geotiff/dist-module/compression/webimage.js

GeoTIFF image decoding in JavaScript

import BaseDecoder from './basedecoder.js';
/**
 * class WebImageDecoder
 *
 * This decoder uses the browsers image decoding facilities to read image
 * formats like WebP when supported.
 */
export default class WebImageDecoder extends BaseDecoder {
    /**
     * @param {import('./basedecoder.js').BaseDecoderParameters} parameters
     */
    constructor(parameters) {
        super(parameters);
        if (typeof createImageBitmap === 'undefined') {
            throw new Error('Cannot decode WebImage as `createImageBitmap` is not available');
        }
        else if (typeof document === 'undefined' && typeof OffscreenCanvas === 'undefined') {
            throw new Error('Cannot decode WebImage as neither `document` nor `OffscreenCanvas` is not available');
        }
    }
    /** @param {ArrayBuffer} buffer */
    async decodeBlock(buffer) {
        const blob = new Blob([buffer]);
        const imageBitmap = await createImageBitmap(blob);
        let canvas;
        if (typeof document !== 'undefined') {
            canvas = document.createElement('canvas');
            canvas.width = imageBitmap.width;
            canvas.height = imageBitmap.height;
        }
        else {
            canvas = new OffscreenCanvas(imageBitmap.width, imageBitmap.height);
        }
        // Draw the image onto the canvas to extract the pixel data.
        // Note: createImageBitmap always returns RGBA data.
        const ctx = /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
        ctx.drawImage(imageBitmap, 0, 0);
        const imageData = ctx.getImageData(0, 0, imageBitmap.width, imageBitmap.height).data;
        // Return the correct channels to the caller
        const samplesPerPixel = this.parameters.samplesPerPixel || 4;
        if (samplesPerPixel === 4) {
            // RGBA, return as is
            return imageData.buffer;
        }
        else if (samplesPerPixel === 3) {
            // RGB, remove alpha channel before returning
            const rgb = new Uint8ClampedArray(imageBitmap.width * imageBitmap.height * 3);
            for (let i = 0, j = 0; i < rgb.length; i += 3, j += 4) {
                rgb[i] = imageData[j];
                rgb[i + 1] = imageData[j + 1];
                rgb[i + 2] = imageData[j + 2];
            }
            return rgb.buffer;
        }
        else {
            throw new Error(`Unsupported SamplesPerPixel value: ${samplesPerPixel}`);
        }
    }
}
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