Add contrast limiting adaptive histogram equalization (CLAHE) operator (#2726)

docs/api-operation.md

@@ -243,6 +243,30 @@ Alternative spelling of normalise.
 
 Returns **Sharp** 
 
+## clahe
+
+Perform contrast limiting adaptive histogram equalization (CLAHE)
+
+This will, in general, enhance the clarity of the image by bringing out
+darker details. Please read more about CLAHE here:
+[https://en.wikipedia.org/wiki/Adaptive_histogram_equalization#Contrast_Limited_AHE][8]
+
+### Parameters
+
+*   `options` **[Object][2]** 
+
+    *   `options.width` **[number][1]** integer width of the region in pixels.
+    *   `options.height` **[number][1]** integer height of the region in pixels.
+    *   `options.maxSlope` **[number][1]** maximum value for the slope of the
+        cumulative histogram. A value of 0 disables contrast limiting. Valid values
+        are integers in the range 0-100 (inclusive) (optional, default `3`)
+
+<!---->
+
+*   Throws **[Error][5]** Invalid parameters
+
+Returns **Sharp** 
+
 ## convolve
 
 Convolve the image with the specified kernel.
@@ -252,7 +276,7 @@ Convolve the image with the specified kernel.
 *   `kernel` **[Object][2]** 
 
     *   `kernel.width` **[number][1]** width of the kernel in pixels.
-    *   `kernel.height` **[number][1]** width of the kernel in pixels.
+    *   `kernel.height` **[number][1]** height of the kernel in pixels.
     *   `kernel.kernel` **[Array][7]<[number][1]>** Array of length `width*height` containing the kernel values.
     *   `kernel.scale` **[number][1]** the scale of the kernel in pixels. (optional, default `sum`)
     *   `kernel.offset` **[number][1]** the offset of the kernel in pixels. (optional, default `0`)
@@ -304,7 +328,7 @@ the selected bitwise boolean `operation` between the corresponding pixels of the
 
 ### Parameters
 
-*   `operand` **([Buffer][8] | [string][3])** Buffer containing image data or string containing the path to an image file.
+*   `operand` **([Buffer][9] | [string][3])** Buffer containing image data or string containing the path to an image file.
 *   `operator` **[string][3]** one of `and`, `or` or `eor` to perform that bitwise operation, like the C logic operators `&`, `|` and `^` respectively.
 *   `options` **[Object][2]?** 
 
@@ -421,4 +445,6 @@ Returns **Sharp**
 
 [7]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array
 
-[8]: https://nodejs.org/api/buffer.html
+[8]: https://en.wikipedia.org/wiki/Adaptive_histogram_equalization#Contrast_Limited_AHE
+
+[9]: https://nodejs.org/api/buffer.html