Extend: default missing edge props to zero #2578

docs/api-resize.md

@@ -137,10 +137,10 @@ This operation will always occur after resizing and extraction, if any.
 ### Parameters
 
 -   `extend` **([number][8] \| [Object][9])** single pixel count to add to all edges or an Object with per-edge counts
-    -   `extend.top` **[number][8]?** 
-    -   `extend.left` **[number][8]?** 
-    -   `extend.bottom` **[number][8]?** 
-    -   `extend.right` **[number][8]?** 
+    -   `extend.top` **[number][8]**  (optional, default `0`)
+    -   `extend.left` **[number][8]**  (optional, default `0`)
+    -   `extend.bottom` **[number][8]**  (optional, default `0`)
+    -   `extend.right` **[number][8]**  (optional, default `0`)
     -   `extend.background` **([String][10] \| [Object][9])** background colour, parsed by the [color][11] module, defaults to black without transparency. (optional, default `{r:0,g:0,b:0,alpha:1}`)
 
 ### Examples
@@ -160,6 +160,16 @@ sharp(input)
   ...
 ```
 
+```javascript
+// Add a row of 10 red pixels to the bottom
+sharp(input)
+  .extend({
+    bottom: 10,
+    background: 'red'
+  })
+  ...
+```
+
 -   Throws **[Error][13]** Invalid parameters
 
 Returns **Sharp** 

docs/changelog.md

@@ -16,6 +16,9 @@ Requires libvips v8.10.6
 
 * Reduce concurrency on glibc-based Linux when using the default memory allocator to help prevent fragmentation.
 
+* Default missing edge properties of extend operation to zero.
+  [#2578](https://github.com/lovell/sharp/issues/2578)
+
 * Ensure composite does not clip top and left offsets.
   [#2594](https://github.com/lovell/sharp/pull/2594)
   [@SHG42](https://github.com/SHG42)