Add removeAlpha op, removes alpha channel if any #1248

docs/api-channel.md

@@ -1,5 +1,21 @@
 <!-- Generated by documentation.js. Update this documentation by updating the source code. -->
 
+## removeAlpha
+
+Remove alpha channel, if any. This is a no-op if the image does not have an alpha channel.
+
+### Examples
+
+```javascript
+sharp('rgba.png')
+  .removeAlpha()
+  .toFile('rgb.png', function(err, info) {
+    // rgb.png is a 3 channel image without an alpha channel
+  });
+```
+
+Returns **Sharp** 
+
 ## extractChannel
 
 Extract a single channel from a multi-channel image.

docs/api-input.md

@@ -79,6 +79,7 @@ A Promise is returned when `callback` is not provided.
     -   `maxX` (x-coordinate of one of the pixel where the maximum lies)
     -   `maxY` (y-coordinate of one of the pixel where the maximum lies)
 -   `isOpaque`: Value to identify if the image is opaque or transparent, based on the presence and use of alpha channel
+-   `entropy`: Histogram-based estimation of greyscale entropy, discarding alpha channel if any (experimental)
 
 ### Parameters
 

docs/changelog.md

@@ -6,6 +6,9 @@ Requires libvips v8.6.1.
 
 #### v0.20.6 - TBD
 
+* Add removeAlpha operation to remove alpha channel, if any.
+  [#1248](https://github.com/lovell/sharp/issues/1248)
+
 * Expose mozjpeg quant_table flag.
   [#1285](https://github.com/lovell/sharp/pull/1285)
   [@rexxars](https://github.com/rexxars)

lib/channel.js

@@ -12,6 +12,23 @@ const bool = {
   eor: 'eor'
 };
 
+/**
+ * Remove alpha channel, if any. This is a no-op if the image does not have an alpha channel.
+ *
+ * @example
+ * sharp('rgba.png')
+ *   .removeAlpha()
+ *   .toFile('rgb.png', function(err, info) {
+ *     // rgb.png is a 3 channel image without an alpha channel
+ *   });
+ *
+ * @returns {Sharp}
+ */
+function removeAlpha () {
+  this.options.removeAlpha = true;
+  return this;
+}
+
 /**
  * Extract a single channel from a multi-channel image.
  *
@@ -102,6 +119,7 @@ function bandbool (boolOp) {
 module.exports = function (Sharp) {
   // Public instance functions
   [
+    removeAlpha,
     extractChannel,
     joinChannel,
     bandbool

lib/constructor.js

@@ -171,6 +171,7 @@ const Sharp = function (input, options) {
     booleanFileIn: '',
     joinChannelIn: [],
     extractChannel: -1,
+    removeAlpha: false,
     colourspace: 'srgb',
     // overlay
     overlayGravity: 0,

src/operations.cc

@@ -28,6 +28,16 @@ using vips::VError;
 
 namespace sharp {
 
+  /*
+    Removes alpha channel, if any.
+  */
+  VImage RemoveAlpha(VImage image) {
+    if (HasAlpha(image)) {
+      image = image.extract_band(0, VImage::option()->set("n", image.bands() - 1));
+    }
+    return image;
+  }
+
   /*
     Composite overlayImage over image at given position
     Assumes alpha channels are already premultiplied and will be unpremultiplied after
@@ -223,10 +233,8 @@ namespace sharp {
   VImage Gamma(VImage image, double const exponent) {
     if (HasAlpha(image)) {
       // Separate alpha channel
-      VImage imageWithoutAlpha = image.extract_band(0,
-        VImage::option()->set("n", image.bands() - 1));
       VImage alpha = image[image.bands() - 1];
-      return imageWithoutAlpha.gamma(VImage::option()->set("exponent", exponent)).bandjoin(alpha);
+      return RemoveAlpha(image).gamma(VImage::option()->set("exponent", exponent)).bandjoin(alpha);
     } else {
       return image.gamma(VImage::option()->set("exponent", exponent));
     }
@@ -374,10 +382,8 @@ namespace sharp {
   VImage Linear(VImage image, double const a, double const b) {
     if (HasAlpha(image)) {
       // Separate alpha channel
-      VImage imageWithoutAlpha = image.extract_band(0,
-        VImage::option()->set("n", image.bands() - 1));
       VImage alpha = image[image.bands() - 1];
-      return imageWithoutAlpha.linear(a, b).bandjoin(alpha);
+      return RemoveAlpha(image).linear(a, b).bandjoin(alpha);
     } else {
       return image.linear(a, b);
     }

src/operations.h

@@ -25,6 +25,11 @@ using vips::VImage;
 
 namespace sharp {
 
+  /*
+    Removes alpha channel, if any.
+  */
+  VImage RemoveAlpha(VImage image);
+
   /*
     Alpha composite src over dst with given gravity.
     Assumes alpha channels are already premultiplied and will be unpremultiplied after.

src/pipeline.cc

@@ -698,6 +698,12 @@ class PipelineWorker : public Nan::AsyncWorker {
           .extract_band(baton->extractChannel)
           .copy(VImage::option()->set("interpretation", VIPS_INTERPRETATION_B_W));
       }
+
+      // Remove alpha channel, if any
+      if (baton->removeAlpha) {
+        image = sharp::RemoveAlpha(image);
+      }
+
       // Convert image to sRGB, if not already
       if (sharp::Is16Bit(image.interpretation())) {
         image = image.cast(VIPS_FORMAT_USHORT);
@@ -1235,6 +1241,7 @@ NAN_METHOD(pipeline) {
   baton->extendLeft = AttrTo<int32_t>(options, "extendLeft");
   baton->extendRight = AttrTo<int32_t>(options, "extendRight");
   baton->extractChannel = AttrTo<int32_t>(options, "extractChannel");
+  baton->removeAlpha = AttrTo<bool>(options, "removeAlpha");
   if (HasAttr(options, "boolean")) {
     baton->boolean = CreateInputDescriptor(AttrAs<v8::Object>(options, "boolean"), buffersToPersist);
     baton->booleanOp = sharp::GetBooleanOperation(AttrAsStr(options, "booleanOp"));

src/pipeline.h

@@ -131,6 +131,7 @@ struct PipelineBaton {
   VipsOperationBoolean booleanOp;
   VipsOperationBoolean bandBoolOp;
   int extractChannel;
+  bool removeAlpha;
   VipsInterpretation colourspace;
   int tileSize;
   int tileOverlap;
@@ -213,6 +214,7 @@ struct PipelineBaton {
     booleanOp(VIPS_OPERATION_BOOLEAN_LAST),
     bandBoolOp(VIPS_OPERATION_BOOLEAN_LAST),
     extractChannel(-1),
+    removeAlpha(false),
     colourspace(VIPS_INTERPRETATION_LAST),
     tileSize(256),
     tileOverlap(0),

test/unit/alpha.js

@@ -81,35 +81,45 @@ describe('Alpha transparency', function () {
       });
   });
 
-  it('Enlargement with non-nearest neighbor interpolation shouldn’t cause dark edges', function (done) {
+  it('Enlargement with non-nearest neighbor interpolation shouldn’t cause dark edges', function () {
     const base = 'alpha-premultiply-enlargement-2048x1536-paper.png';
     const actual = fixtures.path('output.' + base);
     const expected = fixtures.expected(base);
-    sharp(fixtures.inputPngAlphaPremultiplicationSmall)
+    return sharp(fixtures.inputPngAlphaPremultiplicationSmall)
       .resize(2048, 1536)
-      .toFile(actual, function (err) {
-        if (err) {
-          done(err);
-        } else {
+      .toFile(actual)
+      .then(function () {
         fixtures.assertMaxColourDistance(actual, expected, 102);
-          done();
-        }
       });
   });
 
-  it('Reduction with non-nearest neighbor interpolation shouldn’t cause dark edges', function (done) {
+  it('Reduction with non-nearest neighbor interpolation shouldn’t cause dark edges', function () {
     const base = 'alpha-premultiply-reduction-1024x768-paper.png';
     const actual = fixtures.path('output.' + base);
     const expected = fixtures.expected(base);
-    sharp(fixtures.inputPngAlphaPremultiplicationLarge)
+    return sharp(fixtures.inputPngAlphaPremultiplicationLarge)
       .resize(1024, 768)
-      .toFile(actual, function (err) {
-        if (err) {
-          done(err);
-        } else {
+      .toFile(actual)
+      .then(function () {
         fixtures.assertMaxColourDistance(actual, expected, 102);
-          done();
-        }
       });
   });
+
+  it('Removes alpha from fixtures with transparency, ignores those without', function () {
+    return Promise.all([
+      fixtures.inputPngWithTransparency,
+      fixtures.inputPngWithTransparency16bit,
+      fixtures.inputWebPWithTransparency,
+      fixtures.inputJpg,
+      fixtures.inputPng,
+      fixtures.inputWebP
+    ].map(function (input) {
+      return sharp(input)
+        .removeAlpha()
+        .toBuffer({ resolveWithObject: true })
+        .then(function (result) {
+          assert.strictEqual(3, result.info.channels);
+        });
+    }));
+  });
 });