Changing the background color of the canvas using Javascript

Question

Changing the background color of the canvas using Javascript

I want to create a rect on a canvas with a slightly transparent background, but I don't want the drawn rect to have any background.

Here is an example of what I'm looking for:

https://i.stack.imgur.com/axtcE.png

The code I am using is as follows:

var canvas = document.getElementById('canvas');
var img = document.getElementById('photo');
var ctx = canvas.getContext('2d');
var rect = {};
var drag = false;
var update = true; // when true updates canvas
var original_source = img.src;
img.src = original_source;

function init() {
    img.addEventListener('load', function(){
        canvas.width = img.width;
        canvas.height = img.height;
        canvas.addEventListener('mousedown', mouseDown, false);
        canvas.addEventListener('mouseup', mouseUp, false);
        canvas.addEventListener('mousemove', mouseMove, false);
    });
    // start the rendering loop
    requestAnimationFrame(updateCanvas);
}

// main render loop only updates if update is true
function updateCanvas(){
  if(update){
      drawCanvas();
      update = false;
  }

  requestAnimationFrame(updateCanvas);
}

// draws a rectangle with rotation 
function drawRect(){
   ctx.setTransform(1,0,0,1,rect.startX + rect.w / 2, rect.startY + rect.h / 2);
   ctx.rotate(rect.rotate);
   ctx.beginPath();
   ctx.rect(-rect.w/2, -rect.h/2, rect.w, rect.h);
   /* ctx.fill(); */
   ctx.stroke();
}

// clears canvas sets filters and draws rectangles
function drawCanvas(){
    // restore the default transform as rectangle rendering does not restore the transform.
    ctx.setTransform(1,0,0,1,0,0);
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    drawRect()
}

// create new rect add to array 
function mouseDown(e) {
    rect = {
      startX : e.offsetX,
      startY : e.offsetY,
      w : 1,
      h : 1,
      rotate : 0,
    };
    drag = true;
}

function mouseUp() { drag = false; buttons_shown = true; update = true; }

function mouseMove(e) {
    if (drag) {
        rect.w = (e.pageX - this.offsetLeft) - rect.startX;
        rect.h = (e.pageY - this.offsetTop) - rect.startY;
        update = true;
    }
}

init();

.hide{
    display: none !important;
}

canvas{
  position: absolute;
  left: 0; 
  right: 0; 
  top: 0; 
  bottom: 0; 
  display:inline-block;
  background:rgba(0,0,0,0.3);
}

<div style="position: relative; overflow: hidden;display:inline-block;">
    <img id="photo" src="http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg"/>
    <canvas id="canvas"></canvas>
</div>


<div id="buttons" class="hide"></div>

In my scenario, I've set the canvas background to match what I want, but I'm struggling to make the drawn rect appear without that background color being applied.

Here is the fiddle.

Any suggestions on how to solve this issue?

javascript html css canvas

Answer 1

Answer №1

To achieve this effect, there are several methods that can be used such as compositing or clip-path. However, the simplest approach for creating a rectangle with a hole is to utilize the "evenodd" fill-rule parameter of the fill() method in CanvasRenderingContext2D.

The process involves drawing a larger rectangle covering the entire canvas first, and then within the same path declaration, drawing a smaller inner rectangle. By setting the fill-rule to "evenodd", the smaller rectangle will be excluded from the larger one.

function drawRect() {
  ctx.beginPath(); // defines a single path
  ctx.rect(0, 0, ctx.canvas.width, ctx.canvas.height); // draws big rectangle covering canvas
  ctx.setTransform(1, 0, 0, 1, rect.startX + rect.w / 2, rect.startY + rect.h / 2);
  ctx.rotate(rect.rotate);
  ctx.rect(-rect.w / 2, -rect.h / 2, rect.w, rect.h);
  ctx.fill('evenodd'); // sets fill-rule to evenodd
  ctx.beginPath(); // starts a new path declaration
  ctx.rect(-rect.w / 2, -rect.h / 2, rect.w, rect.h);
  ctx.stroke(); // strokes small rectangle
}

var canvas = document.getElementById('canvas');
var img = document.getElementById('photo');
var ctx = canvas.getContext('2d');
var rect = {};
var drag = false;
var update = true; 
var original_source = img.src;

// Event listeners for mouse interactions
function init() {
  img.addEventListener('load', function() {
    canvas.width = img.width;
    canvas.height = img.height;
    canvas.addEventListener('mousedown', mouseDown, false);
    canvas.addEventListener('mouseup', mouseUp, false);
    canvas.addEventListener('mousemove', mouseMove, false);

    // context styles
    ctx.fillStyle = 'rgba(0,0,0,.5)';
    ctx.strokeStyle = 'white';
    ctx.lineWidth = 2;
  });
  
  requestAnimationFrame(updateCanvas); // rendering loop
}

// renders canvas only if update is true
function updateCanvas() {
  if (update) {
    drawCanvas();
    update = false;
  }

  requestAnimationFrame(updateCanvas);
}

// clears canvas, applies filters, and draws rectangles
function drawCanvas() {
  ctx.setTransform(1, 0, 0, 1, 0, 0);
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  drawRect()
}

// creates new rectangle on mouse down
function mouseDown(e) {
  rect = {
    startX: e.offsetX,
    startY: e.offsetY,
    w: 1,
    h: 1,
    rotate: 0,
  };
  drag = true;
}

function mouseUp() {
  drag = false;
  buttons_shown = true;
  update = true;
}

function mouseMove(e) {
  if (drag) {
    rect.w = (e.pageX - this.offsetLeft) - rect.startX;
    rect.h = (e.pageY - this.offsetTop) - rect.startY;
    update = true;
  }
}

init();

.hide {
  display: none !important;
}

canvas {
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  display: inline-block;
  background: rgba(0, 0, 0, 0.3);
}

<div style="position: relative; overflow: hidden;display:inline-block;">
  <img id="photo" src="http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg" />
  <canvas id="canvas"></canvas>
</div>

<div id="buttons" class="hide"></div>

Answer 2

To achieve this effect, there are several methods that can be used such as compositing or clip-path. However, the simplest approach for creating a rectangle with a hole is to utilize the "evenodd" fill-rule parameter of the fill() method in CanvasRenderingContext2D.

The process involves drawing a larger rectangle covering the entire canvas first, and then within the same path declaration, drawing a smaller inner rectangle. By setting the fill-rule to "evenodd", the smaller rectangle will be excluded from the larger one.

function drawRect() {
  ctx.beginPath(); // defines a single path
  ctx.rect(0, 0, ctx.canvas.width, ctx.canvas.height); // draws big rectangle covering canvas
  ctx.setTransform(1, 0, 0, 1, rect.startX + rect.w / 2, rect.startY + rect.h / 2);
  ctx.rotate(rect.rotate);
  ctx.rect(-rect.w / 2, -rect.h / 2, rect.w, rect.h);
  ctx.fill('evenodd'); // sets fill-rule to evenodd
  ctx.beginPath(); // starts a new path declaration
  ctx.rect(-rect.w / 2, -rect.h / 2, rect.w, rect.h);
  ctx.stroke(); // strokes small rectangle
}

var canvas = document.getElementById('canvas');
var img = document.getElementById('photo');
var ctx = canvas.getContext('2d');
var rect = {};
var drag = false;
var update = true; 
var original_source = img.src;

// Event listeners for mouse interactions
function init() {
  img.addEventListener('load', function() {
    canvas.width = img.width;
    canvas.height = img.height;
    canvas.addEventListener('mousedown', mouseDown, false);
    canvas.addEventListener('mouseup', mouseUp, false);
    canvas.addEventListener('mousemove', mouseMove, false);

    // context styles
    ctx.fillStyle = 'rgba(0,0,0,.5)';
    ctx.strokeStyle = 'white';
    ctx.lineWidth = 2;
  });
  
  requestAnimationFrame(updateCanvas); // rendering loop
}

// renders canvas only if update is true
function updateCanvas() {
  if (update) {
    drawCanvas();
    update = false;
  }

  requestAnimationFrame(updateCanvas);
}

// clears canvas, applies filters, and draws rectangles
function drawCanvas() {
  ctx.setTransform(1, 0, 0, 1, 0, 0);
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  drawRect()
}

// creates new rectangle on mouse down
function mouseDown(e) {
  rect = {
    startX: e.offsetX,
    startY: e.offsetY,
    w: 1,
    h: 1,
    rotate: 0,
  };
  drag = true;
}

function mouseUp() {
  drag = false;
  buttons_shown = true;
  update = true;
}

function mouseMove(e) {
  if (drag) {
    rect.w = (e.pageX - this.offsetLeft) - rect.startX;
    rect.h = (e.pageY - this.offsetTop) - rect.startY;
    update = true;
  }
}

init();

.hide {
  display: none !important;
}

canvas {
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  display: inline-block;
  background: rgba(0, 0, 0, 0.3);
}

<div style="position: relative; overflow: hidden;display:inline-block;">
  <img id="photo" src="http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg" />
  <canvas id="canvas"></canvas>
</div>

<div id="buttons" class="hide"></div>

Answer 3

Answer №2

Make sure to fill your rectangle before using ctx.stroke(), as demonstrated below:

ctx.fillStyle = "rgba(255, 255, 255, 0.3)";
ctx.fill();

By following this method, you can achieve a similar effect to what was displayed in the original question. Instead of just applying a css background, filling both inside and outside of the rectangle with the desired style would result in an even more enhanced effect.

Answer 4

Make sure to fill your rectangle before using ctx.stroke(), as demonstrated below:

ctx.fillStyle = "rgba(255, 255, 255, 0.3)";
ctx.fill();

By following this method, you can achieve a similar effect to what was displayed in the original question. Instead of just applying a css background, filling both inside and outside of the rectangle with the desired style would result in an even more enhanced effect.

Answer 5

Answer №3

To start, create a complete canvas with a slightly transparent background by using the following code:

ctx.fillStyle = 'rgba(50, 50, 50, 0.6)';
ctx.fillRect(0, 0, width, height);

Next, simply clear a specific rectangular area from this canvas using the following code:

ctx.clearRect(x, y, rect_width, rect_height);

Answer 6

To start, create a complete canvas with a slightly transparent background by using the following code:

ctx.fillStyle = 'rgba(50, 50, 50, 0.6)';
ctx.fillRect(0, 0, width, height);

Next, simply clear a specific rectangular area from this canvas using the following code:

ctx.clearRect(x, y, rect_width, rect_height);

Changing the background color of the canvas using Javascript

Answer №1

Answer №2

Answer №3

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