Question 171274 in Javascript Question posted in
A very good W3school tutorial can be found here.

Can HTML Canvas generate a single channel image in Javascript?

I want to reduce the dimensions of the image obtained from a canvas

const imgData = this.ctx.getImageData(
  0,
  0,
  280,
  280
);
const img_arr = new ImageData(imgData.data, imgData.width, imgData.height);
// ImageData { width: 280, height: 280, data: Uint8ClampedArray(313600) }
console.log(img_arr);

length 313600 = 280 x 280 x 4 (RGBA)

Is there a way to obtain single chanel image directly rather than looping the rgba image array??

var cnt = 0;
for (var i = 0; i < imgData.data.length; i += 4) {
  grayImgData.data[cnt] = imgData.data[i];
cnt+=1;
}
Tags:

2

Answers


  1. 0

    Yes, you can obtain a single channel image directly from the RGB image array without explicitly looping over the pixels. The ImageData object in JavaScript stores pixel data in a one-dimensional array where each pixel consists of consecutive red, green, blue, and alpha values.

    To extract a single channel, such as the red channel, you can access every fourth element in the data array. Here’s an example:

    const imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
const redChannel = new Uint8ClampedArray(imgData.width *         
imgData.height);

for (let i = 0; i < imgData.data.length; i += 4) {
const redValue = imgData.data[i];
redChannel[i / 4] = redValue;
}

console.log(redChannel);

    In this example, we create a new Uint8ClampedArray called redChannel to store the red channel values. We iterate over the data array with a step of 4 (since every fourth element represents the red value) and assign the red value to the corresponding index in the redChannel array.

    After the loop, the redChannel array will contain only the red channel values from the original image. You can then use this array for further processing or display.

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  2. 0

    You are correct, the ImageData() constructor in JavaScript requires an array with four channels (RGBA) for each pixel. If you attempt to create an ImageData object with a single-channel array, you will encounter an error.

    To create an ImageData object with a single-channel array, you can create a new array with four channels and set the desired channel value for each pixel. Here’s an example of creating an ImageData object with a single-channel red array:

    const redChannel = [/* your red channel values */];
const imageData = new ImageData(redChannel.length, 1);

for (let i = 0; i < redChannel.length; i++) {
imageData.data[i * 4] = redChannel[i]; // Set red channel value
imageData.data[i * 4 + 1] = 0; // Set green channel to 0
imageData.data[i * 4 + 2] = 0; // Set blue channel to 0
imageData.data[i * 4 + 3] = 255; // Set alpha channel to 255 (fully 
opaque)
}

console.log(imageData);

    In this example, we create a new ImageData object with a width equal to the length of the redChannel array and a height of 1. Then, we iterate over the redChannel array and assign the red channel value to the appropriate index in the imageData.data array. We set the green and blue channels to 0 and the alpha channel to 255 for each pixel.

    After this process, you can use the resulting ImageData object for further processing or display.

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