visualize Mat

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@@ -36,7 +36,7 @@ int main()
 
 ### print Mat content
 ```
-void pretty_print(const Mat& m)
+void pretty_print(const ncnn::Mat& m)
 {
     for (int q=0; q<m.c; q++)
     {
@@ -55,6 +55,60 @@ void pretty_print(const Mat& m)
 }
 ```
 
+### visualize Mat content
+```
+void visualize(const char* title, const ncnn::Mat& m)
+{
+    std::vector<cv::Mat> normed_feats(m.c);
+
+    for (int i=0; i<m.c; i++)
+    {
+        cv::Mat tmp(m.h, m.w, CV_32FC1, (void*)(const float*)m.channel(i));
+
+        cv::normalize(tmp, normed_feats[i], 0, 255, cv::NORM_MINMAX, CV_8U);
+
+        cv::cvtColor(normed_feats[i], normed_feats[i], cv::COLOR_GRAY2BGR);
+
+        // check NaN
+        for (int y=0; y<m.h; y++)
+        {
+            const float* tp = tmp.ptr<float>(y);
+            uchar* sp = normed_feats[i].ptr<uchar>(y);
+            for (int x=0; x<m.w; x++)
+            {
+                float v = tp[x];
+                if (v != v)
+                {
+                    sp[0] = 0;
+                    sp[1] = 0;
+                    sp[2] = 255;
+                }
+
+                sp += 3;
+            }
+        }
+    }
+
+    int tw = m.w < 10 ? 32 : m.w < 20 ? 16 : m.w < 40 ? 8 : m.w < 80 ? 4 : m.w < 160 ? 2 : 1;
+    int th = (m.c - 1) / tw + 1;
+
+    cv::Mat show_map(m.h * th, m.w * tw, CV_8UC3);
+    show_map = cv::Scalar(127);
+
+    // tile
+    for (int i=0; i<m.c; i++)
+    {
+        int ty = i / tw;
+        int tx = i % tw;
+
+        normed_feats[i].copyTo(show_map(cv::Rect(tx * m.w, ty * m.h, m.w, m.h)));
+    }
+
+    cv::resize(show_map, show_map, cv::Size(0,0), 2, 2, cv::INTER_NEAREST);
+    cv::imshow(title, show_map);
+}
+```
+
 ### caffe-android-lib+openblas vs ncnn
 use squeezenet v1.1, nexus6p, android 7.1.2