Pixel manipulation with canvas
Hasta ahora, no habíamos mirado los píxeles reales de nuestrocanvas. Con el objetoImageData, puedes leer y escribir directamente unarray de datos para manipular píxeles.
También veremos cómo se puede controlar el suavizado de la imagen (antialiasing) y cómo guardar imágenes de tucanvas.
El objetoImageData
El objetoImageData representa los datos pixelados subyacentes de un área de un objeto lienzo. Contiene los siguientes atributos de sólo lectura:
widthEl ancho de la imagen en píxeles.
heightLa altura de la imagen en píxeles.
dataUn objeto
Uint8ClampedArrayque representa un array unidimensional, contiene información en formato RGBA, con valores desde0hasta255(incluído).
La propiedaddata devuelve unUint8ClampedArray, al que se puede acceder para ver los datos originales del pixel; cada pixel está representado por cuatro valores (rojo, verde, azul, y alfa, en ese orden; esto es, formato "RGBA"). Cada componente de color se representa con un valor entero entre 0 y 255. Dentro del array, cada componente ocupa un índice consecutivo, comenzando con 0 desde el punto superior izquierdo, continuando de izquierda a derecha y de arriba hacia abajo, a través del array.
ElUint8ClampedArray contienealto ×ancho × 4 bytes de datos, con valores de índice en el rango entre 0 y (alto ×ancho × 4)-1.
Por ejemplo, para leer el valor del componente azul del pixel en la columna 200, fila 50 de una imagen, deberías hacer lo siguiente:
blueComponent = imageData.data[((50 * (imageData.width * 4)) + (200 * 4)) + 2];
Si se le da un conjunto de coordenadas (X e Y), puede que termine haciendo algo así:
var xCoord = 50;var yCoord = 100;var canvasWidth = 1024;function getColorIndicesForCoord(x, y, width) { var red = y * (width * 4) + x * 4; return [red, red + 1, red + 2, red + 3];}var colorIndices = getColorIndicesForCoord(xCoord, yCoord, canvasWidth);var redIndex = colorIndices[0];var greenIndex = colorIndices[1];var blueIndex = colorIndices[2];var alphaIndex = colorIndices[3];var redForCoord = imageData.data[redIndex];var greenForCoord = imageData.data[greenIndex];var blueForCoord = imageData.data[blueIndex];var alphaForCoord = imageData.data[alphaIndex];O, en ES6 sería algo así:
const xCoord = 50;const yCoord = 100;const canvasWidth = 1024;const getColorIndicesForCoord = (x, y, width) => { const red = y * (width * 4) + x * 4; return [red, red + 1, red + 2, red + 3];};const colorIndices = getColorIndicesForCoord(xCoord, yCoord, canvasWidth);const [redIndex, greenIndex, blueIndex, alphaIndex] = colorIndices;You may also access the size of the pixel array in bytes by reading theUint8ClampedArray.length attribute:
var numBytes = imageData.data.length;Creando un objetoImageData
Para crear un objeto nuevo y vacío tipoImageData, debes usar el métodocreateImageData(). Hay dos versiones del métodocreateImageData():
var myImageData = ctx.createImageData(width, height);Esto crea un nuevo objetoImageData con las dimensiones especificadas. Todos los pixels tienen valor correspondiente a negro - transparente (0,0,0,0).
También puedes crear un nuevo objetoImageData con las mismas dimensiones que otro objeto, especificado poranotherImageData. Los píxels del nuevo objeto tienen valor negro - transparente.¡Esto no es una copia de los datos de la imagen!
var myImageData = ctx.createImageData(anotherImageData);Getting the pixel data for a context
To obtain anImageData object containing a copy of the pixel data for a canvas context, you can use thegetImageData() method:
var myImageData = ctx.getImageData(left, top, width, height);This method returns anImageData object representing the pixel data for the area of the canvas whose corners are represented by the points (left,top), (left+width,top), (left,top+height), and (left+width,top+height). The coordinates are specified in canvas coordinate space units.
Nota:Any pixels outside the canvas are returned as transparent black in the resultingImageData object.
This method is also demonstrated in the articleManipulating video using canvas.
A color picker
In this example we are using thegetImageData() method to display the color under the mouse cursor. For this, we need the current position of the mouse withlayerX andlayerY, then we look up the pixel data on that position in the pixel array thatgetImageData() provides us. Finally, we use the array data to set a background color and a text in the<div> to display the color.
const img = new Image();img.crossOrigin = "anonymous";img.src = "./assets/rhino.jpg";const canvas = document.getElementById("canvas");const ctx = canvas.getContext("2d");img.addEventListener("load", () => { ctx.drawImage(img, 0, 0); img.style.display = "none";});const hoveredColor = document.getElementById("hovered-color");const selectedColor = document.getElementById("selected-color");function pick(event, destination) { const bounding = canvas.getBoundingClientRect(); const x = event.clientX - bounding.left; const y = event.clientY - bounding.top; const pixel = ctx.getImageData(x, y, 1, 1); const data = pixel.data; const rgba = `rgba(${data[0]}, ${data[1]}, ${data[2]}, ${data[3] / 255})`; destination.style.background = rgba; destination.textContent = rgba; return rgba;}canvas.addEventListener("mousemove", (event) => pick(event, hoveredColor));canvas.addEventListener("click", (event) => pick(event, selectedColor));Painting pixel data into a context
You can use theputImageData()method to paint pixel data into a context:
ctx.putImageData(myImageData, dx, dy);Thedx anddy parameters indicate the device coordinates within the context at which to paint the top left corner of the pixel data you wish to draw.
For example, to paint the entire image represented bymyImageData to the top left corner of the context, you can simply do the following:
ctx.putImageData(myImageData, 0, 0);Grayscaling and inverting colors
In this example we iterate over all pixels to change their values, then we put the modified pixel array back to the canvas usingputImageData(). The invert function simply subtracts each color from the max value 255. The grayscale function simply uses the average of red, green and blue. You can also use a weighted average, given by the formulax = 0.299r + 0.587g + 0.114b, for example. SeeGrayscale on Wikipedia for more information.
const img = new Image();img.crossOrigin = "anonymous";img.src = "./assets/rhino.jpg";const canvas = document.getElementById("canvas");const ctx = canvas.getContext("2d");img.onload = () => { ctx.drawImage(img, 0, 0);};const original = () => { ctx.drawImage(img, 0, 0);};const invert = () => { ctx.drawImage(img, 0, 0); const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height); const data = imageData.data; for (let i = 0; i < data.length; i += 4) { data[i] = 255 - data[i]; // red data[i + 1] = 255 - data[i + 1]; // green data[i + 2] = 255 - data[i + 2]; // blue } ctx.putImageData(imageData, 0, 0);};const grayscale = () => { ctx.drawImage(img, 0, 0); const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height); const data = imageData.data; for (let i = 0; i < data.length; i += 4) { const avg = (data[i] + data[i + 1] + data[i + 2]) / 3; data[i] = avg; // red data[i + 1] = avg; // green data[i + 2] = avg; // blue } ctx.putImageData(imageData, 0, 0);};const inputs = document.querySelectorAll("[name=color]");for (const input of inputs) { input.addEventListener("change", (evt) => { switch (evt.target.value) { case "inverted": return invert(); case "grayscale": return grayscale(); default: return original(); } });}Zooming and anti-aliasing
With the help of thedrawImage() method, a second canvas and theimageSmoothingEnabled property, we are able to zoom into our picture and see the details.
We get the position of the mouse and crop an image of 5 pixels left and above to 5 pixels right and below. Then we copy that one over to another canvas and resize the image to the size we want it to. In the zoom canvas we resize a 10×10 pixel crop of the original canvas to 200×200.
zoomctx.drawImage( canvas, Math.min(Math.max(0, x - 5), img.width - 10), Math.min(Math.max(0, y - 5), img.height - 10), 10, 10, 0, 0, 200, 200,);Because anti-aliasing is enabled by default, we might want to disable the smoothing to see clear pixels. You can toggle the checkbox to see the effect of theimageSmoothingEnabled property (which needs prefixes for different browsers).
Zoom example
const img = new Image();img.crossOrigin = "anonymous";img.src = "./assets/rhino.jpg";img.onload = () => { draw(this);};function draw(img) { const canvas = document.getElementById("canvas"); const ctx = canvas.getContext("2d"); ctx.drawImage(img, 0, 0); const smoothedZoomCtx = document .getElementById("smoothed-zoom") .getContext("2d"); smoothedZoomCtx.imageSmoothingEnabled = true; smoothedZoomCtx.mozImageSmoothingEnabled = true; smoothedZoomCtx.webkitImageSmoothingEnabled = true; smoothedZoomCtx.msImageSmoothingEnabled = true; const pixelatedZoomCtx = document .getElementById("pixelated-zoom") .getContext("2d"); pixelatedZoomCtx.imageSmoothingEnabled = false; pixelatedZoomCtx.mozImageSmoothingEnabled = false; pixelatedZoomCtx.webkitImageSmoothingEnabled = false; pixelatedZoomCtx.msImageSmoothingEnabled = false; const zoom = (ctx, x, y) => { ctx.drawImage( canvas, Math.min(Math.max(0, x - 5), img.width - 10), Math.min(Math.max(0, y - 5), img.height - 10), 10, 10, 0, 0, 200, 200, ); }; canvas.addEventListener("mousemove", (event) => { const x = event.layerX; const y = event.layerY; zoom(smoothedZoomCtx, x, y); zoom(pixelatedZoomCtx, x, y); });}Guardando las imágenes
TheHTMLCanvasElement provides atoDataURL() method, which is useful when saving images. It returns adata URI containing a representation of the image in the format specified by thetype parameter (defaults toPNG). The returned image is in a resolution of 96 dpi.
canvas.toDataURL('image/png')Default setting. Creates a PNG image.
canvas.toDataURL('image/jpeg', quality)Creates a JPG image. Optionally, you can provide a quality in the range from 0 to 1, with one being the best quality and with 0 almost not recognizable but small in file size.
Once you have generated a data URI from you canvas, you are able to use it as the source of any<image> or put it into a hyper link with adownload attribute to save it to disc, for example.
You can also create aBlob from the canvas.
canvas.toBlob(callback, type, encoderOptions)Creates a
Blobobject representing the image contained in the canvas.