image-js/lib/compute/histogram.js

Image processing and manipulation in JavaScript

import { validateChannel } from '../utils/validators/validators.js';
/**
 * Returns a histogram of pixel intensities.
 * @param image - The original image.
 * @param options - Histogram options.
 * @returns - The histogram.
 */
export function histogram(image, options = {}) {
    let { channel } = options;
    const { slots = 2 ** image.bitDepth } = options;
    if (!(slots !== 0 && (slots & (slots - 1)) === 0)) {
        throw new RangeError('slots must be a power of 2, for example: 64, 256, 1024');
    }
    if (typeof channel !== 'number') {
        if (image.channels !== 1) {
            throw new TypeError('channel option is mandatory for multi-channel images');
        }
        channel = 0;
    }
    validateChannel(channel, image);
    const hist = new Uint32Array(slots);
    let bitShift = 0;
    const bitSlots = Math.log2(slots);
    bitShift = image.bitDepth - bitSlots;
    for (let i = 0; i < image.size; i++) {
        hist[image.getValueByIndex(i, channel) >> bitShift]++;
    }
    return hist;
}
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