CarterLi · June 29, 2017 14:11
diff --git a/canvas.html b/canvas.html
 <!doctype html>
 <meta charset="utf-8" />
 <style type="text/css">
 	canvas {
 		display: block;
 	}
 </style>
 <input id="file" type="file" accept="image/*" multiple />
 <canvas id="canvas"></canvas>
 <script>
 /**
 * @param {string} url
 * @return {Promise<HTMLImageElement>} 
 */
 function loadImageFromUrl(url) {
    return new Promise(resolve => {
        const image = new Image();
        image.crossOrigin = 'Anonymous';
        image.addEventListener('load', function (e) {
            resolve(image);
        }, { once: true });
        image.src = url;
    });
 }

 /**
 * @param {File} file 
 * @return {Promise<string>}
 */
 function loadUrlFromFile(file) {
    return new Promise(resolve => {
        const reader = new FileReader();
        reader.addEventListener('load', function (e) {
            resolve(reader.result);
        }, { once: true });
        reader.readAsDataURL(file);
    });
 }

 /**
 * @param {ImageData} imgData
 * @param {number} length 
 */
 function createMosaic(imgData, length) {
    for (let row = 0; row < imgData.height; row += length) {
        for (let column = 0; column < imgData.width; column += length) {
            const index = (row * imgData.width + column) * 4;

            for (let x = column + 1; x < column + length; ++x) {
                const targetIndex = (row * imgData.width + x) * 4;
                imgData.data.copyWithin(targetIndex, index, index + 4);
            }
            for (let y = row + 1; y < row + length; ++y) {
                const targetIndex = (y * imgData.width + column) * 4;
                imgData.data.copyWithin(targetIndex, index, index + length * 4);
            }
        }
    }

    return imgData;
 }

 /**
 * @type {HTMLCanvasElement}
 */
 const canvas = document.getElementById('canvas');

 /**
 * @type {HTMLInputElement}
 */
 const file = document.getElementById('file');
 file.addEventListener('change', async function (e) {
    const file = this.files[0];
    const url = await loadUrlFromFile(file);
    const image = await loadImageFromUrl(url);
    canvas.width = image.naturalWidth;
    canvas.height = image.naturalHeight;
    const ctx = canvas.getContext('2d');
    ctx.drawImage(image, 0, 0);

    const imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
    const originalData = new Uint8ClampedArray(imgData.data);

    let start = 0, lastLength, flag = false;
    function render(t) {
        imgData.data.set(originalData);
        start = start || t;
        const percent = Math.min(1, (t - start) / 2000);
        let length = ~~(canvas.width / 10 * (flag ? 1 - percent : percent) ** 3);
        if (lastLength !== length) {
            if (length < 2) {
                ctx.putImageData(imgData, 0, 0);
            } else {
                ctx.putImageData(createMosaic(imgData, length), 0, 0);
            }
            
            lastLength = length;
        }
        requestAnimationFrame(render);
        if (percent === 1) {
            flag = !flag;
            start = t;
        }
    }
    requestAnimationFrame(render);
 });
 </script>
	<!doctype html>
	<meta charset="utf-8" />
	<style type="text/css">
	canvas {
	display: block;
	}
	</style>
	<input id="file" type="file" accept="image/*" multiple />
	<canvas id="canvas"></canvas>
	<script>
	/**
	* @param {string} url
	* @return {Promise<HTMLImageElement>}
	*/
	function loadImageFromUrl(url) {
	return new Promise(resolve => {
	const image = new Image();
	image.crossOrigin = 'Anonymous';
	image.addEventListener('load', function (e) {
	resolve(image);
	}, { once: true });
	image.src = url;
	});
	}

	/**
	* @param {File} file
	* @return {Promise<string>}
	*/
	function loadUrlFromFile(file) {
	return new Promise(resolve => {
	const reader = new FileReader();
	reader.addEventListener('load', function (e) {
	resolve(reader.result);
	}, { once: true });
	reader.readAsDataURL(file);
	});
	}

	/**
	* @param {ImageData} imgData
	* @param {number} length
	*/
	function createMosaic(imgData, length) {
	for (let row = 0; row < imgData.height; row += length) {
	for (let column = 0; column < imgData.width; column += length) {
	const index = (row * imgData.width + column) * 4;

	for (let x = column + 1; x < column + length; ++x) {
	const targetIndex = (row * imgData.width + x) * 4;
	imgData.data.copyWithin(targetIndex, index, index + 4);
	}
	for (let y = row + 1; y < row + length; ++y) {
	const targetIndex = (y * imgData.width + column) * 4;
	imgData.data.copyWithin(targetIndex, index, index + length * 4);
	}
	}
	}

	return imgData;
	}

	/**
	* @type {HTMLCanvasElement}
	*/
	const canvas = document.getElementById('canvas');

	/**
	* @type {HTMLInputElement}
	*/
	const file = document.getElementById('file');
	file.addEventListener('change', async function (e) {
	const file = this.files[0];
	const url = await loadUrlFromFile(file);
	const image = await loadImageFromUrl(url);
	canvas.width = image.naturalWidth;
	canvas.height = image.naturalHeight;
	const ctx = canvas.getContext('2d');
	ctx.drawImage(image, 0, 0);

	const imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
	const originalData = new Uint8ClampedArray(imgData.data);

	let start = 0, lastLength, flag = false;
	function render(t) {
	imgData.data.set(originalData);
	start = start \|\| t;
	const percent = Math.min(1, (t - start) / 2000);
	let length = ~~(canvas.width / 10 * (flag ? 1 - percent : percent) ** 3);
	if (lastLength !== length) {
	if (length < 2) {
	ctx.putImageData(imgData, 0, 0);
	} else {
	ctx.putImageData(createMosaic(imgData, length), 0, 0);
	}

	lastLength = length;
	}
	requestAnimationFrame(render);
	if (percent === 1) {
	flag = !flag;
	start = t;
	}
	}
	requestAnimationFrame(render);
	});
	</script>