Ensure background is premultiplied when compositing #2858

docs/changelog.md

@@ -9,6 +9,9 @@ Requires libvips v8.11.3
 * Ensure correct PNG bitdepth is set based on number of colours.
   [#2855](https://github.com/lovell/sharp/issues/2855)
 
+* Ensure background is always premultiplied when compositing.
+  [#2858](https://github.com/lovell/sharp/issues/2858)
+
 ### v0.29.0 - 17th August 2021
 
 * Drop support for Node.js 10, now requires Node.js >= 12.13.0.

src/common.cc

@@ -759,23 +759,27 @@ namespace sharp {
   /*
     Convert RGBA value to another colourspace
   */
-  std::vector<double> GetRgbaAsColourspace(std::vector<double> const rgba, VipsInterpretation const interpretation) {
+  std::vector<double> GetRgbaAsColourspace(std::vector<double> const rgba,
+    VipsInterpretation const interpretation, bool premultiply) {
     int const bands = static_cast<int>(rgba.size());
-    if (bands < 3 || interpretation == VIPS_INTERPRETATION_sRGB || interpretation == VIPS_INTERPRETATION_RGB) {
+    if (bands < 3) {
       return rgba;
-    } else {
+    }
     VImage pixel = VImage::new_matrix(1, 1);
     pixel.set("bands", bands);
-      pixel = pixel.new_from_image(rgba);
-      pixel = pixel.colourspace(interpretation, VImage::option()->set("source_space", VIPS_INTERPRETATION_sRGB));
-      return pixel(0, 0);
+    pixel = pixel
+      .new_from_image(rgba)
+      .colourspace(interpretation, VImage::option()->set("source_space", VIPS_INTERPRETATION_sRGB));
+    if (premultiply) {
+      pixel = pixel.premultiply();
     }
+    return pixel(0, 0);
   }
 
   /*
     Apply the alpha channel to a given colour
   */
-  std::tuple<VImage, std::vector<double>> ApplyAlpha(VImage image, std::vector<double> colour) {
+  std::tuple<VImage, std::vector<double>> ApplyAlpha(VImage image, std::vector<double> colour, bool premultiply) {
     // Scale up 8-bit values to match 16-bit input image
     double const multiplier = sharp::Is16Bit(image.interpretation()) ? 256.0 : 1.0;
     // Create alphaColour colour
@@ -799,7 +803,7 @@ namespace sharp {
       alphaColour.push_back(colour[3] * multiplier);
     }
     // Ensure alphaColour colour uses correct colourspace
-    alphaColour = sharp::GetRgbaAsColourspace(alphaColour, image.interpretation());
+    alphaColour = sharp::GetRgbaAsColourspace(alphaColour, image.interpretation(), premultiply);
     // Add non-transparent alpha channel, if required
     if (colour[3] < 255.0 && !HasAlpha(image)) {
       image = image.bandjoin(

src/common.h

@@ -288,12 +288,13 @@ namespace sharp {
   /*
     Convert RGBA value to another colourspace
   */
-  std::vector<double> GetRgbaAsColourspace(std::vector<double> const rgba, VipsInterpretation const interpretation);
+  std::vector<double> GetRgbaAsColourspace(std::vector<double> const rgba,
+    VipsInterpretation const interpretation, bool premultiply);
 
   /*
     Apply the alpha channel to a given colour
    */
-  std::tuple<VImage, std::vector<double>> ApplyAlpha(VImage image, std::vector<double> colour);
+  std::tuple<VImage, std::vector<double>> ApplyAlpha(VImage image, std::vector<double> colour, bool premultiply);
 
   /*
     Removes alpha channel, if any.

src/pipeline.cc

@@ -90,7 +90,7 @@ class PipelineWorker : public Napi::AsyncWorker {
         }
         if (baton->rotationAngle != 0.0) {
           std::vector<double> background;
-          std::tie(image, background) = sharp::ApplyAlpha(image, baton->rotationBackground);
+          std::tie(image, background) = sharp::ApplyAlpha(image, baton->rotationBackground, FALSE);
           image = image.rotate(baton->rotationAngle, VImage::option()->set("background", background));
         }
       }
@@ -423,7 +423,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       if (image.width() != baton->width || image.height() != baton->height) {
         if (baton->canvas == Canvas::EMBED) {
           std::vector<double> background;
-          std::tie(image, background) = sharp::ApplyAlpha(image, baton->resizeBackground);
+          std::tie(image, background) = sharp::ApplyAlpha(image, baton->resizeBackground, shouldPremultiplyAlpha);
 
           // Embed
 
@@ -480,7 +480,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       // Rotate post-extract non-90 angle
       if (!baton->rotateBeforePreExtract && baton->rotationAngle != 0.0) {
         std::vector<double> background;
-        std::tie(image, background) = sharp::ApplyAlpha(image, baton->rotationBackground);
+        std::tie(image, background) = sharp::ApplyAlpha(image, baton->rotationBackground, shouldPremultiplyAlpha);
         image = image.rotate(baton->rotationAngle, VImage::option()->set("background", background));
       }
 
@@ -493,7 +493,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       // Affine transform
       if (baton->affineMatrix.size() > 0) {
         std::vector<double> background;
-        std::tie(image, background) = sharp::ApplyAlpha(image, baton->affineBackground);
+        std::tie(image, background) = sharp::ApplyAlpha(image, baton->affineBackground, shouldPremultiplyAlpha);
         image = image.affine(baton->affineMatrix, VImage::option()->set("background", background)
           ->set("idx", baton->affineIdx)
           ->set("idy", baton->affineIdy)
@@ -505,7 +505,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       // Extend edges
       if (baton->extendTop > 0 || baton->extendBottom > 0 || baton->extendLeft > 0 || baton->extendRight > 0) {
         std::vector<double> background;
-        std::tie(image, background) = sharp::ApplyAlpha(image, baton->extendBackground);
+        std::tie(image, background) = sharp::ApplyAlpha(image, baton->extendBackground, shouldPremultiplyAlpha);
 
         // Embed
         baton->width = image.width() + baton->extendLeft + baton->extendRight;

test/unit/extend.js

@@ -124,4 +124,30 @@ describe('Extend', function () {
         fixtures.assertSimilar(fixtures.expected('extend-2channel.png'), data, done);
       });
   });
+
+  it('Premultiply background when compositing', async () => {
+    const background = '#bf1942cc';
+    const data = await sharp({
+      create: {
+        width: 1, height: 1, channels: 4, background: '#fff0'
+      }
+    })
+      .composite([{
+        input: {
+          create: {
+            width: 1, height: 1, channels: 4, background
+          }
+        }
+      }])
+      .extend({
+        left: 1, background
+      })
+      .raw()
+      .toBuffer();
+    const [r1, g1, b1, a1, r2, g2, b2, a2] = data;
+    assert.strictEqual(true, Math.abs(r2 - r1) < 2);
+    assert.strictEqual(true, Math.abs(g2 - g1) < 2);
+    assert.strictEqual(true, Math.abs(b2 - b1) < 2);
+    assert.strictEqual(true, Math.abs(a2 - a1) < 2);
+  });
 });