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Fix rounding when un-premultiplying alpha in getImageData #2622

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Fix rounding when un-premultiplying alpha in getImageData #2622

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1 change: 1 addition & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
### Added
### Fixed
* Load images from Node.js object URLs (#2525)
* Fix off-by-one RGB values from `getImageData` on semi-transparent pixels (#1933)

3.2.3
==================
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7 changes: 3 additions & 4 deletions src/CanvasRenderingContext2d.cc
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Expand Up @@ -1079,10 +1079,9 @@ Context2d::GetImageData(const Napi::CallbackInfo& info) {
dst[bx + 2] = b;
} else {
// Undo alpha pre-multiplication
float alphaR = (float)255 / a;
dst[bx + 0] = (int)((float)r * alphaR);
dst[bx + 1] = (int)((float)g * alphaR);
dst[bx + 2] = (int)((float)b * alphaR);
dst[bx + 0] = (r * 255 + a / 2) / a;
dst[bx + 1] = (g * 255 + a / 2) / a;
dst[bx + 2] = (b * 255 + a / 2) / a;
}

}
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9 changes: 9 additions & 0 deletions test/canvas.test.js
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Expand Up @@ -2224,6 +2224,15 @@ describe('Canvas', function () {
assert.equal(result.data[255], 255) // not black or junk data
assert.equal(result.data[255 * 256], 255) // not black or junk data
})

it('round-trips semi-transparent pixels through getImageData (#1933)', function () {
const canvas = createCanvas(1, 1)
const ctx = canvas.getContext('2d')
ctx.putImageData(createImageData(new Uint8ClampedArray([252, 252, 252, 252]), 1), 0, 0)
assert.deepEqual(Array.from(ctx.getImageData(0, 0, 1, 1).data), [252, 252, 252, 252])
ctx.putImageData(createImageData(new Uint8ClampedArray([255, 128, 1, 2]), 1), 0, 0)
assert.deepEqual(Array.from(ctx.getImageData(0, 0, 1, 1).data), [255, 128, 0, 2])
})
})

it('Canvas#createPNGStream()', function (done) {
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