feat: exposing vips_affine operation

docs/api-operation.md

@@ -65,6 +65,61 @@ The use of `flop` implies the removal of the EXIF `Orientation` tag, if any.
 
 Returns **Sharp** 
 
+## affine
+
+Perform an affine transform on an image. This operation will always occur after resizing, extraction and rotation, if any.
+
+You must provide an array of length 4 or a 2x2 affine transformation matrix.
+By default, new pixels are filled with a black background. You can provide a background color with the `background` option.
+A particular interpolator may also be specified. Set the `interpolator` option to an attribute of the `sharp.interpolator` Object e.g. `sharp.interpolator.nohalo`.
+
+In the case of a 2x2 matrix, the transform is:
+
+-   X = `matrix[0, 0]` \* (x + `idx`) + `matrix[0, 1]` \* (y + `idy`) + `odx`
+-   Y = `matrix[1, 0]` \* (x + `idx`) + `matrix[1, 1]` \* (y + `idy`) + `ody`
+
+where:
+
+-   x and y are the coordinates in input image.
+-   X and Y are the coordinates in output image.
+-   (0,0) is the upper left corner.
+
+### Parameters
+
+-   `matrix` **([Array][7]<[Array][7]<[number][1]>> | [Array][7]<[number][1]>)** affine transformation matrix
+-   `options` **[Object][2]?** if present, is an Object with optional attributes.
+    -   `options.background` **([String][3] \| [Object][2])** parsed by the [color][4] module to extract values for red, green, blue and alpha. (optional, default `"#000000"`)
+    -   `options.idx` **[Number][1]** input horizontal offset (optional, default `0`)
+    -   `options.idy` **[Number][1]** input vertical offset (optional, default `0`)
+    -   `options.odx` **[Number][1]** output horizontal offset (optional, default `0`)
+    -   `options.ody` **[Number][1]** output vertical offset (optional, default `0`)
+    -   `options.interpolator` **[String][3]** interpolator (optional, default `sharp.interpolators.bicubic`)
+
+### Examples
+
+```javascript
+const pipeline = sharp()
+  .affine([[1, 0.3], [0.1, 0.7]], {
+     background: 'white',
+     interpolate: sharp.interpolators.nohalo
+  })
+  .toBuffer((err, outputBuffer, info) => {
+     // outputBuffer contains the transformed image
+     // info.width and info.height contain the new dimensions
+  });
+
+inputStream
+  .pipe(pipeline);
+```
+
+-   Throws **[Error][5]** Invalid parameters
+
+Returns **Sharp** 
+
+**Meta**
+
+-   **since**: 0.27.0
+
 ## sharpen
 
 Sharpen the image.

docs/api-utility.md

@@ -12,6 +12,36 @@ console.log(sharp.format);
 
 Returns **[Object][1]** 
 
+## interpolators
+
+An Object containing the available interpolators and their proper values
+
+Type: [string][2]
+
+### nearest
+
+[Nearest neighbour interpolation][3]. Suitable for image enlargement only.
+
+### bilinear
+
+[Bilinear interpolation][4]. Faster than bicubic but with less smooth results.
+
+### bicubic
+
+[Bicubic interpolation][5] (the default).
+
+### locallyBoundedBicubic
+
+[LBB interpolation][6]. Prevents some "[acutance][7]" but typically reduces performance by a factor of 2.
+
+### nohalo
+
+[Nohalo interpolation][8]. Prevents acutance but typically reduces performance by a factor of 3.
+
+### vertexSplitQuadraticBasisSpline
+
+[VSQBS interpolation][9]. Prevents "staircasing" when enlarging.
+
 ## versions
 
 An Object containing the version numbers of libvips and its dependencies.
@@ -31,10 +61,10 @@ useful for determining how much working memory is required for a particular task
 
 ### Parameters
 
--   `options` **([Object][1] \| [boolean][2])** Object with the following attributes, or boolean where true uses default cache settings and false removes all caching (optional, default `true`)
-    -   `options.memory` **[number][3]** is the maximum memory in MB to use for this cache (optional, default `50`)
-    -   `options.files` **[number][3]** is the maximum number of files to hold open (optional, default `20`)
-    -   `options.items` **[number][3]** is the maximum number of operations to cache (optional, default `100`)
+-   `options` **([Object][1] \| [boolean][10])** Object with the following attributes, or boolean where true uses default cache settings and false removes all caching (optional, default `true`)
+    -   `options.memory` **[number][11]** is the maximum memory in MB to use for this cache (optional, default `50`)
+    -   `options.files` **[number][11]** is the maximum number of files to hold open (optional, default `20`)
+    -   `options.items` **[number][11]** is the maximum number of operations to cache (optional, default `100`)
 
 ### Examples
 
@@ -64,7 +94,7 @@ This method always returns the current concurrency.
 
 ### Parameters
 
--   `concurrency` **[number][3]?** 
+-   `concurrency` **[number][11]?** 
 
 ### Examples
 
@@ -74,7 +104,7 @@ sharp.concurrency(2); // 2
 sharp.concurrency(0); // 4
 ```
 
-Returns **[number][3]** concurrency
+Returns **[number][11]** concurrency
 
 ## queue
 
@@ -116,7 +146,7 @@ by taking advantage of the SIMD vector unit of the CPU, e.g. Intel SSE and ARM N
 
 ### Parameters
 
--   `simd` **[boolean][2]**  (optional, default `true`)
+-   `simd` **[boolean][10]**  (optional, default `true`)
 
 ### Examples
 
@@ -130,10 +160,26 @@ const simd = sharp.simd(false);
 // prevent libvips from using liborc at runtime
 ```
 
-Returns **[boolean][2]** 
+Returns **[boolean][10]** 
 
 [1]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object
 
-[2]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Boolean
+[2]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/String
+
+[3]: http://en.wikipedia.org/wiki/Nearest-neighbor_interpolation
+
+[4]: http://en.wikipedia.org/wiki/Bilinear_interpolation
+
+[5]: http://en.wikipedia.org/wiki/Bicubic_interpolation
+
+[6]: https://github.com/jcupitt/libvips/blob/master/libvips/resample/lbb.cpp#L100
+
+[7]: http://en.wikipedia.org/wiki/Acutance
+
+[8]: http://eprints.soton.ac.uk/268086/
+
+[9]: https://github.com/jcupitt/libvips/blob/master/libvips/resample/vsqbs.cpp#L48
+
+[10]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Boolean
 
-[3]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number
+[11]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number