Looping through pixels in an image

Question

My project is to input an image into a canvas tag in an HTML page, and then loop through the pixels and RGBA values of the pixels. While looping through the red values,so ev

Answer 1

It's straightforward.

All the pixel data for a canvas are stored sequentially in an array.

The first pixel's data occupy array elements #0-3 (red=0/green=1/blue=2/alpha=3).

The second pixel's data occupy array elements #4-7 (red=4/green=5/blue=6/alpha=7).

And so on...

You can load that pixel data by using context.getImageData() and enumerating through the array:

const imgData = context.getImageData(0, 0, canvas.width, canvas.height);
const data = imgData.data;

// enumerate all pixels
// each pixel's r,g,b,a datum are stored in separate sequential array elements

for(const i = 0; i < data.length; i += 4) {
  const red = data[i];
  const green = data[i + 1];
  const blue = data[i + 2];
  const alpha = data[i + 3];
}

You can also change those array values and then save the array back to the image using context.putImageData().

// save any altered pixel data back to the context
// the image will reflect any changes you made

context.putImageData(imgData, 0, 0);

The image will then change according to the changes you made to its pixel array.

Each pixel contains 4 components red, green, blue, alpha - each of them is number 0-255. The loop starts from top-left to bottom-right.