OpenCV實現簡單的錄屏功能

OpenCV中VideoCapture和VideoWriter用於讀寫視頻文件，這裏的錄屏功能用到VideoWriter，用於將捕獲的屏幕的每一幀數據保存到視頻文件。
VideoWriter寫視頻文件的步驟

1. bool open(const String& filename, int fourcc, double fps,Size frameSize, bool isColor = true);
2. void write(InputArray image);或者VideoWriter& operator << (const Mat& image);
3. void release();

下列代碼用於獲取屏幕的截圖

int width = GetSystemMetrics(SM_CXSCREEN);
int height = GetSystemMetrics(SM_CYSCREEN);
HDC hdcScreen = GetDC(NULL);
HDC hdcMemDC = CreateCompatibleDC(hdcScreen);
HBITMAP hbmScreen = CreateCompatibleBitmap(hdcScreen, width, height);

BITMAPINFO bi;
bi.bmiHeader.biSize = sizeof(bi.bmiHeader);
bi.bmiHeader.biWidth = width;
bi.bmiHeader.biHeight = height;
bi.bmiHeader.biPlanes = 1;
bi.bmiHeader.biBitCount = 24;
bi.bmiHeader.biCompression = BI_RGB;
bi.bmiHeader.biSizeImage = 0;
bi.bmiHeader.biXPelsPerMeter = 0;
bi.bmiHeader.biYPelsPerMeter = 0;
bi.bmiHeader.biClrUsed = 0;
bi.bmiHeader.biClrImportant = 0;
SelectObject(hdcMemDC, hbmScreen);
int lineBytes = ((width * bi.bmiHeader.biBitCount + 31) / 32) * 4;//每行字節數必須是4字節的整數倍
int bmpSize = lineBytes * height;
char* lpbitmap = new char[bmpSize];
BitBlt(hdcMemDC, 0, 0, width, height, hdcScreen, 0, 0, SRCCOPY);
GetDIBits(hdcMemDC, hbmScreen, 0, height, lpbitmap, &bi, DIB_RGB_COLORS);

lpbitmap爲屏幕的像素顏色數據，下列代碼將lpbitmap作爲一幀寫到視頻中（假設VideoWriter爲已正常打開的VideoWriter實例）

cv::Mat bmpMat(height, width, CV_8UC3);
for (int i = 0; i < height; i++)
{
    int srcIndex = (height-i-1) * lineBytes;
    int destIndex = i * width * 3;
    memcpy(&bmpMat.data[destIndex],&lpbitmap[srcIndex],width*3);
}
videoWriter.write(bmpMat);//或videoWriter << bmpMat;

因爲lpbitmap中的數據是從左下角到右上角排列，而視頻幀圖像的數據是從左上角到右下角排列，所以要將數據按行上下翻轉，即lpbitmap第一行對應視頻圖像的最後一行。另外BMP圖像數據每行的字節數必須是4字節的整數倍，而寫入視頻的Mat數據沒有這個要求，即每行數據大小是圖像實際寬度乘以每個顏色佔用的字節數，所以實際每行拷貝的數據是width*3節字。
下面是一段測試代碼，這裏只錄制100幀，實際使用中可通過命令行參數、快捷鍵或按鈕等自行決定開始和結束時間，幀率這裏也設爲固定的25，其實也應該根據具體形況設定合適的值。最後別忘了將opencv_ffmpegXXX.dll文件放到可執行文件目錄下。

#include<windows.h>
#include"opencv2/opencv.hpp"
int main()
{

    cv::VideoWriter videoWriter;
    double fps = 25;
    int codec = cv::VideoWriter::fourcc('m', 'p', '4', 'v');
    int width =  GetSystemMetrics(SM_CXSCREEN);
    int height = GetSystemMetrics(SM_CYSCREEN);

    time_t seconds = time(0);
    int s = seconds % 60;
    int m = (seconds % 3600) / 60;
    int h = (seconds % (3600 * 24)) / 3600 + 8;
    char timeBuf[128] = { 0 };
    sprintf_s(timeBuf, "CaptureScreen-%d-%d-%d.mp4", h, m, s);
    cv::String filePath = timeBuf;
    videoWriter.open(filePath, codec, fps, cv::Size(width, height), true);
    if (!videoWriter.isOpened())
    {
        return -1;
    }

    HDC hdcScreen = GetDC(NULL);
    HDC hdcMemDC = CreateCompatibleDC(hdcScreen);
    HBITMAP hbmScreen = CreateCompatibleBitmap(hdcScreen, width, height);

    BITMAPINFO bi;
    bi.bmiHeader.biSize = sizeof(bi.bmiHeader);
    bi.bmiHeader.biWidth = width;
    bi.bmiHeader.biHeight = height;
    bi.bmiHeader.biPlanes = 1;
    bi.bmiHeader.biBitCount = 24;
    bi.bmiHeader.biCompression = BI_RGB;
    bi.bmiHeader.biSizeImage = 0;
    bi.bmiHeader.biXPelsPerMeter = 0;
    bi.bmiHeader.biYPelsPerMeter = 0;
    bi.bmiHeader.biClrUsed = 0;
    bi.bmiHeader.biClrImportant = 0;
    SelectObject(hdcMemDC, hbmScreen);

    int lineBytes = ((width * bi.bmiHeader.biBitCount + 31) / 32) * 4;
    int bmpSize = lineBytes * height;
    char* lpbitmap = new char[bmpSize];
    cv::Mat bmpMat(height, width, CV_8UC3);
    for (int i=0;i<100;i++)
    {
        if (BitBlt(hdcMemDC, 0, 0, width, height, hdcScreen, 0, 0, SRCCOPY))
        {
            GetDIBits(hdcMemDC, hbmScreen, 0, height, lpbitmap, &bi, DIB_RGB_COLORS);
            for (int i = 0; i < height; i++)
            {
                int srcIndex = (height-i-1) * lineBytes;
                int destIndex = i * width * 3;
                memcpy(&bmpMat.data[destIndex],&lpbitmap[srcIndex],width*3);
            }
            videoWriter.write(bmpMat);//videoWriter << bmpMat;
        }
    }
    delete[] lpbitmap;
    if (videoWriter.isOpened())
    {
        videoWriter.release();
    }
    return 0;
}