FFmpeg原始碼簡單分析：libavdevice的gdigrab，

FFmpeg原始碼簡單分析：libavdevice的gdigrab，

本文記錄FFmpeg的libavdevice中GDIGrab組件的原始碼。GDIGrab用於在Windows下螢幕錄影（抓屏）。在ffmpeg.exe中使用可以參考文章：

FFmpeg擷取DirectShow裝置資料（網路攝影機，錄屏）

編程使用可以參考文章：

最簡單的基於FFmpeg的AVDevice例子（螢幕錄製）

gdigrab的原始碼位於libavdevice\gdigrab.c。關鍵函數的呼叫歷程圖如所示。圖中綠色背景的函數代表原始碼中自己聲明的函數，紫色背景的函數代表Win32的API函數。

ff_gdigrab_demuxer在FFmpeg中Device也被當做是一種Format，因為GDIGrab是一個輸入裝置，因此被當作一個AVInputFormat。GDIGrab對應的AVInputFormat結構體如下所示。

AVInputFormat ff_gdigrab_demuxer = {    .name           = "gdigrab",    .long_name      = NULL_IF_CONFIG_SMALL("GDI API Windows frame grabber"),    .priv_data_size = sizeof(struct gdigrab),    .read_header    = gdigrab_read_header,    .read_packet    = gdigrab_read_packet,    .read_close     = gdigrab_read_close,    .flags          = AVFMT_NOFILE,    .priv_class     = &gdigrab_class,};

從該結構體可以看出：

裝置名稱是“gdigrab”；
裝置完整名稱是“GDI API Windows frame grabber”；
初始化函數指標read_header()指向gdigrab_read_header()；
讀取資料函數指標read_packet()指向gdigrab_read_packet()；
關閉函數指標read_close()指向gdigrab_read_close()；
Flags設定為AVFMT_NOFILE；
AVClass指定為gdigrab_class。

下面分析一下這些資料。

gdigrab_classff_gdigrab_demuxer指定它的AVClass為一個名稱為“gdigrab_class”的靜態變數。有關AVClass的概念之前已經記錄過，在這裡不再重複。gdigrab_class的定義如下。

static const AVClass gdigrab_class = {    .class_name = "GDIgrab indev",    .item_name  = av_default_item_name,    .option     = options,    .version    = LIBAVUTIL_VERSION_INT,};

從gdigrab_class的定義可以看出，它指定了一個名稱為“options”的數組作為它的選項數組（賦值給AVClass的option變數）。

options下面看一下這個options數組的定義，如下所示。

#define OFFSET(x) offsetof(struct gdigrab, x)#define DEC AV_OPT_FLAG_DECODING_PARAMstatic const AVOption options[] = {    { "draw_mouse", "draw the mouse pointer", OFFSET(draw_mouse), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, DEC },    { "show_region", "draw border around capture area", OFFSET(show_region), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, DEC },    { "framerate", "set video frame rate", OFFSET(framerate), AV_OPT_TYPE_VIDEO_RATE, {.str = "ntsc"}, 0, 0, DEC },    { "video_size", "set video frame size", OFFSET(width), AV_OPT_TYPE_IMAGE_SIZE, {.str = NULL}, 0, 0, DEC },    { "offset_x", "capture area x offset", OFFSET(offset_x), AV_OPT_TYPE_INT, {.i64 = 0}, INT_MIN, INT_MAX, DEC },    { "offset_y", "capture area y offset", OFFSET(offset_y), AV_OPT_TYPE_INT, {.i64 = 0}, INT_MIN, INT_MAX, DEC },    { NULL },};

options數組中包含了該Device支援的選項。可以看出GDIGrab支援如下選項：

draw_mouse：畫出滑鼠指標。
show_region：划出抓屏地區的邊界。
framerate：抓屏幀率。
video_size：抓屏的大小。
offset_x：抓屏起始點x軸座標。
offset_y：抓屏起始點y軸座標。

從宏定義“#define OFFSET(x) offsetof(struct gdigrab, x)”中可以看出，這些選項都儲存在一個名稱為“gdigrab”的結構體中。

Gdigrab 上下文結構體Gdigrab上下文結構體中儲存了GDIGrab裝置用到的各種變數，定義如下所示。

/** * GDI Device Demuxer context */struct gdigrab {    const AVClass *class;   /**< Class for private options */    int        frame_size;  /**< Size in bytes of the frame pixel data */    int        header_size; /**< Size in bytes of the DIB header */    AVRational time_base;   /**< Time base */    int64_t    time_frame;  /**< Current time */    int        draw_mouse;  /**< Draw mouse cursor (private option) */    int        show_region; /**< Draw border (private option) */    AVRational framerate;   /**< Capture framerate (private option) */    int        width;       /**< Width of the grab frame (private option) */    int        height;      /**< Height of the grab frame (private option) */    int        offset_x;    /**< Capture x offset (private option) */    int        offset_y;    /**< Capture y offset (private option) */    HWND       hwnd;        /**< Handle of the window for the grab */    HDC        source_hdc;  /**< Source device context */    HDC        dest_hdc;    /**< Destination, source-compatible DC */    BITMAPINFO bmi;         /**< Information describing DIB format */    HBITMAP    hbmp;        /**< Information on the bitmap captured */    void      *buffer;      /**< The buffer containing the bitmap image data */    RECT       clip_rect;   /**< The subarea of the screen or window to clip */    HWND       region_hwnd; /**< Handle of the region border window */    int cursor_error_printed;};

gdigrab_read_header()gdigrab_read_header()用於初始化gdigrab。函數的定義如下所示。

/** * Initializes the gdi grab device demuxer (public device demuxer API). * * @param s1 Context from avformat core * @return AVERROR_IO error, 0 success */static intgdigrab_read_header(AVFormatContext *s1){    struct gdigrab *gdigrab = s1->priv_data;    //視窗控制代碼    HWND hwnd;    HDC source_hdc = NULL;    HDC dest_hdc   = NULL;    BITMAPINFO bmi;    HBITMAP hbmp   = NULL;    void *buffer   = NULL;    const char *filename = s1->filename;    const char *name     = NULL;    AVStream   *st       = NULL;    int bpp;    RECT virtual_rect;    //視窗的位置和大小    RECT clip_rect;    BITMAP bmp;    int ret;    //filename為視窗名稱    if (!strncmp(filename, "title=", 6)) {        name = filename + 6;        //尋找視窗的控制代碼        hwnd = FindWindow(NULL, name);        if (!hwnd) {            av_log(s1, AV_LOG_ERROR,                   "Can't find window '%s', aborting.\n", name);            ret = AVERROR(EIO);            goto error;        }        if (gdigrab->show_region) {            av_log(s1, AV_LOG_WARNING,                    "Can't show region when grabbing a window.\n");            gdigrab->show_region = 0;        }        //filename為desktop    } else if (!strcmp(filename, "desktop")) {    //視窗控制代碼為NULL        hwnd = NULL;    } else {        av_log(s1, AV_LOG_ERROR,               "Please use \"desktop\" or \"title=<windowname>\" to specify your target.\n");        ret = AVERROR(EIO);        goto error;    }    if (hwnd) {        GetClientRect(hwnd, &virtual_rect);    } else {    //視窗控制代碼為NULL，代表是全屏        virtual_rect.left = GetSystemMetrics(SM_XVIRTUALSCREEN);        virtual_rect.top = GetSystemMetrics(SM_YVIRTUALSCREEN);        virtual_rect.right = virtual_rect.left + GetSystemMetrics(SM_CXVIRTUALSCREEN);        virtual_rect.bottom = virtual_rect.top + GetSystemMetrics(SM_CYVIRTUALSCREEN);    }    /* If no width or height set, use full screen/window area */    if (!gdigrab->width || !gdigrab->height) {        clip_rect.left = virtual_rect.left;        clip_rect.top = virtual_rect.top;        clip_rect.right = virtual_rect.right;        clip_rect.bottom = virtual_rect.bottom;    } else {        clip_rect.left = gdigrab->offset_x;        clip_rect.top = gdigrab->offset_y;        clip_rect.right = gdigrab->width + gdigrab->offset_x;        clip_rect.bottom = gdigrab->height + gdigrab->offset_y;    }    if (clip_rect.left < virtual_rect.left ||            clip_rect.top < virtual_rect.top ||            clip_rect.right > virtual_rect.right ||            clip_rect.bottom > virtual_rect.bottom) {            av_log(s1, AV_LOG_ERROR,                    "Capture area (%li,%li),(%li,%li) extends outside window area (%li,%li),(%li,%li)",                    clip_rect.left, clip_rect.top,                    clip_rect.right, clip_rect.bottom,                    virtual_rect.left, virtual_rect.top,                    virtual_rect.right, virtual_rect.bottom);            ret = AVERROR(EIO);            goto error;    }    /* This will get the device context for the selected window, or if     * none, the primary screen */    //得到某個視窗控制代碼的DC     source_hdc = GetDC(hwnd);    if (!source_hdc) {        WIN32_API_ERROR("Couldn't get window device context");        ret = AVERROR(EIO);        goto error;    }    bpp = GetDeviceCaps(source_hdc, BITSPIXEL);    if (name) {        av_log(s1, AV_LOG_INFO,               "Found window %s, capturing %lix%lix%i at (%li,%li)\n",               name,               clip_rect.right - clip_rect.left,               clip_rect.bottom - clip_rect.top,               bpp, clip_rect.left, clip_rect.top);    } else {        av_log(s1, AV_LOG_INFO,               "Capturing whole desktop as %lix%lix%i at (%li,%li)\n",               clip_rect.right - clip_rect.left,               clip_rect.bottom - clip_rect.top,               bpp, clip_rect.left, clip_rect.top);    }    if (clip_rect.right - clip_rect.left <= 0 ||            clip_rect.bottom - clip_rect.top <= 0 || bpp%8) {        av_log(s1, AV_LOG_ERROR, "Invalid properties, aborting\n");        ret = AVERROR(EIO);        goto error;    }    //建立一個與指定裝置相容的HDC    dest_hdc = CreateCompatibleDC(source_hdc);    if (!dest_hdc) {        WIN32_API_ERROR("Screen DC CreateCompatibleDC");        ret = AVERROR(EIO);        goto error;    }    /* Create a DIB and select it into the dest_hdc */    //BMP    bmi.bmiHeader.biSize          = sizeof(BITMAPINFOHEADER);    bmi.bmiHeader.biWidth         = clip_rect.right - clip_rect.left;    bmi.bmiHeader.biHeight        = -(clip_rect.bottom - clip_rect.top);    bmi.bmiHeader.biPlanes        = 1;    bmi.bmiHeader.biBitCount      = bpp;    bmi.bmiHeader.biCompression   = BI_RGB;    bmi.bmiHeader.biSizeImage     = 0;    bmi.bmiHeader.biXPelsPerMeter = 0;    bmi.bmiHeader.biYPelsPerMeter = 0;    bmi.bmiHeader.biClrUsed       = 0;    bmi.bmiHeader.biClrImportant  = 0;    hbmp = CreateDIBSection(dest_hdc, &bmi, DIB_RGB_COLORS,            &buffer, NULL, 0);    if (!hbmp) {        WIN32_API_ERROR("Creating DIB Section");        ret = AVERROR(EIO);        goto error;    }    if (!SelectObject(dest_hdc, hbmp)) {        WIN32_API_ERROR("SelectObject");        ret = AVERROR(EIO);        goto error;    }    /* Get info from the bitmap */    GetObject(hbmp, sizeof(BITMAP), &bmp);    //建立AVStream    st = avformat_new_stream(s1, NULL);    if (!st) {        ret = AVERROR(ENOMEM);        goto error;    }    avpriv_set_pts_info(st, 64, 1, 1000000); /* 64 bits pts in us */    //儲存資訊到GDIGrab上下文結構體    gdigrab->frame_size  = bmp.bmWidthBytes * bmp.bmHeight * bmp.bmPlanes;    gdigrab->header_size = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) +                           (bpp <= 8 ? (1 << bpp) : 0) * sizeof(RGBQUAD) /* palette size */;    gdigrab->time_base   = av_inv_q(gdigrab->framerate);    gdigrab->time_frame  = av_gettime() / av_q2d(gdigrab->time_base);    gdigrab->hwnd       = hwnd;    gdigrab->source_hdc = source_hdc;    gdigrab->dest_hdc   = dest_hdc;    gdigrab->hbmp       = hbmp;    gdigrab->bmi        = bmi;    gdigrab->buffer     = buffer;    gdigrab->clip_rect  = clip_rect;    gdigrab->cursor_error_printed = 0;    if (gdigrab->show_region) {        if (gdigrab_region_wnd_init(s1, gdigrab)) {            ret = AVERROR(EIO);            goto error;        }    }    st->codec->codec_type = AVMEDIA_TYPE_VIDEO;    st->codec->codec_id   = AV_CODEC_ID_BMP;    st->codec->time_base  = gdigrab->time_base;    st->codec->bit_rate   = (gdigrab->header_size + gdigrab->frame_size) * 1/av_q2d(gdigrab->time_base) * 8;    return 0;error://如果出錯了    if (source_hdc)        ReleaseDC(hwnd, source_hdc);    if (dest_hdc)        DeleteDC(dest_hdc);    if (hbmp)        DeleteObject(hbmp);    if (source_hdc)        DeleteDC(source_hdc);    return ret;}

從原始碼可以看出，gdigrab_read_header()的流程大致如下所示：
（1）確定視窗的控制代碼hwnd。如果指定了“title=”的話，調用FindWindow()擷取hwnd；如果指定了“desktop”，則設定hwnd為NULL。
（2）根據視窗的控制代碼hwnd確定抓屏的矩形地區。如果抓取指定視窗，則通過GetClientRect()函數；否則就抓取整個螢幕。
（3）調用GDI的API完成抓屏的一些初始化工作。包括：

a)通過GetDC()獲得某個視窗控制代碼的HDC（在這裡是source_hdc）。
b)通過CreateCompatibleDC()建立一個與指定裝置相容的HDC（在這裡是dest_hdc）
c)通過CreateDIBSection()建立HBITMAP
d)通過SelectObject()綁定HBITMAP和HDC（指的是dest_hdc）

（4）通過avformat_new_stream()建立一個AVStream。
（5）將初始化時候的一些參數儲存至GDIGrab的上下文結構體。

gdigrab_read_packet()gdigrab_read_packet()用於讀取一幀抓屏資料。該函數的定義如下所示。

/** * Grabs a frame from gdi (public device demuxer API). * * @param s1 Context from avformat core * @param pkt Packet holding the grabbed frame * @return frame size in bytes */static int gdigrab_read_packet(AVFormatContext *s1, AVPacket *pkt){    struct gdigrab *gdigrab = s1->priv_data;    //讀取參數    HDC        dest_hdc   = gdigrab->dest_hdc;    HDC        source_hdc = gdigrab->source_hdc;    RECT       clip_rect  = gdigrab->clip_rect;    AVRational time_base  = gdigrab->time_base;    int64_t    time_frame = gdigrab->time_frame;    BITMAPFILEHEADER bfh;    int file_size = gdigrab->header_size + gdigrab->frame_size;    int64_t curtime, delay;    /* Calculate the time of the next frame */    time_frame += INT64_C(1000000);    /* Run Window message processing queue */    if (gdigrab->show_region)        gdigrab_region_wnd_update(s1, gdigrab);    /* wait based on the frame rate */    //延時    for (;;) {        curtime = av_gettime();        delay = time_frame * av_q2d(time_base) - curtime;        if (delay <= 0) {            if (delay < INT64_C(-1000000) * av_q2d(time_base)) {                time_frame += INT64_C(1000000);            }            break;        }        if (s1->flags & AVFMT_FLAG_NONBLOCK) {            return AVERROR(EAGAIN);        } else {            av_usleep(delay);        }    }    //建立一個AVPacket    if (av_new_packet(pkt, file_size) < 0)        return AVERROR(ENOMEM);    pkt->pts = curtime;    /* Blit screen grab */    //關鍵：BitBlt()完成抓屏功能    if (!BitBlt(dest_hdc, 0, 0,                clip_rect.right - clip_rect.left,                clip_rect.bottom - clip_rect.top,                source_hdc,                clip_rect.left, clip_rect.top, SRCCOPY | CAPTUREBLT)) {        WIN32_API_ERROR("Failed to capture image");        return AVERROR(EIO);    }    //畫滑鼠指標？    if (gdigrab->draw_mouse)        paint_mouse_pointer(s1, gdigrab);    /* Copy bits to packet data */    //BMP檔案頭BITMAPFILEHEADER    bfh.bfType = 0x4d42; /* "BM" in little-endian */    bfh.bfSize = file_size;    bfh.bfReserved1 = 0;    bfh.bfReserved2 = 0;    bfh.bfOffBits = gdigrab->header_size;    //往AVPacket中拷貝資料    //拷貝BITMAPFILEHEADER    memcpy(pkt->data, &bfh, sizeof(bfh));    //拷貝BITMAPINFOHEADER    memcpy(pkt->data + sizeof(bfh), &gdigrab->bmi.bmiHeader, sizeof(gdigrab->bmi.bmiHeader));    //不常見    if (gdigrab->bmi.bmiHeader.biBitCount <= 8)        GetDIBColorTable(dest_hdc, 0, 1 << gdigrab->bmi.bmiHeader.biBitCount,                (RGBQUAD *) (pkt->data + sizeof(bfh) + sizeof(gdigrab->bmi.bmiHeader)));    //拷貝像素資料    memcpy(pkt->data + gdigrab->header_size, gdigrab->buffer, gdigrab->frame_size);    gdigrab->time_frame = time_frame;    return gdigrab->header_size + gdigrab->frame_size;}

從原始碼可以看出，gdigrab_read_packet()的流程大致如下所示：
（1）從GDIGrab上下文結構體讀取初始化時候設定的參數。
（2）根據幀率參數進行延時。
（3）通過av_new_packet()建立一個AVPacket。
（4）通過BitBlt()完成抓屏功能。
（5）如果需要畫滑鼠指標的話，調用paint_mouse_pointer()，這裡不做分析。
（6）按照順序拷貝以下3項內容至AVPacket的data指向的記憶體：

a)BITMAPFILEHEADER
b)BITMAPINFOHEADER
c)抓屏的到的像素資料

gdigrab_read_close()gdigrab_read_close()用於關閉gdigrab。該函數的定義如下所示。

/** * Closes gdi frame grabber (public device demuxer API). * * @param s1 Context from avformat core * @return 0 success, !0 failure */static int gdigrab_read_close(AVFormatContext *s1){    struct gdigrab *s = s1->priv_data;    if (s->show_region)        gdigrab_region_wnd_destroy(s1, s);    if (s->source_hdc)        ReleaseDC(s->hwnd, s->source_hdc);    if (s->dest_hdc)        DeleteDC(s->dest_hdc);    if (s->hbmp)        DeleteObject(s->hbmp);    if (s->source_hdc)        DeleteDC(s->source_hdc);    return 0;}

從原始碼可以看出，gdigrab_read_close ()完成了各種變數的清理工作。

雷霄驊
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020