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https://github.com/andrewstephens75/as-dithered-image.git
synced 2026-04-14 12:29:30 -07:00
Rewriten history to make me look less of an idiot. The first version worthy to be seen.
This commit is contained in:
Andrew Stephens
199
as-dithered-image.js
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199
as-dithered-image.js
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const DITHERED_IMAGE_STYLE = `
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.ditheredImageStyle {
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width: 100%;
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height: 100%;
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padding: 0;
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margin: 0;
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}
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`
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class ASDitheredImage extends HTMLElement {
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constructor() {
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super()
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// the canvas API is confusing if you want pixel accurate drawing. The canvas backing store must be set to the screen size * the devicePixelRatio
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// The crunch factor is how "chunky" the dither should be, ie how many css pixels to dithered pixels
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this.crunchFactor = this.getAutoCrunchFactor()
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this.drawTimestamp = 0
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this.drawRect = undefined
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this.altText = ""
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}
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connectedCallback() {
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if (!this.isConnected) {
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return
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}
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const shadowDOM = this.attachShadow({ mode: "open" })
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const style = document.createElement("style")
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style.innerHTML = DITHERED_IMAGE_STYLE
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shadowDOM.appendChild(style)
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this.canvas = document.createElement("canvas")
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this.canvas.setAttribute("role", "image")
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this.canvas.setAttribute("aria-label", this.altText)
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this.canvas.classList.add("ditheredImageStyle")
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shadowDOM.appendChild(this.canvas)
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this.context = this.canvas.getContext("2d")
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const resizeObserver = new ResizeObserver((entries) => {
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for (const e of entries) {
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if (e.contentBoxSize) {
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this.requestUpdate()
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}
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}
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})
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resizeObserver.observe(this.canvas)
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this.requestUpdate()
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}
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static get observedAttributes() { return ["src", "crunch", "alt"] }
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attributeChangedCallback(name, oldValue, newValue) {
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if (oldValue === newValue) return
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if ((name === "src")) {
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this.src = newValue
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this.requestUpdate()
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} else if (name === "crunch") {
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if (newValue === "auto") {
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this.crunchFactor = this.getAutoCrunchFactor()
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} else if (newValue === "pixel") {
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this.crunchFactor = 1.0 / window.devicePixelRatio
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} else {
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this.crunchFactor = parseInt(newValue, 10)
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if (isNaN(this.crunchFactor)) {
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this.crunchFactor = this.getAutoCrunchFactor()
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}
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}
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this.requestUpdate()
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} else if (name === "alt") {
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this.altText = newValue;
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if (this.canvas != undefined) {
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let currentAltText = this.canvas.getAttribute("aria-label")
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if (currentAltText != this.altText) {
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this.canvas.setAttribute("aria-label", this.altText)
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}
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}
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}
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}
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// all drawing is funneled through requestUpdate so that multiple calls are coalesced to prevent
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// processing the image multiple times for no good reason
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requestUpdate() {
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window.requestAnimationFrame(((timestamp) => {
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if (this.drawTimestamp != timestamp) {
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this.drawImage()
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this.drawTimestamp = timestamp
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}
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}).bind(this))
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}
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// The crunch factor defaults 1 css pixel to 1 dither pixel which I think looks best when the device pixel ratio is 1 or 2
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// If the pixel ratio is 3 or above (like on my iPhone) then even css pixels are too small to make dithering
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// look effective, so I double the pixels again
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getAutoCrunchFactor() {
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if (window.devicePixelRatio < 3) {
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return 1
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} else {
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return 2
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}
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}
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drawImage() {
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if ((this.canvas === undefined) || (this.src === undefined)) {
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return
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}
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const rect = this.canvas.getBoundingClientRect()
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if ((this.drawRect != undefined) && (rect.width == this.drawRect.width) && (rect.height == this.drawRect.height)) {
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return // already drawn the image at this size
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}
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this.drawRect = rect;
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// to get really crisp pixels on retina-type displays (window.devicePixelRatio > 1) we have to set the
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// canvas backing store to the element size times the devicePixelRatio
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// Then, once the image has loaded we draw it manually scaled to only part of the canvas (since the canvas is bigger than the element)
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// The dithering algorythm will scale up the image to the canvas size
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const logicalPixelSize = window.devicePixelRatio * this.crunchFactor
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this.canvas.width = rect.width * window.devicePixelRatio
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this.canvas.height = rect.height * window.devicePixelRatio
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const image = new Image()
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image.onload = (() => {
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this.context.imageSmoothingEnabled = true
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this.context.drawImage(image, 0, 0, this.canvas.width / logicalPixelSize, this.canvas.height / logicalPixelSize)
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const original = this.context.getImageData(0, 0, this.canvas.width / logicalPixelSize, this.canvas.height / logicalPixelSize)
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const dithered = this.dither(original, logicalPixelSize)
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this.context.imageSmoothingEnabled = false
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this.context.putImageData(dithered, 0, 0)
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}).bind(this)
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image.src = this.src
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}
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dither(imageData, scaleFactor) {
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let output = new ImageData(imageData.width * scaleFactor, imageData.height * scaleFactor)
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for (let i = 0; i < imageData.data.length; i += 4) {
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imageData.data[i] = imageData.data[i + 1] = imageData.data[i + 2] = Math.floor(imageData.data[i] * 0.3 + imageData.data[i + 1] * 0.59 + imageData.data[i + 2] * 0.11)
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}
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// most implementations I see just distibute error into the existing image, wrapping around edge pixels
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// this implementation uses a sliding window of floats for more accuracy (probably not needed really)
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let slidingErrorWindow = [new Float32Array(imageData.width), new Float32Array(imageData.width), new Float32Array(imageData.width)]
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const offsets = [[1, 0], [2, 0], [-1, 1], [0, 1], [1, 1], [0, 2]]
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for (let y = 0, limY = imageData.height; y < limY; ++y) {
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for (let x = 0, limX = imageData.width; x < limX; ++x) {
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let i = ((y * limX) + x) * 4;
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let accumulatedError = Math.floor(slidingErrorWindow[0][x])
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let expectedMono = imageData.data[i] + accumulatedError
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let monoValue = expectedMono
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if (monoValue <= 127) {
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monoValue = 0
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} else {
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monoValue = 255
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}
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let error = (expectedMono - monoValue) / 8.0
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for (let q = 0; q < offsets.length; ++q) {
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let offsetX = offsets[q][0] + x
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let offsetY = offsets[q][1] + y
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if ((offsetX >= 0) && (offsetX < slidingErrorWindow[0].length))
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slidingErrorWindow[offsets[q][1]][offsetX] += error
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}
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// this is stupid but we have to do the pixel scaling ourselves because safari insists on interpolating putImageData
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// which gives us blury pixels (and it doesn't support the createImageBitmap call with an ImageData instance which
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// would make this easy)
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for (let scaleY = 0; scaleY < scaleFactor; ++scaleY) {
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let pixelOffset = (((y * scaleFactor + scaleY) * output.width) + (x * scaleFactor)) * 4
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for (let scaleX = 0; scaleX < scaleFactor; ++scaleX) {
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output.data[pixelOffset] = output.data[pixelOffset + 1] = output.data[pixelOffset + 2] = monoValue
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output.data[pixelOffset + 3] = 255
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pixelOffset += 4
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}
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}
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}
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// move the sliding window
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slidingErrorWindow.push(slidingErrorWindow.shift())
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slidingErrorWindow[2].fill(0, 0, slidingErrorWindow[2].length)
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}
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return output
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}
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}
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window.customElements.define('as-dithered-image', ASDitheredImage);
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