How can I grayscale a canvas image in JavaScript?

Question

I have an image inside of an HTML5 canvas with the size of 28x28 pixels. I get the imageData of the canvas as an array of RGBA (red, green, blue, alpha) values using this code:

canvas = document.getElementById('canvas');
ctx = canvas.getContext("2d");
imgData = ctx.getImageData(0, 0, 28, 28);

Now I want to grayscale the image, so that I get an array of 784 values (28x28 pixels) where each pixel has one value (instead of four).

I've found a lot of different formulas for grayscaling, some are multiplying the rgb values, some are just calculating the average - I really don't know which of them to use...

I'm also stuck at getting 784 values - it's always 3136 (because of the 4 channels)...

Thanks in advance!

Answer 1

The main idea is to have the same value for the red green and blue component of the color. For this you need to calculate the lightness of every pixel. There are several ways to calculate the lightness. This is one of them.

window.onload = function() {
  let canvas = document.getElementById("c");

  let ctx = canvas.getContext("2d");
  canvas.width = 50;
  canvas.height = 50;

  let srcImg = document.getElementById("sof");
  ctx.drawImage(srcImg, 0, 0, ctx.canvas.width, ctx.canvas.height);
  let imgData = ctx.getImageData(0, 0, ctx.canvas.width, ctx.canvas.height);
  let pixels = imgData.data;
  for (var i = 0; i < pixels.length; i += 4) {

    let lightness = parseInt((pixels[i] + pixels[i + 1] + pixels[i + 2]) / 3);

    pixels[i] = lightness;
    pixels[i + 1] = lightness;
    pixels[i + 2] = lightness;
  }
  ctx.putImageData(imgData, 0, 0);
}

<canvas id="c"></canvas>
<img src="data:image/jpeg;base64,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"
  id="sof" />

UPDATE:
Alternative lightness Calculations:

• Wikipedia (Luma):
  let lightness = parseInt(pixels[i]*.299 + pixels[i + 1]*.587 + pixels[i + 2]*.114);

• elsewhere (source unknown):
  let lightness = parseInt(3*pixels[i] + 4*pixels[i + 1] + pixels[i + 2] >>> 3);

• Wikipedia (Linear Luminance):
  let lightness = 0.2126 * pixels[i] + 0.715 * pixels[i+1] + 0.0722 * pixels[i+2];