Setting a Clipping Region

Having wanted to find this out for a recent project I thought I would share this here.  When drawing to an HTML5 Canvas the default clipping region is the size of the canvas.  But what if you want to change the clipping area?

<body onload="drawCanvas();">
    <div>
        <canvas id="myCanvas" width="300" height="150">
            <p>Your browser doesn't support canvas.</p>
        </canvas>
    </div>
</body>

function drawCanvas() {
    // Get our Canvas element
    var surface = document.getElementById("myCanvas");

    if (surface.getContext) {
        // If Canvas is supported
        var context = surface.getContext('2d');

        // Draw a red circle
        context.beginPath();
        context.arc(60, 60, 40, 0, Math.PI * 2, false);
        context.closePath();
        context.fillStyle = "#ff0000";
        context.fill();

        // Set the clipping area
        context.beginPath();
        context.rect(0, 0, 300, 60);
        context.clip();

        // Draw the circle again
        context.beginPath();
        context.arc(150, 60, 40, 0, Math.PI * 2, false);
        context.closePath();
        context.fillStyle = "#ff0000";
        context.fill();
    }
}

Here we draw two Red circles, the first is drawn with the default clipping region set which starts at 0,0 and finishes at 300,150 (the dimensions of our Canvas element).

Canvas Text

Before we draw the 2nd circle we define our new clipping region using the rect() method which takes the following parameters: left, top, right, bottom.  When we have defined our clipping area we call clip() which sets it.  All future rendering will have that clipping region applied.

If you want to save the current rendering state (which includes the clipping region) you can call the save() method on the graphics context before you change the state.  Using restore() can then be used to quickly set the rendering state back to what it was when you last called save().

Post your Comments

 
 
Latest Games
Zombie Escape
Apr 19, 2016
Plays: 2,741

Zombie Escape ScreenshotDrive fast before the crazy mutant zombies get you!

6 Ratings/4.1 Average
Car Parking
Jan 16, 2016
Plays: 2,592

Car Parking ScreenshotGuide the car to its parking space in this fun Car Parking game.

1 Rating/5 Average
Trash It
Jan 11, 2016
Plays: 2,459

Trash It ScreenshotAim for the Trash Can and get the various items of Trash in the bin.

4 Ratings/5 Average
Sky Fly
Jan 11, 2016
Plays: 2,667

Sky Fly ScreenshotFly your plane in this colorful vertical scrolling shoot-em-up. Blast the bad guys and collect any bonus's they leave behind.

1 Rating/5 Average
Professor Snappy
Jan 11, 2016
Plays: 2,137

Professor Snappy ScreenshotPop as many bubbles as possible in this fun and colorful bubble popping game. The levels start off easy enough but gradually get harder!

1 Rating/5 Average
Monster Match Saga
Jan 10, 2016
Plays: 2,461

Monster Match Saga ScreenshotHere we have a bunch of monsters that need to be matched up. Look out for the bomb and spinning monsters that will cause special damage!

3 Ratings/4.6 Average
Fly Bird Fly
Jan 10, 2016
Plays: 2,288

Fly Bird Fly ScreenshotGuide your friendly Bird through the maze of pipes and other obstacles collecting the Stars in this cool arcade game inspired by the legendary Flappy Bird.

1 Rating/5 Average
Life In One
Jan 10, 2016
Plays: 2,465

Life In One ScreenshotYou are stranded on an Alien planet. Your goal is to build a space rocket and escape. Start by building units to create power and mine the metal patches. Build defenses to defend your base from the advancing Aliens and Zombies!

2 Ratings/3 Average
X Pool
Jan 02, 2016
Plays: 3,099

X Pool ScreenshotPlay Pool against the computer or battle against your friends in the online mode!

3 Ratings/3 Average
Fruit Slicer
Jan 02, 2016
Plays: 2,172

Fruit Slicer ScreenshotSlice the fruit that is thrown up onto the screen. Slice the fruit into multiple pieces for maximum points!

1 Rating/5 Average