2021年4月7日 星期三

Logistics regression in Chinese

Here are recommended posts in Chinese for understanding logistics regressions. The readers can read them step by step. 

2. 你可能不知道的邏輯回歸(Logistics Regression), URL: https://taweihuang.hpd.io/2017/12/22/logreg101/

2020年8月5日 星期三

科技的進步,學無止境

在2006年左右,我還在唸嘉義大學數學系時,跟同學分工合作,用PHP 5+Dreamweaver 寫學校處室網站,那時候學校IT不給MySQL/Sql Server,我自己還默默用很簡單的檔案系統處理函數,定義好資料結構,一行一行把「最新消息」等訊息,存在單一檔案作為offline database使用(那時候學藝不精,不知道有Sqlite可以用)。就這樣把學校處室的消息發布功能做了出來,還具備CRUD功能。 
在那個大學年代,身兼學校的BBS系統站長,在那邊辛苦學習FreeBSD , Linux⋯當起MIS,三不五時就是去修伺服器,怎麼用fsck修硬碟, Make tools, build kernel 還自己去學怎麼剪RJ 45網路線,讓Server可以正常運作(可見那時候多窮,還要自己剪網路線)。

就這樣懵懵懂懂地大學畢業。

碩士班時期,實驗室還沒有積累一些關於data mining智慧資產,自己寫了一些演算法,像是PrefixSpan sequential pattern mining. 為了求效能,還全部用C++搭配STL寫。自己排crontab job,搭配bash script來產生自己要的實驗記錄。

博士班時期,老實說蠻精彩的。
當了三年的兼任助理,幫忙編hadoop教材...。為了自己的生計,還幫學校處室架設Wordpress,改別人的theme,甚至去看wordpress template怎麼寫,修掉別人模板的bug。也稍微懂得怎麼調整Apache的參數讓系統效能能夠跟得上來。
因為過去都用PHP寫網站,也順便接了幾個案子,像是做一些報名系統,多語系網站建置。
去當了幾家公司的兼任工程師,都是以C# + ASP.Net/MVC 作為主要工作技能。
曾經接了某個專利事務所的案子,幫別人寫Chrome外掛(後續做得不錯,顧客還有回鍋要我再幫忙開發,但是要顧學業,就婉拒了。)
曾幫博弈平台公司維護模組(這期間眼睛也去動手術了)。
為了打軟體競賽,還去用MongoDB,寫寫jQuery and Bootstrap。
為了做某公司的軟體專案,自己去玩Zebra ZPL印表機語言, WCF跟實作軟體序號與數位簽章演算法。
為了幫德國實驗室的同事做Big Data Benchmark,學怎麼用HBase & MapReduce API來做一些ETL工作(後來這也成為我的論文參考文獻之一)。
那時候政府還很流行Open Data,就去學架設CKAN,以及去研究HTTP協議,寫一些CKAN的prototype plugin。

出社會工作了,這些學到的技能也逐漸成為我的技術基石,不過是這些都是後話了。

2020年7月27日 星期一

Raspberry Pi Camera + OpenCV

If you are interested in how to use Raspberry Pi Camera + OpenCV to capture images, here is a great post in Chinese. 

However, the post had something wrong. If you want to install OpenCV packages of Python 3, please use the command “apt-get install python3-opencv”. 

2020年6月25日 星期四

FFMPEG: Decode and then encode frames to JPEG images

I've used FFMPEG library for a while. Actually, the FFMPEG library's decoding process flow can be described as the following picture. If you want to read the videos and then save to jpeg file, you can take a look on my programming code (tested on FFMPEG ver. 4 library).


// VideoProcessing.cpp 
//
#pragma once
using namespace std;
#include <iostream>
extern "C" 
{
    #include <libavcodec/avcodec.h>
    #include <libavdevice/avdevice.h>
    #include <libavformat/avformat.h>
    #include <libavfilter/avfilter.h>
    #include <libavutil/avutil.h>
    #include <libswscale/swscale.h>
    #pragma comment(lib, "avcodec.lib")
    #pragma comment(lib, "avdevice.lib")
    #pragma comment(lib, "avformat.lib")
    #pragma comment(lib, "avfilter.lib")
    #pragma comment(lib, "avutil.lib")
    #pragma comment(lib, "swscale.lib")
}

void SaveToJPEG(AVFrame* pFrame, const char * folderName, int index)

{

    // Setup Output Path
    char outFile[256] = { 0 };
    sprintf_s(outFile, sizeof(outFile)/sizeof(outFile[0]), "%s\\OutputImages-%d.jpg", folderName, index);


    AVFormatContext* pFormatCtx = avformat_alloc_context();

    // Setup the output format
    pFormatCtx->oformat = av_guess_format("mjpeg", NULL, NULL);

    // Initializext 
    if (avio_open(&pFormatCtx->pb, outFile, AVIO_FLAG_READ_WRITE) < 0) {

        printf("Couldn't open output file.");

        return;

    }

    // Get a new Stream from the indicated format context
    AVStream* pAVStream = avformat_new_stream(pFormatCtx, 0);

    if (pAVStream == NULL) {

        return;

    }

    // Find encoder from the codec identifier.

    AVCodec* pCodec = avcodec_find_encoder(pFormatCtx->oformat->video_codec);
    
    if (!pCodec) {
        printf("Codec not found.");
        return;
    }
    // Setup the codec context
    AVCodecContext* codecCtx = avcodec_alloc_context3(pCodec);
    codecCtx->codec_id =  pFormatCtx->oformat->video_codec;
    codecCtx->codec_type = AVMEDIA_TYPE_VIDEO;
    codecCtx->pix_fmt = AV_PIX_FMT_YUVJ420P;
    codecCtx->width = pFrame->width;
    codecCtx->height = pFrame->height;
    codecCtx->time_base = AVRational{ 1,25 };
    
    // Open the codec
    if (avcodec_open2(codecCtx, pCodec, NULL) < 0) {

        printf("Could not open codec.");

        return;

    }
    // assign the codec context to the stream parameters.
    avcodec_parameters_from_context(pAVStream->codecpar, codecCtx);


    //Write Header 
    avformat_write_header(pFormatCtx, NULL);

    int y_size = (codecCtx->width) * (codecCtx->height);

    // assign large enough space
    AVPacket pkt;

    av_new_packet(&pkt, y_size); 

    int got_picture = 0;

    // Use avcodec_send_frame()/avcodec_receive_packet() instead
    int ret = avcodec_send_frame(codecCtx, pFrame);

    if (ret < 0) {

        printf("Encode Error.\n");

        return;

    }
    else
    {
        ret = avcodec_receive_packet(codecCtx, &pkt);

        ret = av_write_frame(pFormatCtx, &pkt);

    }

    av_packet_unref(&pkt);

    //Write Trailer 
    av_write_trailer(pFormatCtx);

    printf("Encode Successful.\n");

    avcodec_close(codecCtx);

    avio_close(pFormatCtx->pb);
    
    avformat_free_context(pFormatCtx);
    avcodec_free_context(&codecCtx);
}

int main(int argc, char * argv[])
{

    if (argc < 2) {
        cout << "You need to specify a media file." << endl;
        cout << "Command line : VideoProcessing.exe [input_video_path] [output_folder]" << endl;
        return -1;
    }

    AVFormatContext* pFormatContext = avformat_alloc_context();
    if (!pFormatContext) {
        cout << "ERROR could not allocate memory for Format Context" << endl;
        return -1;
    }

    if (avformat_open_input(&pFormatContext, argv[1], NULL, NULL) != 0) {
        cout << "ERROR could not open the file" << endl;
        return -1;
    }

    if (avformat_find_stream_info(pFormatContext, NULL) < 0) {
        cout << "ERROR could not get the stream info" << endl;
        return -1;
    }

    // Initialize the codec, paramters for subsequent useage. 
    AVCodec* pCodec = NULL;
    AVCodecParameters* pCodecParameters = NULL;
    int videoStreamIndex = -1;

    for (int i = 0; i < pFormatContext->nb_streams; i++)
    {
        AVCodecParameters* pLocalCodecParameters = NULL;
        // Read the codec parameters corresponding to each stream.
        pLocalCodecParameters = pFormatContext->streams[i]->codecpar;

        AVCodec* pLocalCodec = NULL;
        pLocalCodec = avcodec_find_decoder(pLocalCodecParameters->codec_id);

        if (pLocalCodec == NULL)
        {
            cout << "[ERROR] Cannot find the codec" << endl;
        }

        if (pLocalCodecParameters->codec_type == AVMEDIA_TYPE_VIDEO)
        {
            if (videoStreamIndex == -1)
            {
                videoStreamIndex = i;
                pCodec = pLocalCodec;
                pCodecParameters = pLocalCodecParameters;
            }
        }
    }

    AVCodecContext* pCodecContext = avcodec_alloc_context3(pCodec);
    if (pCodecContext == NULL)
    {
        cout << "Fail to allocate the memoery to the Codec Context." << endl;
        return -1;
    }

    if (avcodec_parameters_to_context(pCodecContext, pCodecParameters) < 0)
    {
        cout << "failed to copy codec params to codec context" << endl;
        return -1;
    }

    if (avcodec_open2(pCodecContext, pCodec, NULL) < 0)
    {
        cout << "failed to open codec through avcodec_open2" << endl;
        return -1;
    }

    AVFrame* pFrame = av_frame_alloc();
    if (!pFrame)
    {
        cout << "failed to allocated memory for AVFrame" << endl;
        return -1;
    }

    AVPacket* pPacket = av_packet_alloc();
    if (!pPacket)
    {
        cout << "failed to allocated memory for AVPacket" << endl;
        return -1;
    }

    int indexOfFrame = 0;
    while (av_read_frame(pFormatContext, pPacket) >= 0)
    {
        // if it's the video stream
        if (pPacket->stream_index == videoStreamIndex) {
            int response = avcodec_send_packet(pCodecContext, pPacket);
            if (response < 0)
            {
                break;
            }
            else
            {
                response = avcodec_receive_frame(pCodecContext, pFrame);

                if (response >= 0)
                {
                    indexOfFrame++;
                    SaveToJPEG(pFrame, argv[2], indexOfFrame);
                }
            }
        }
        av_packet_unref(pPacket);
        // Limit the number of output frame to be 5.
        if (indexOfFrame == 5)
        {
            break;
        }

    }
    // https://ffmpeg.org/doxygen/trunk/group__lavc__packet.html#ga63d5a489b419bd5d45cfd09091cbcbc2
    avformat_close_input(&pFormatContext);
    av_frame_free(&pFrame);
    avcodec_free_context(&pCodecContext);
}
Reference:
  1. FFMPEG libav decode note
  2. FFMPEG libav tutorial
  3. 用AVCodecParameters代替AVCodecContext
  4. 用FFmpeg保存JPEG图片



2018年12月24日 星期一

Materials for Neural Network

Some materials are enclosed here.
1. 深度學習(二): 反向傳播 URL:http://chansh518.github.io/deep%20learning/2016/08/08/Deep-Learning-Notes-Backpropagation.html
2. 一文看懂常用的梯度下降算法 URL: https://blog.csdn.net/u013709270/article/details/78667531
3. 邏輯回歸代價函數及其梯度下降公式 URL: https://blog.csdn.net/Mr_HHH/article/details/78934793
4. The Back Propagation Algorithm. URL: https://page.mi.fu-berlin.de/rojas/neural/chapter/K7.pdf
5. Derivation of Back Propagation Algorithm for Forward Neural Networks. URL:  http://www.cs.put.poznan.pl/pliskowski/pub/teaching/eio/lab1/eio-supplementary.pdf
6. 凸優化 梯度下降。URL: http://www.hanlongfei.com/凸优化/2015/09/29/cmu-10725-gradient/
7. Geadient Descent demystified. URL:  https://towardsdatascience.com/gradient-descent-demystified-bc30b26e432a
8. An introduction to gradient descent and linear regression. URL: https://spin.atomicobject.com/2014/06/24/gradient-descent-linear-regression/

2018年5月1日 星期二

It's time to push my career forward

After a long time, almost 3 months, I've done my PhD degree. In the past, I could not image that I get my PhD. Thanks for my supervisor's and family's great supports. I did it!
Through the training of PhD, making things clear and be systematic are not the difficult parts anymore for me. But, I deeply felt that doing research tasks and engineering works was the most difficult part!

Now, I am a software engineer of an international corporate.
For me, doing both engineering and research work is better than doing only researches!
Now, it's time to push my career forward.