ncnn/docs/Home.md

### input data and extract output
```cpp
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include "net.h"

int main()
{
    cv::Mat img = cv::imread("image.ppm", CV_LOAD_IMAGE_GRAYSCALE);
    int w = img.cols;
    int h = img.rows;

    // subtract 128, norm to -1 ~ 1
    ncnn::Mat in = ncnn::Mat::from_pixels_resize(img.data, ncnn::Mat::PIXEL_GRAY, w, h, 60, 60);
    float mean[1] = { 128.f };
    float norm[1] = { 1/128.f };
    in.substract_mean_normalize(mean, norm);

    ncnn::Net net;
    net.load_param("model.param");
    net.load_model("model.bin");

    ncnn::Extractor ex = net.create_extractor();
    ex.set_light_mode(true);
    ex.set_num_threads(4);

    ex.input("data", in);

    ncnn::Mat feat;
    ex.extract("output", feat);

    return 0;
}

```

### print Mat content
```cpp
void pretty_print(const ncnn::Mat& m)
{
    for (int q=0; q<m.c; q++)
    {
        const float* ptr = m.channel(q);
        for (int y=0; y<m.h; y++)
        {
            for (int x=0; x<m.w; x++)
            {
                printf("%f ", ptr[x]);
            }
            ptr += m.w;
            printf("\n");
        }
        printf("------------------------\n");
    }
}
```

### visualize Mat content
```cpp
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

memory usage is the RSS item in top utility output

|compare item|caffe-android-lib+openblas|ncnn|
|---|---|---|
|inference time(1 thread)|228ms|88ms|
|inference time(8 thread)|152ms|38ms|
|memory usage|138.16M|21.56M|
|library binary size|6.9M|<500K|
|compatibility|armeabi-v7a-hard with neon or arm64-v8a|armeabi-v7a with neon or arm64-v8a|
|thirdparty dependency|boost gflags glog lmdb openblas opencv protobuf|none|

### FAQ
Q ncnn的起源

A 深度学习算法要在手机上落地，caffe依赖太多，手机上也没有cuda，需要个又快又小的前向网络实现


Q ncnn名字的来历

A cnn就是卷积神经网络的缩写，开头的n算是一语n关。比如new/next(全新的实现)，naive(ncnn是naive实现)，neon(ncnn最初为手机优化)，up主名字(←_←)


Q 支持哪些平台

A 跨平台，主要支持 android，次要支持 ios / linux / windows


Q 计算精度如何

A armv7 neon float 不遵照 ieee754 标准，有些采用快速实现(如exp sin等)，速度快但确保精度足够高


Q pc 上的速度很慢

A pc都是x86架构的，基本没做什么优化，主要用来核对结果，毕竟up主精力是有限的（


Q logo

A up主是mc玩家，所以灵魂手绘像素猫，还可以找到ncnn...