Add ability to extend (pad) the edges of an image

docs/api.md

@@ -280,7 +280,7 @@ sharp(input)
 
 #### background(rgba)
 
-Set the background for the `embed` and `flatten` operations.
+Set the background for the `embed`, `flatten` and `extend` operations.
 
 `rgba` is parsed by the [color](https://www.npmjs.org/package/color) module to extract values for red, green, blue and alpha.
 
@@ -292,6 +292,25 @@ The default background is `{r: 0, g: 0, b: 0, a: 1}`, black without transparency
 
 Merge alpha transparency channel, if any, with `background`.
 
+#### extend(extension)
+
+Extends/pads the edges of the image with `background`, where `extension` is one of:
+
+* a Number representing the pixel count to add to each edge, or
+* an Object containing `top`, `left`, `bottom` and `right` attributes, each a Number of pixels to add to that edge.
+
+This operation will always occur after resizing and extraction, if any.
+
+```javascript
+// Resize to 140 pixels wide, then add 10 transparent pixels
+// to the top, left and right edges and 20 to the bottom edge
+sharp(input)
+  .resize(140)
+  .background({r: 0, g: 0, b: 0, a: 0})
+  .extend({top: 10, bottom: 20, left: 10, right: 10})
+  ...
+```
+
 #### negate()
 
 Produces the "negative" of the image.  White => Black, Black => White, Blue => Yellow, etc.

docs/changelog.md

@@ -2,6 +2,10 @@
 
 ### v0.14 - "*needle*"
 
+* Add ability to extend (pad) the edges of an image.
+  [#128](https://github.com/lovell/sharp/issues/128)
+  [@blowsie](https://github.com/blowsie)
+
 * Improvements to overlayWith: differing sizes/formats, gravity, buffer input.
   [#239](https://github.com/lovell/sharp/issues/239)
   [@chrisriley](https://github.com/chrisriley)