Drawing soft brush

Question

I'm trying to create a smooth brush in HTML5, an example is below.

This is what I tried, it's something. But it's not as smooth as the image above.

Editor.Drawing.Context.globalAlpha = 0.3;
var amount = 3;

for(var t = -amount; t <= amount; t += 3) {
    for(var n = -amount; n <= amount; n += 3) {
        Editor.Drawing.Context.drawImage(Editor.Drawing.ClipCanvas, -(n-1), -(t-1));
    }
}

And it looks like this.

Answer 1

Using brushes

Choose a brush, this can be an image with predefined brushes or you can make one using an off-screen canvas and draw a radial gradient into it. For simplicity I made a simple image brush such as these:

Then for each new point drawn to the canvas:

• Calculate the diff between the previous and current point
• Calculate the length of the line so we can use an absolute step value independent of length
• Iterate over the length using a normalized value and the previously calculated step value

The step value can be anything that looks good as a result - it largely depends on the smoothness of the brush as well as its general size (smoother brushes will require smaller steps to blend into each other).

For this demo I used brush-width, the smaller values that are used, the more brushes will be drawn along the line, nicer result, but can also slow down the program, so find a value that compromises quality and speed.

For example:

This will be called every time a new point is registered when drawing:

function brushLine(ctx, x1, y1, x2, y2) {

    var diffX = Math.abs(x2 - x1),                       // calc diffs
        diffY = Math.abs(y2 - y1),
        dist = Math.sqrt(diffX * diffX + diffY * diffY), // find length
        step = 20 / (dist ? dist : 1),                   // "resolution"
        i = 0,                                           // iterator for length
        t = 0,                                           // t [0, 1]
        b, x, y;

    while (i <= dist) {
      t = Math.max(0, Math.min(1, i / dist));
      x = x1 + (x2 - x1) * t;
      y = y1 + (y2 - y1) * t;
      b = (Math.random() * 3) | 0;
      ctx.drawImage(brush, x - bw * 0.5, y - bh * 0.5);  // draw brush
      i += step;
    }
}

Demo

var brush = new Image();
brush.onload = ready;
brush.src = "//i.sstatic.net/HsbVA.png";

function ready() {

  var c = document.querySelector("canvas"),
      ctx = c.getContext("2d"),
      isDown = false, px, py, 
      bw = this.width, bh = this.height;

  c.onmousedown = c.ontouchstart = function(e) {
    isDown = true;
    var pos = getPos(e);
    px = pos.x;
    py = pos.y;
  };

  window.onmousemove = window.ontouchmove = function(e) {
    if (isDown) draw(e);
  };

  window.onmouseup = window.ontouchend = function(e) {
    e.preventDefault();
    isDown = false
  };

  function getPos(e) {
    e.preventDefault();
    if (e.touches) e = e.touches[0];
    var r = c.getBoundingClientRect();
    return {
      x: e.clientX - r.left,
      y: e.clientY - r.top
    }
  }

  function draw(e) {
    var pos = getPos(e);
    brushLine(ctx, px, py, pos.x, pos.y);
    px = pos.x;
    py = pos.y;
  }

  function brushLine(ctx, x1, y1, x2, y2) {

    var diffX = Math.abs(x2 - x1),
      diffY = Math.abs(y2 - y1),
      dist = Math.sqrt(diffX * diffX + diffY * diffY),
      step = bw / (dist ? dist : 1),
      i = 0,
      t = 0,
      b, x, y;

    while (i <= dist) {
      t = Math.max(0, Math.min(1, i / dist));
      x = x1 + (x2 - x1) * t;
      y = y1 + (y2 - y1) * t;
      b = (Math.random() * 3) | 0;
      ctx.drawImage(brush, x - bw * 0.5, y - bh * 0.5);
      i += step
    }
  }
}

body {background: #777}
canvas {background: #fff;cursor:crosshair}

<canvas width=630 height=500></canvas>

You can use this technique to simulate a variety of brushes.

Tip: with a small modification you can also variate the width depending on velocity to increase realism (not shown).