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fix(blit): preserve source colors over transparency #1409

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fix(blit): preserve source colors over transparency #1409

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17 changes: 17 additions & 0 deletions plugins/plugin-blit/src/index.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -69,4 +69,21 @@ describe("Blit over image", function () {
targetImg.clone().blit({ src: srcImg, x: 3, y: 3 }),
).toMatchSnapshot();
});

test("blit preserves source colors over transparent pixels", () => {
const target = Jimp.fromBitmap({
width: 1,
height: 1,
data: new Uint8Array([0, 0, 0, 0]),
});
const source = Jimp.fromBitmap({
width: 1,
height: 1,
data: new Uint8Array([255, 255, 255, 128]),
});

target.blit(source);

expect(Array.from(target.bitmap.data)).toEqual([255, 255, 255, 128]);
});
});
31 changes: 24 additions & 7 deletions plugins/plugin-blit/src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -99,13 +99,30 @@ export const methods = {
a: image.bitmap.data[dstIdx + 3] || 0,
};

image.bitmap.data[dstIdx] =
((srcColor.a * (srcColor.r - dst.r) - dst.r + 255) >> 8) + dst.r;
image.bitmap.data[dstIdx + 1] =
((srcColor.a * (srcColor.g - dst.g) - dst.g + 255) >> 8) + dst.g;
image.bitmap.data[dstIdx + 2] =
((srcColor.a * (srcColor.b - dst.b) - dst.b + 255) >> 8) + dst.b;
image.bitmap.data[dstIdx + 3] = limit255(dst.a + srcColor.a);
const srcAlpha = srcColor.a / 255;
const dstAlpha = dst.a / 255;
const outAlpha = srcAlpha + dstAlpha * (1 - srcAlpha);

if (outAlpha === 0) {
image.bitmap.data[dstIdx] = 0;
image.bitmap.data[dstIdx + 1] = 0;
image.bitmap.data[dstIdx + 2] = 0;
image.bitmap.data[dstIdx + 3] = 0;
} else {
image.bitmap.data[dstIdx] = limit255(
(srcColor.r * srcAlpha + dst.r * dstAlpha * (1 - srcAlpha)) /
outAlpha,
);
image.bitmap.data[dstIdx + 1] = limit255(
(srcColor.g * srcAlpha + dst.g * dstAlpha * (1 - srcAlpha)) /
outAlpha,
);
image.bitmap.data[dstIdx + 2] = limit255(
(srcColor.b * srcAlpha + dst.b * dstAlpha * (1 - srcAlpha)) /
outAlpha,
);
image.bitmap.data[dstIdx + 3] = limit255(outAlpha * 255);
}
}
});

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