Ensure background metadata can be parsed #4090

test/fixtures/index.js

@@ -80,6 +80,8 @@ module.exports = {
   inputPngWithGreyAlpha: getPath('grey-8bit-alpha.png'),
   inputPngWithOneColor: getPath('2x2_fdcce6.png'),
   inputPngWithTransparency16bit: getPath('tbgn2c16.png'), // http://www.schaik.com/pngsuite/tbgn2c16.png
+  inputPng8BitGreyBackground: getPath('bgbn4a08.png'), // http://www.schaik.com/pngsuite/bgbn4a08.png
+  inputPng16BitGreyBackground: getPath('bggn4a16.png'), // http://www.schaik.com/pngsuite/bggn4a16.png
   inputPng16BitGreyAlpha: getPath('16-bit-grey-alpha.png'), // CC-BY-NC-SA florc http://www.colourlovers.com/pattern/50713/pat
   inputPngOverlayLayer0: getPath('alpha-layer-0-background.png'),
   inputPngOverlayLayer1: getPath('alpha-layer-1-fill.png'),

test/unit/metadata.js

@@ -201,6 +201,48 @@ describe('Image metadata', function () {
     });
   });
 
+  it('PNG with greyscale bKGD chunk - 8 bit', async () => {
+    const data = await sharp(fixtures.inputPng8BitGreyBackground).metadata();
+    assert.deepStrictEqual(data, {
+      background: {
+        gray: 0
+      },
+      bitsPerSample: 8,
+      channels: 2,
+      density: 72,
+      depth: 'uchar',
+      format: 'png',
+      hasAlpha: true,
+      hasProfile: false,
+      height: 32,
+      isPalette: false,
+      isProgressive: false,
+      space: 'b-w',
+      width: 32
+    });
+  });
+
+  it('PNG with greyscale bKGD chunk - 16 bit', async () => {
+    const data = await sharp(fixtures.inputPng16BitGreyBackground).metadata();
+    assert.deepStrictEqual(data, {
+      background: {
+        gray: 67
+      },
+      bitsPerSample: 16,
+      channels: 2,
+      density: 72,
+      depth: 'ushort',
+      format: 'png',
+      hasAlpha: true,
+      hasProfile: false,
+      height: 32,
+      isPalette: false,
+      isProgressive: false,
+      space: 'grey16',
+      width: 32
+    });
+  });
+
   it('WebP', function (done) {
     sharp(fixtures.inputWebP).metadata(function (err, metadata) {
       if (err) throw err;