Windows客户端开发--截图并保存为JPG文件

以你之姓@ 2022-09-25 14:21 182阅读 0赞

### **JPG和PNG的区别：** ###

PNG is a true color lossless format. In practice it can accomplish a compression on standard photos of a factor of 2-3x with no quality loss with respect to the raw bitmap data.  
JPEG is a true color lossy format. Depending on the settings the user can determine the tradeoff between quality and amount of compression, which can usually achieve a compression ratio well above 10x.

### **何为GetEncoderClsid** ###

CLSID是指windows系统对于不同的应用程序，文件类型，OLE对象，特殊文件夹以及各种系统组件分配一个唯一表示它的ID代码，用于对其身份的标示和与其他对象进行区分。

msdn上有例子：  
[https://msdn.microsoft.com/en-us/library/windows/desktop/ms533843(v=vs.85).aspx][https_msdn.microsoft.com_en-us_library_windows_desktop_ms533843_v_vs.85_.aspx]

The function GetEncoderClsid in the following example receives the [MIME][] type of an encoder and returns the class identifier (CLSID) of that encoder. The MIME types of the encoders built into Windows GDI+ are as follows:

image/bmp
    image/jpeg
    image/gif
    image/tiff
    image/png

介绍类和一些方法：

**ImageCodecInfo类**  
The ImageCodecInfo class provides the necessary storage members and methods to retrieve all pertinent information about the installed image encoders and decoders (called codecs). Not inheritable.

**Clsid方法**  
Gets or sets a Guid structure that contains a GUID that identifies a specific codec.

**GetImageEncodersSize**  
The GetImageEncodersSize function gets the number of available image encoders and the total size of the array of ImageCodecInfo objects that is returned by the GetImageEncoders function.

**GetImageEncoders**  
The GetImageEncoders function gets an array of ImageCodecInfo objects that contain information about the available image encoders.

int GetEncoderClsid(const WCHAR* format, CLSID* pClsid)
    {
        UINT num = 0;          // number of image encoders
        UINT size = 0;         // size of the image encoder array in bytes
    
        ImageCodecInfo* pImageCodecInfo = NULL;
    
        GetImageEncodersSize(&num, &size);
        if(size == 0)
        {
            return -1;  // Failure
        }
    
        pImageCodecInfo = (ImageCodecInfo*)(malloc(size));
        if(pImageCodecInfo == NULL)
        {
            return -1;  // Failure
        }
    
        GetImageEncoders(num, size, pImageCodecInfo);
    
        for(UINT j = 0; j < num; ++j)
        {
            if( wcscmp(pImageCodecInfo[j].MimeType, format) == 0 )
            {
                *pClsid = pImageCodecInfo[j].Clsid;
                free(pImageCodecInfo);
                return j;  // Success
            }    
        }
    
        free(pImageCodecInfo);
        return -1;  // Failure
    }

### **bitmap转为jpg** ###

先介绍几个函数：

**Bitmap::FromHBITMAP**  
The Bitmap::FromHBITMAP method creates a Bitmap object based on a handle to a Windows Graphics Device Interface (GDI) bitmap and a handle to a GDI palette.

**Bitmap的save方法**  
Saves this Image to the specified file, with the specified encoder and image-encoder parameters.

void BitmapToJpg(HBITMAP hbmpImage, int width, int height)
    {
        Bitmap *p_bmp = Bitmap::FromHBITMAP(hbmpImage, NULL);
    
        CLSID jpgClsid;
        int result = GetEncoderClsid(L"image/jpeg", &jpgClsid);
        if(result != -1)
            std::cout << "Encoder succeeded" << std::endl;
        else
            std::cout << "Encoder failed" << std::endl;
        p_bmp->Save(L"screen.jpg", &pngClsid, NULL);
        delete p_bmp;
    }

### **ScreenCapture** ###

先介绍几个函数：

**CreateCompatibleDC**  
The CreateCompatibleDC function creates a memory device context (DC) compatible with the specified device.

**CreateCompatibleBitmap**  
The CreateCompatibleBitmap function creates a bitmap compatible with the device that is associated with the specified device context.

**BitBlt**  
The BitBlt function performs a bit-block transfer of the color data corresponding to a rectangle of pixels from the specified source device context into a destination device context.

bool ScreenCapture(int x, int y, int width, int height)
    {
        HDC hDc = CreateCompatibleDC(0);
        HBITMAP hBmp = CreateCompatibleBitmap(GetDC(0), width, height);
        SelectObject(hDc, hBmp);
        BitBlt(hDc, 0, 0, width, height, GetDC(0), x, y, SRCCOPY);
    
        BitmapToJpg(hBmp, width, height);
    
        DeleteObject(hBmp);
        DeleteDC(hDc);
        return true;
    }

[https_msdn.microsoft.com_en-us_library_windows_desktop_ms533843_v_vs.85_.aspx]: https://msdn.microsoft.com/en-us/library/windows/desktop/ms533843%28v=vs.85%29.aspx
[MIME]: http://blog.csdn.net/wangshubo1989/article/details/50890080