PngParser: Handle gAMA chunk

Author: codedread

image/parsers/png.js

@@ -14,12 +14,14 @@ import { ByteStream } from '../../io/bytestream.js';
 // https://en.wikipedia.org/wiki/PNG#File_format
 // https://www.w3.org/TR/2003/REC-PNG-20031110
 
+// let DEBUG = true;
 let DEBUG = false;
 const SIG = new Uint8Array([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A]);
 
 /** @enum {string} */
 export const PngParseEventType = {
   IHDR: 'image_header',
+  gAMA: 'image_gamma',
   PLTE: 'palette',
   IDAT: 'image_data',
 };
@@ -59,6 +61,15 @@ export class PngImageHeaderEvent extends Event {
   }
 }
 
+export class PngImageGammaEvent extends Event {
+  /** @param {number} */
+  constructor(gamma) {
+    super(PngParseEventType.gAMA);
+    /** @type {number} */
+    this.gamma = gamma;
+  }
+}
+
 /**
  * @typedef PngColor
  * @property {number} red
@@ -132,6 +143,16 @@ export class PngParser extends EventTarget {
     return this;
   }
 
+  /**
+   * Type-safe way to bind a listener for a PngImageGammaEvent.
+   * @param {function(PngImageGammaEvent): void} listener
+   * @returns {PngParser} for chaining
+   */
+  onGamma(listener) {
+    super.addEventListener(PngParseEventType.gAMA, listener);
+    return this;
+  }
+
   /**
    * Type-safe way to bind a listener for a PngPaletteEvent.
    * @param {function(PngPaletteEvent): void} listener
@@ -204,6 +225,12 @@ export class PngParser extends EventTarget {
           this.dispatchEvent(new PngImageHeaderEvent(header));
           break;
 
+        // https://www.w3.org/TR/2003/REC-PNG-20031110/#11gAMA
+        case 'gAMA':
+          if (length !== 4) throw `Bad length for gAMA: ${length}`;
+          this.dispatchEvent(new PngImageGammaEvent(chStream.readNumber(4)));
+          break;
+
         // https://www.w3.org/TR/2003/REC-PNG-20031110/#11PLTE
         case 'PLTE':
           if (this.colorType === undefined) throw `PLTE before IHDR`;
@@ -282,8 +309,11 @@ async function main() {
     parser.onImageHeader(evt => {
       // console.dir(evt.imageHeader);
     });
+    parser.onGamma(evt => {
+      // console.dir(evt.imageGamma);
+    });
     parser.onPalette(evt => {
-      console.dir(evt.palette);
+      // console.dir(evt.palette);
     });
     parser.onImageData(evt => {
       // console.dir(evt);

tests/image-parsers-png.spec.js

@@ -5,6 +5,7 @@ import { PngColorType, PngInterlaceMethod, PngParser } from '../image/parsers/pn
 
 /** @typedef {import('../image/parsers/png.js').PngImageHeader} PngImageHeader */
 /** @typedef {import('../image/parsers/png.js').PngImageData} PngImageData */
+/** @typedef {import('../image/parsers/png.js').PngImageGamma} PngImageGamma */
 /** @typedef {import('../image/parsers/png.js').PngPalette} PngPalette */
 
 function getPngParser(fileName) {
@@ -47,6 +48,15 @@ describe('bitjs.image.parsers.PngParser', () => {
     });
   });
 
+  it('extracts gAMA', async () => {
+    /** @type {number} */
+    let gamma;
+    await getPngParser('tests/image-testfiles/g05n3p04.png')
+        .onGamma(evt => gamma = evt.gamma)
+        .start();
+    expect(gamma).equals(55000);
+  });
+
   it('extracts PLTE', async () => {
     /** @type {PngPalette} */
     let palette;