add point of interest API

experimental/skpdiff/diff_pixels.cl

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #pragma OPENCL_EXTENSION cl_khr_global_int32_base_atomics
 
 const sampler_t gInSampler = CLK_NORMALIZED_COORDS_FALSE |
                              CLK_ADDRESS_CLAMP_TO_EDGE   |
                              CLK_FILTER_NEAREST;
 
-__kernel void diff(read_only image2d_t baseline, read_only image2d_t test, __global int* result) {
+__kernel void diff(read_only image2d_t baseline, read_only image2d_t test,
+                   __global int* result, __global int2* poi) {
     int2 coord = (int2)(get_global_id(0), get_global_id(1));
     uint4 baselinePixel = read_imageui(baseline, gInSampler, coord);
     uint4 testPixel = read_imageui(test, gInSampler, coord);
     int4 pixelCompare = baselinePixel == testPixel;
     if (baselinePixel.x != testPixel.x ||
         baselinePixel.y != testPixel.y ||
         baselinePixel.z != testPixel.z ||
         baselinePixel.w != testPixel.w) {
 
-        atom_inc(result);
+        int poiIndex = atomic_inc(result);
+        poi[poiIndex] = coord;
     }
 }