How To Recognize An Area In An Canvas Via Javascript

This image is loaded in an html5 canvas. If a user presses at any point inside the square, I want to fill this area with a color. But just inside, the color stops at the black line

Solution 1:

What you are looking for is called a flood fill algorithm. Which basically will take the color of the pixel you clicked on as a seed. It will check for all the pixels that are connected to the seed pixel that have the same color of the seed and fill them to a desired color. There are two types of this algorithm: 8-directional and 4-directional. Where the neighbours are defined as such ( S = Seed, X = Connected ):

4-directional

----------------
|    |  X |    |
----------------
|  X |  S |  X |
----------------
|    |  X |    |
----------------

8-directional

----------------
|  X |  X |  X |
----------------
|  X |  S |  X |
----------------
|  X |  X |  X |
----------------

The recursive algorithm is like so (From Wikipedia):

Flood-fill (node, target-color, replacement-color):
 1. If target-color is equal to replacement-color, return.
 2. If the color of node is not equal to target-color, return.
 3. Set the color of node to replacement-color.
 4. Perform Flood-fill (one step to the west of node, target-color, replacement-color).
    Perform Flood-fill (one step to the east of node, target-color, replacement-color).
    Perform Flood-fill (one step to the north of node, target-color, replacement-color).
    Perform Flood-fill (one step to the south of node, target-color, replacement-color).
 5. Return.

Solution 2:

You might want to use the CanvasRenderingContext2D.isPointInPath()

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.clicked = false;

function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  ctx.save();
  ctx.rotate(45);
  ctx.beginPath();
  ctx.rect(100, -100, 100, 100);
  if (ctx.clicked) {
    ctx.fillStyle = "blue";
    ctx.fill()
  }
  ctx.stroke();
  ctx.restore();
}
draw();
canvas.addEventListener('click', function(e) {
  var bounds= canvas.getBoundingClientRect();
  var mouseXY = [e.clientX-bounds.left, e.clientY-bounds.top]
  ctx.clicked = ctx.isPointInPath(mouseXY[0], mouseXY[1]);
  draw();
});

canvas {
  border: 1px solid
}

<canvas id="canvas" height="500" width="500"></canvas>