Is it possible to query a HTML Canvas object to get the color at a specific location?
If you want to extract a particular color of pixel by passing the coordinates of pixel into the function, this will come in handy:
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
function detectColor(x, y){
data=ctx.getImageData(x, y, 1, 1).data;
col={
r:data[0],
g:data[1],
b:data[2]
};
return col;
}
x
, y
is the coordinate you want to filter out color.
var color = detectColor(x, y)
The color is the object, you will get the RGB value by color.r
, color.g
, color.b
.
Try the getImageData method:
var data = context.getImageData(x, y, 1, 1).data;
var rgb = [ data[0], data[1], data[2] ];
Yes sure, provided you have its context. (See how to get canvas context here.)
var imgData = context.getImageData(0,0,canvas.width,canvas.height);
// { data: [r,g,b,a,r,g,b,a,r,g,..], ... }
function getPixel(imgData, index) {
var i = index*4, d = imgData.data;
return [d[i],d[i+1],d[i+2],d[i+3]] // Returns array [R,G,B,A]
}
// AND/OR
function getPixelXY(imgData, x, y) {
return getPixel(imgData, y*imgData.width+x);
}
PS: If you plan to mutate the data and draw them back on the canvas, you can use subarray
var
idt = imgData, // See previous code snippet
a = getPixel(idt, 188411), // Array(4) [0, 251, 0, 255]
b = idt.data.subarray(188411*4, 188411*4 + 4) // Uint8ClampedArray(4) [0, 251, 0, 255]
a[0] = 255 // Does nothing
getPixel(idt, 188411), // Array(4) [0, 251, 0, 255]
b[0] = 255 // Mutates the original imgData.data
getPixel(idt, 188411), // Array(4) [255, 251, 0, 255]
// Or use it in the function
function getPixel(imgData, index) {
var i = index*4, d = imgData.data;
return imgData.data.subarray(index, index+4) // Returns subarray [R,G,B,A]
}
You can experiment with this on http://qry.me/xyscope/, the code for this is in the source, just copy/paste it in the console.
Note that getImageData returns a snapshot. Implications are:
// Get pixel data
var imageData = context.getImageData(x, y, width, height);
// Color at (x,y) position
var color = [];
color['red'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 0];
color['green'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 1];
color['blue'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 2];
color['alpha'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 3];
There's a section about pixel manipulation in the W3C documentation.
Here's an example on how to invert an image:
var context = document.getElementById('myCanvas').getContext('2d');
// Get the CanvasPixelArray from the given coordinates and dimensions.
var imgd = context.getImageData(x, y, width, height);
var pix = imgd.data;
// Loop over each pixel and invert the color.
for (var i = 0, n = pix.length; i < n; i += 4) {
pix[i ] = 255 - pix[i ]; // red
pix[i+1] = 255 - pix[i+1]; // green
pix[i+2] = 255 - pix[i+2]; // blue
// i+3 is alpha (the fourth element)
}
// Draw the ImageData at the given (x,y) coordinates.
context.putImageData(imgd, x, y);