Document reduction kernels

2 files changed, 37 insertions, 1 deletions

diff --git a/docs/filters.rst b/docs/filters.rst
index f6b914d..719719f 100644
--- a/docs/filters.rst
+++ b/docs/filters.rst
@@ -518,7 +518,7 @@ Reducing with OpenCL
         kernel void sum (global float *in, global float *out)
         {
             size_t idx = get_global_id (1) * get_global_size (0) + get_global_id (0);
-            in[idx] += out[idx];
+            out[idx] += in[idx];
         }
 
         kernel void divide (global float *out, uint count)
@@ -531,6 +531,9 @@ Reducing with OpenCL
 
         ufo-launch ... ! opencl-reduce kernel=sum finish=divide ! ...
 
+    If :gobj:prop:`filename` is not set, a default kernel file is loaded. See
+    :ref:`opencl-reduction-default-kernels` for a list of possible kernels.
+
     .. gobj:prop:: filename:string
 
         Filename with kernel sources to load.
diff --git a/docs/kernels.rst b/docs/kernels.rst
index f2f77e1..cd0020c 100644
--- a/docs/kernels.rst
+++ b/docs/kernels.rst
@@ -17,3 +17,36 @@ This section lists all kernel functions that are available to the
 .. c:function:: void nlm_noise_reduction ()
 
     Smooths data within a local neighbourhood.
+
+
+.. _opencl-reduction-default-kernels:
+
+
+OpenCL reduction default kernels
+================================
+
+This section lists all kernel functions that are available to the
+:gobj:class:`opencl-reduce` filter if no filename is specified. These kernels
+are supposed to be used for the ``kernel`` argument.
+
+.. c:function:: void minimum ()
+
+    Computes the minimum of each pixel in the stream.
+
+.. c:function:: void maximum ()
+
+    Computes the maximum of each pixel in the stream.
+
+.. c:function:: void sum ()
+
+    Computes the sum of each pixel in the stream.
+
+
+These kernels are supposed to be used in the ``finish`` argument:
+
+.. c:function:: void divide ()
+
+    Divides each pixel by the stream count. Together with ``sum`` this can be
+    used to compute the average, i.e.::
+
+        ufo-launch .. ! opencl-reduce kernel=sum finish=divide ! ..