Allow ensureAlpha to set alpha transparency level #2634

docs/api-channel.md

@@ -18,18 +18,33 @@ Returns **Sharp**
 
 ## ensureAlpha
 
-Ensure alpha channel, if missing. The added alpha channel will be fully opaque. This is a no-op if the image already has an alpha channel.
+Ensure the output image has an alpha transparency channel.
+If missing, the added alpha channel will have the specified
+transparency level, defaulting to fully-opaque (1).
+This is a no-op if the image already has an alpha channel.
+
+### Parameters
+
+-   `alpha` **[number][1]** alpha transparency level (0=fully-transparent, 1=fully-opaque) (optional, default `1`)
 
 ### Examples
 
 ```javascript
-sharp('rgb.jpg')
+// rgba.png will be a 4 channel image with a fully-opaque alpha channel
+await sharp('rgb.jpg')
   .ensureAlpha()
-  .toFile('rgba.png', function(err, info) {
+  .toFile('rgba.png')
-    // rgba.png is a 4 channel image with a fully opaque alpha channel
-  });
 ```
 
+```javascript
+// rgba is a 4 channel image with a fully-transparent alpha channel
+const rgba = await sharp(rgb)
+  .ensureAlpha(0)
+  .toBuffer();
+```
+
+-   Throws **[Error][2]** Invalid alpha transparency level
+
 Returns **Sharp** 
 
 **Meta**
@@ -42,7 +57,7 @@ Extract a single channel from a multi-channel image.
 
 ### Parameters
 
--   `channel` **([number][1] \| [string][2])** zero-indexed channel/band number to extract, or `red`, `green`, `blue` or `alpha`.
+-   `channel` **([number][1] \| [string][3])** zero-indexed channel/band number to extract, or `red`, `green`, `blue` or `alpha`.
 
 ### Examples
 
@@ -56,7 +71,7 @@ sharp(input)
    });
 ```
 
--   Throws **[Error][3]** Invalid channel
+-   Throws **[Error][2]** Invalid channel
 
 Returns **Sharp** 
 
@@ -75,11 +90,11 @@ For raw pixel input, the `options` object should contain a `raw` attribute, whic
 
 ### Parameters
 
--   `images` **([Array][4]<([string][2] \| [Buffer][5])> | [string][2] \| [Buffer][5])** one or more images (file paths, Buffers).
+-   `images` **([Array][4]<([string][3] \| [Buffer][5])> | [string][3] \| [Buffer][5])** one or more images (file paths, Buffers).
 -   `options` **[Object][6]** image options, see `sharp()` constructor.
 
 
--   Throws **[Error][3]** Invalid parameters
+-   Throws **[Error][2]** Invalid parameters
 
 Returns **Sharp** 
 
@@ -89,7 +104,7 @@ Perform a bitwise boolean operation on all input image channels (bands) to produ
 
 ### Parameters
 
--   `boolOp` **[string][2]** one of `and`, `or` or `eor` to perform that bitwise operation, like the C logic operators `&`, `|` and `^` respectively.
+-   `boolOp` **[string][3]** one of `and`, `or` or `eor` to perform that bitwise operation, like the C logic operators `&`, `|` and `^` respectively.
 
 ### Examples
 
@@ -103,15 +118,15 @@ sharp('3-channel-rgb-input.png')
   });
 ```
 
--   Throws **[Error][3]** Invalid parameters
+-   Throws **[Error][2]** Invalid parameters
 
 Returns **Sharp** 
 
 [1]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number
 
-[2]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/String
+[2]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Error
 
-[3]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Error
+[3]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/String
 
 [4]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array
 

docs/changelog.md

@@ -8,6 +8,9 @@ Requires libvips v8.10.6
 
 * Ensure all installation errors are logged with a more obvious prefix.
 
+* Allow `ensureAlpha` to set the alpha transparency level.
+ [#2634](https://github.com/lovell/sharp/issues/2634)
+
 ### v0.28.0 - 29th March 2021
 
 * Prebuilt binaries now include mozjpeg and libimagequant (BSD 2-Clause).

lib/channel.js

@@ -30,21 +30,39 @@ function removeAlpha () {
 }
 
 /**
- * Ensure alpha channel, if missing. The added alpha channel will be fully opaque. This is a no-op if the image already has an alpha channel.
+ * Ensure the output image has an alpha transparency channel.
+ * If missing, the added alpha channel will have the specified
+ * transparency level, defaulting to fully-opaque (1).
+ * This is a no-op if the image already has an alpha channel.
  *
  * @since 0.21.2
  *
  * @example
- * sharp('rgb.jpg')
+ * // rgba.png will be a 4 channel image with a fully-opaque alpha channel
+ * await sharp('rgb.jpg')
  *   .ensureAlpha()
- *   .toFile('rgba.png', function(err, info) {
+ *   .toFile('rgba.png')
- *     // rgba.png is a 4 channel image with a fully opaque alpha channel
- *   });
  *
+ * @example
+ * // rgba is a 4 channel image with a fully-transparent alpha channel
+ * const rgba = await sharp(rgb)
+ *   .ensureAlpha(0)
+ *   .toBuffer();
+ *
+ * @param {number} [alpha=1] - alpha transparency level (0=fully-transparent, 1=fully-opaque)
  * @returns {Sharp}
+ * @throws {Error} Invalid alpha transparency level
  */
-function ensureAlpha () {
+function ensureAlpha (alpha) {
-  this.options.ensureAlpha = true;
+  if (is.defined(alpha)) {
+    if (is.number(alpha) && is.inRange(alpha, 0, 1)) {
+      this.options.ensureAlpha = alpha;
+    } else {
+      throw is.invalidParameterError('alpha', 'number between 0 and 1', alpha);
+    }
+  } else {
+    this.options.ensureAlpha = 1;
+  }
   return this;
 }
 

lib/constructor.js

@@ -221,7 +221,7 @@ const Sharp = function (input, options) {
     joinChannelIn: [],
     extractChannel: -1,
     removeAlpha: false,
-    ensureAlpha: false,
+    ensureAlpha: -1,
     colourspace: 'srgb',
     composite: [],
     // output

src/common.cc

@@ -799,10 +799,10 @@ namespace sharp {
   /*
     Ensures alpha channel, if missing.
   */
-  VImage EnsureAlpha(VImage image) {
+  VImage EnsureAlpha(VImage image, double const value) {
     if (!HasAlpha(image)) {
       std::vector<double> alpha;
-      alpha.push_back(sharp::MaximumImageAlpha(image.interpretation()));
+      alpha.push_back(value * sharp::MaximumImageAlpha(image.interpretation()));
       image = image.bandjoin_const(alpha);
     }
     return image;

src/common.h

@@ -296,7 +296,7 @@ namespace sharp {
   /*
     Ensures alpha channel, if missing.
   */
-  VImage EnsureAlpha(VImage image);
+  VImage EnsureAlpha(VImage image, double const value);
 
 }  // namespace sharp
 

src/pipeline.cc

@@ -346,7 +346,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       bool const shouldModulate = baton->brightness != 1.0 || baton->saturation != 1.0 || baton->hue != 0.0;
 
       if (shouldComposite && !sharp::HasAlpha(image)) {
-        image = sharp::EnsureAlpha(image);
+        image = sharp::EnsureAlpha(image, 1);
       }
 
       bool const shouldPremultiplyAlpha = sharp::HasAlpha(image) &&
@@ -594,7 +594,7 @@ class PipelineWorker : public Napi::AsyncWorker {
           // Ensure image to composite is sRGB with premultiplied alpha
           compositeImage = compositeImage.colourspace(VIPS_INTERPRETATION_sRGB);
           if (!sharp::HasAlpha(compositeImage)) {
-            compositeImage = sharp::EnsureAlpha(compositeImage);
+            compositeImage = sharp::EnsureAlpha(compositeImage, 1);
           }
           if (!composite->premultiplied) compositeImage = compositeImage.premultiply();
           // Calculate position
@@ -691,8 +691,8 @@ class PipelineWorker : public Napi::AsyncWorker {
       }
 
       // Ensure alpha channel, if missing
-      if (baton->ensureAlpha) {
+      if (baton->ensureAlpha != -1) {
-        image = sharp::EnsureAlpha(image);
+        image = sharp::EnsureAlpha(image, baton->ensureAlpha);
       }
 
       // Convert image to sRGB, if not already
@@ -1341,7 +1341,7 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   baton->affineInterpolator = vips::VInterpolate::new_from_name(sharp::AttrAsStr(options, "affineInterpolator").data());
 
   baton->removeAlpha = sharp::AttrAsBool(options, "removeAlpha");
-  baton->ensureAlpha = sharp::AttrAsBool(options, "ensureAlpha");
+  baton->ensureAlpha = sharp::AttrAsDouble(options, "ensureAlpha");
   if (options.Has("boolean")) {
     baton->boolean = sharp::CreateInputDescriptor(options.Get("boolean").As<Napi::Object>());
     baton->booleanOp = sharp::GetBooleanOperation(sharp::AttrAsStr(options, "booleanOp"));

src/pipeline.h

@@ -178,7 +178,7 @@ struct PipelineBaton {
   VipsOperationBoolean bandBoolOp;
   int extractChannel;
   bool removeAlpha;
-  bool ensureAlpha;
+  double ensureAlpha;
   VipsInterpretation colourspace;
   int pageHeight;
   std::vector<int> delay;
@@ -297,7 +297,7 @@ struct PipelineBaton {
     bandBoolOp(VIPS_OPERATION_BOOLEAN_LAST),
     extractChannel(-1),
     removeAlpha(false),
-    ensureAlpha(false),
+    ensureAlpha(-1.0),
     colourspace(VIPS_INTERPRETATION_LAST),
     pageHeight(0),
     delay{-1},

test/unit/alpha.js

@@ -155,4 +155,27 @@ describe('Alpha transparency', function () {
         });
     }));
   });
+
+  it('Valid ensureAlpha value used for alpha channel', async () => {
+    const background = { r: 255, g: 0, b: 0 };
+    const [r, g, b, alpha] = await sharp({
+      create: {
+        width: 8,
+        height: 8,
+        channels: 3,
+        background
+      }
+    })
+      .ensureAlpha(0.5)
+      .raw()
+      .toBuffer();
+
+    assert.deepStrictEqual({ r, g, b, alpha }, { ...background, alpha: 127 });
+  });
+
+  it('Invalid ensureAlpha value throws', async () => {
+    assert.throws(() => {
+      sharp().ensureAlpha('fail');
+    });
+  });
 });