Add Extract Quantisation Tables operation

src/core/config/Categories.json

@@ -379,6 +379,7 @@
             "Remove EXIF",
             "Extract EXIF",
             "Extract RGBA",
+            "Extract Quantisation Tables",
             "View Bit Plane",
             "Randomize Colour Palette",
             "Extract LSB"

src/core/lib/QuantisationTable.mjs

@@ -0,0 +1,68 @@
+/**
+ * Parses a JPEG quantisation table.
+ *
+ * @param {Uint8Array} rawTable
+ * @param {boolean} doublePrec - true if 16-bit precision, false if 8-bit
+ * @returns {number[][]}
+ *
+ */
+export function parseQTable(rawTable, doublePrec = false) {
+    // The raw table is flattened in zig-zag order, similar to enumerating a countable set.
+    // This lookup table tells the index in the natural order for each element in the zig-zag order
+    const unZigZagIndex = [
+        0,  1,  8, 16,  9,  2,  3, 10,
+        17, 24, 32, 25, 18, 11,  4,  5,
+        12, 19, 26, 33, 40, 48, 41, 34,
+        27, 20, 13,  6,  7, 14, 21, 28,
+        35, 42, 49, 56, 57, 50, 43, 36,
+        29, 22, 15, 23, 30, 37, 44, 51,
+        58, 59, 52, 45, 38, 31, 39, 46,
+        53, 60, 61, 54, 47, 55, 62, 63];
+    const ret = [
+        [1, 1, 1, 1, 1, 1, 1, 1],
+        [1, 1, 1, 1, 1, 1, 1, 1],
+        [1, 1, 1, 1, 1, 1, 1, 1],
+        [1, 1, 1, 1, 1, 1, 1, 1],
+        [1, 1, 1, 1, 1, 1, 1, 1],
+        [1, 1, 1, 1, 1, 1, 1, 1],
+        [1, 1, 1, 1, 1, 1, 1, 1],
+        [1, 1, 1, 1, 1, 1, 1, 1]];
+    let i = 0;
+    let elemCount = 0;
+    while (i < rawTable.length) {
+        const row = ~~(unZigZagIndex[elemCount] / 8); // floor
+        const col = unZigZagIndex[elemCount] % 8;
+        if (doublePrec) {
+            ret[row][col] = rawTable[i++] << 8 + rawTable[i++];
+        } else {
+            ret[row][col] = rawTable[i++];
+        }
+        elemCount++;
+    }
+    return ret;
+}
+
+/**
+ * Finds the index of first appearance of a JPEG marker
+ *
+ * @param {Uint8Array} image
+ * @param {number} markerByte - doesn't include 0xFF
+ * @param {number} fromIndex - the index to start the search at
+ * @returns {number}
+ *
+ */
+export function indexOfMarker(image, markerByte, fromIndex = 0) {
+    let ptr = fromIndex;
+    let markerIndex = 0;
+    do {
+        // look for the byte that changes across markers first, as these are less common than 0xFF
+        markerIndex = image.indexOf(markerByte, ptr);
+        if (markerIndex === 0) return -1; // marker byte must follow 0xFF
+        if (image[markerIndex-1] === 0xFF) {
+            return markerIndex - 1;
+        } else {
+            ptr = markerIndex + 1;
+        }
+    } while (markerIndex !== -1);
+    return -1;
+}

src/core/operations/ExtractQuantisationTables.mjs

@@ -0,0 +1,98 @@
+/**
+ * @author cbeuw [cbeuw.andy@gmail.com]
+ * @copyright Crown Copyright 2020
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import OperationError from "../errors/OperationError.mjs";
+import {parseQTable, indexOfMarker} from "../lib/QuantisationTable.mjs";
+
+/**
+ * Extract Quantisation Tables operation
+ */
+class ExtractQuantisationTables extends Operation {
+
+    /**
+     * ExtractQuantisationTables constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "Extract Quantisation Tables";
+        this.module = "Default";
+        this.description = "Extracts quantisation tables embedded in a JPEG image.";
+        this.infoURL = "https://en.wikipedia.org/wiki/Quantization_(image_processing)";
+        this.inputType = "ArrayBuffer";
+        this.outputType = "string";
+        this.args = [];
+    }
+
+    /**
+     * @param {ArrayBuffer} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const image = new Uint8Array(input);
+        // maximally 4 tables are allowed in a JPEG file
+
+        let ptr = indexOfMarker(image, 0xD8);
+        if (ptr === -1) throw new OperationError("Malformed image file: Start of Image not found");
+
+        let ret = "";
+
+        do {
+            const dqtIndex = indexOfMarker(image, 0xDB, ptr);
+            if (dqtIndex === -1) {
+                break;
+            }
+            // -2 here because the length bytes include the two bytes used to record length
+            // -1 to exclude the byte used to record the table's precision and id
+            const tableLength = (image[dqtIndex+2] << 8) + image[dqtIndex+3] - 2 - 1;
+            // if the table is 16-bit precision
+            const doublePrecision = (image[dqtIndex+4] >> 4) === 1;
+            if (tableLength !== (doublePrecision? 128:64)) throw new OperationError(`Invalid table length ${tableLength} at table definition beginning at ${dqtIndex}`);
+            const tableId = image[dqtIndex+4] & 0x0F;
+            if (tableId >= 4) throw new OperationError(`Invalid table identifier ${tableId} at table definition beginning at ${dqtIndex}`);
+            const table = parseQTable(image.slice(dqtIndex + 5, dqtIndex + 5 + tableLength), doublePrecision);
+
+            ret += `Quantisation table at position ${dqtIndex}. ID ${tableId}, ${(doublePrecision? "16":"8")}-bit precision:\n`;
+            ret += this.prettifyMatrix(table);
+            ret += "\n";
+
+            ptr = dqtIndex + 5 + tableLength + 1;
+        } while (ptr < image.length);
+
+        return ret;
+    }
+
+    /**
+     * @param {number[][]} mat
+     * @returns {string}
+     */
+    prettifyMatrix(mat) {
+        const nRows = mat.length;
+        const nCols = mat[0].length;
+        let maxLen = 0;
+        for (let i = 0; i < nRows; i++) {
+            for (let j = 0; j < nCols; j++) {
+                const elemLen = mat[i][j].toString().length;
+                if (elemLen > maxLen) {
+                    maxLen = elemLen;
+                }
+            }
+        }
+
+        let ret = "";
+        for (let i = 0; i < nRows; i++) {
+            for (let j = 0; j < nCols; j++) {
+                ret += mat[i][j].toString().padStart(maxLen) + " ";
+            }
+            ret += "\n";
+        }
+        return ret;
+    }
+}
+
+export default ExtractQuantisationTables;