
    ctx.beginPath();
    var i = 0;
    var w = canvas.width;
    var h = canvas.height;
    ctx.moveTo(poly[i++] * w,poly[i++] * h);
    while(i < poly.length){
        ctx.lineTo(poly[i++] * w,poly[i++] * h);
    }
    ctx.closePath();
    var col1 = rgb2Array(rgb1);
    var col2 = rgb2Array(rgb2);
    
    i = width * 2;
    var col = [];
    while(i > 0){
        ctx.lineWidth = i;
        ctx.strokeStyle = array2rgb(lerpArr(col1,col2,1- i / (width * 2),col));
        ctx.stroke();
        i -= 1;
    }
    ctx.fillStyle = fillStyle;
    ctx.fill();
}
ctx.clearRect(0,0,canvas.width,canvas.height)
ctx.lineJoin = "miter";
drawGradientOutline(poly,20,"black","rgb(255,0,0)","rgb(255,255,0)")
ctx.lineJoin = "round";
drawGradientOutline(poly1,20,"black","rgb(255,0,0)","rgb(255,255,0)")