Faster: Using context.getImageData() to find "colored" pixels (non-zero values)
// returns true if all color channels in each pixel are 0 (or "blank")
function isCanvasBlank(canvas) {
return !canvas.getContext('2d')
.getImageData(0, 0, canvas.width, canvas.height).data
.some(channel => channel !== 0);
}
As @Kaiido points out, you can get even better performance by enumerating over a Uint32Array of pixels instead of each color channel in every pixel.
// returns true if every pixel's uint32 representation is 0 (or "blank")
function isCanvasBlank(canvas) {
const context = canvas.getContext('2d');
const pixelBuffer = new Uint32Array(
context.getImageData(0, 0, canvas.width, canvas.height).data.buffer
);
return !pixelBuffer.some(color => color !== 0);
}
Slower: Comparing data URLs with a blank canvas
function isCanvasBlank(canvas) {
const blank = document.createElement('canvas');
blank.width = canvas.width;
blank.height = canvas.height;
return canvas.toDataURL() === blank.toDataURL();
}
Benchmark
Demo
document.getElementById('check').addEventListener('click', function() {
const blank = isCanvasBlank(document.getElementById('canvas'));
alert(blank ? 'blank' : 'not blank');
});
document.getElementById('draw').addEventListener('click', function() {
drawOnCanvas(document.getElementById('canvas'));
});
document.getElementById('clear').addEventListener('click', function() {
const canvas = document.getElementById('canvas');
canvas.getContext('2d').clearRect(0, 0, canvas.width, canvas.height);
});
function isCanvasBlank(canvas) {
const context = canvas.getContext('2d');
const pixelBuffer = new Uint32Array(
context.getImageData(0, 0, canvas.width, canvas.height).data.buffer
);
return !pixelBuffer.some(color => color !== 0);
}
function drawOnCanvas(canvas) {
const context = canvas.getContext('2d');
context.fillStyle = '#' + Math.floor(Math.random() * 0xFFFFFF).toString(16);
context.fillRect(Math.floor(Math.random() * canvas.width),
Math.floor(Math.random() * canvas.height),
Math.floor(Math.random() * canvas.width),
Math.floor(Math.random() * canvas.height));
}
canvas {
display: block;
margin-top: 10px;
border: 1px solid black;
}
<button id="check"> Check </button>
<button id="draw"> Draw </button>
<button id="clear"> Clear </button>
<canvas id="canvas" width="200" height="200"></canvas>
