Black image when creating a Bitmap from a selection of the screen in C++

Question

I am trying to get a screen capture of a selected region of the desktop.

The problem I am having is that the output (.bmp file) is just black.

I took CreateBitmapInfoStruct(); and CreateBMPFile(); from MSDN. The rest has been snips from the internet Frankenstein-ed together by me.

Here is what my code looks like presently:

#include <windows.h>
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

PBITMAPINFO CreateBitmapInfoStruct(HBITMAP hBmp)
{ 
    BITMAP bmp; 
    PBITMAPINFO pbmi; 
    WORD    cClrBits; 

    // Retrieve the bitmap color format, width, and height.  
    if (!GetObject(hBmp, sizeof(BITMAP), (LPSTR)&bmp)) 
        cout << "ERROR:1";

    // Convert the color format to a count of bits.  
    cClrBits = (WORD)(bmp.bmPlanes * bmp.bmBitsPixel); 
    if (cClrBits == 1) 
        cClrBits = 1; 
    else if (cClrBits <= 4) 
        cClrBits = 4; 
    else if (cClrBits <= 8) 
        cClrBits = 8; 
    else if (cClrBits <= 16) 
        cClrBits = 16; 
    else if (cClrBits <= 24) 
        cClrBits = 24; 
    else cClrBits = 32; 

    // Allocate memory for the BITMAPINFO structure. (This structure  
    // contains a BITMAPINFOHEADER structure and an array of RGBQUAD  
    // data structures.)  

     if (cClrBits < 24) 
         pbmi = (PBITMAPINFO) LocalAlloc(LPTR, 
                    sizeof(BITMAPINFOHEADER) + 
                    sizeof(RGBQUAD) * (1<< cClrBits)); 

     // There is no RGBQUAD array for these formats: 24-bit-per-pixel or 32-bit-per-pixel 

     else 
         pbmi = (PBITMAPINFO) LocalAlloc(LPTR, 
                    sizeof(BITMAPINFOHEADER)); 

    // Initialize the fields in the BITMAPINFO structure.  

    pbmi->bmiHeader.biSize = sizeof(BITMAPINFOHEADER); 
    pbmi->bmiHeader.biWidth = bmp.bmWidth; 
    pbmi->bmiHeader.biHeight = bmp.bmHeight; 
    pbmi->bmiHeader.biPlanes = bmp.bmPlanes; 
    pbmi->bmiHeader.biBitCount = bmp.bmBitsPixel; 
    if (cClrBits < 24) 
        pbmi->bmiHeader.biClrUsed = (1<<cClrBits); 

    // If the bitmap is not compressed, set the BI_RGB flag.  
    pbmi->bmiHeader.biCompression = BI_RGB; 

    // Compute the number of bytes in the array of color  
    // indices and store the result in biSizeImage.  
    // The width must be DWORD aligned unless the bitmap is RLE 
    // compressed. 
    pbmi->bmiHeader.biSizeImage = ((pbmi->bmiHeader.biWidth * cClrBits +31) & ~31) /8
                                  * pbmi->bmiHeader.biHeight; 
    // Set biClrImportant to 0, indicating that all of the  
    // device colors are important.  
     pbmi->bmiHeader.biClrImportant = 0; 
     return pbmi; 
 } 

void CreateBMPFile(LPTSTR pszFile, PBITMAPINFO pbi, 
                  HBITMAP hBMP, HDC hDC) 
 { 
     HANDLE hf;                 // file handle  
    BITMAPFILEHEADER hdr;       // bitmap file-header  
    PBITMAPINFOHEADER pbih;     // bitmap info-header  
    LPBYTE lpBits;              // memory pointer  
    DWORD dwTotal;              // total count of bytes  
    DWORD cb;                   // incremental count of bytes  
    BYTE *hp;                   // byte pointer  
    DWORD dwTmp; 

    pbih = (PBITMAPINFOHEADER) pbi; 
    lpBits = (LPBYTE) GlobalAlloc(GMEM_FIXED, pbih->biSizeImage);

    if (!lpBits) 
         cout << "ERROR:2"; 

    // Retrieve the color table (RGBQUAD array) and the bits  
    // (array of palette indices) from the DIB.  
    GetDIBits(hDC, hBMP, 0, (WORD) pbih->biHeight, lpBits, pbi, 
        DIB_RGB_COLORS);

    // Create the .BMP file.  
    hf = CreateFile(pszFile, 
                   GENERIC_READ | GENERIC_WRITE, 
                   (DWORD) 0, 
                    NULL, 
                   CREATE_ALWAYS, 
                   FILE_ATTRIBUTE_NORMAL, 
                   (HANDLE) NULL); 
    if (hf == INVALID_HANDLE_VALUE) 
        cout << "ERROR:4"; 
    hdr.bfType = 0x4d42;        // 0x42 = "B" 0x4d = "M"  
    // Compute the size of the entire file.  
    hdr.bfSize = (DWORD) (sizeof(BITMAPFILEHEADER) + 
                 pbih->biSize + pbih->biClrUsed 
                 * sizeof(RGBQUAD) + pbih->biSizeImage); 
    hdr.bfReserved1 = 0; 
    hdr.bfReserved2 = 0; 

    // Compute the offset to the array of color indices.  
    hdr.bfOffBits = (DWORD) sizeof(BITMAPFILEHEADER) + 
                    pbih->biSize + pbih->biClrUsed 
                    * sizeof (RGBQUAD); 

    // Copy the BITMAPFILEHEADER into the .BMP file.  
    if (!WriteFile(hf, (LPVOID) &hdr, sizeof(BITMAPFILEHEADER), 
        (LPDWORD) &dwTmp,  NULL)) 
    {
       cout << "ERROR:5";
    }

    // Copy the BITMAPINFOHEADER and RGBQUAD array into the file.  
    if (!WriteFile(hf, (LPVOID) pbih, sizeof(BITMAPINFOHEADER) 
                  + pbih->biClrUsed * sizeof (RGBQUAD), 
                  (LPDWORD) &dwTmp, ( NULL)))
        cout << "ERROR:6";

    // Copy the array of color indices into the .BMP file.  
    dwTotal = cb = pbih->biSizeImage; 
    hp = lpBits; 
    if (!WriteFile(hf, (LPSTR) hp, (int) cb, (LPDWORD) &dwTmp,NULL)) 
           cout << "ERROR:7";

    // Close the .BMP file.  
     if (!CloseHandle(hf)) 
           cout << "ERROR:8"; 

    // Free memory.  
    GlobalFree((HGLOBAL)lpBits);
}

void getBit(string name) {
    int nX = 300;
    int nX2 = 600;
    int nY = 300;
    int nY2 = 700;
     HDC hScrDC  =  GetDC(NULL); 
     HDC hMemDC  =  CreateCompatibleDC(hScrDC);
     int nWidth  =  nX2  -  nX; 
     int nHeight  =  nY2  -  nY; 
     HBITMAP hBitmap  =  CreateCompatibleBitmap(hScrDC,  nWidth,  nHeight); 
     HBITMAP hOldBitmap  =  (HBITMAP)SelectObject(hMemDC,  hBitmap); 

     BitBlt(hMemDC,  0,  0,  nWidth,  nHeight, 
     hScrDC,  nX,  nY,  SRCCOPY); 
     hBitmap  =  (HBITMAP)SelectObject(hMemDC,  hOldBitmap); 
     DeleteDC(hScrDC); 
     DeleteDC(hMemDC); 
    // now your image is held in hBitmap. You can save it or do whatever with it
    PBITMAPINFO pbi = CreateBitmapInfoStruct(hBitmap);
    //File name
    LPTSTR path = L"test.bmp";
    CreateBMPFile(path,pbi,hBitmap,hMemDC);
}

void main(void) {
    getBit("SAVE");
    cout << "Done";
    int wait;
    cin >> wait;
}

Answer 1

The biggest problem is that GetDIBits fails because you're passing a handle to a deleted memory context instead of a handle to a valid device context. If you fix that, it works.

When debugging, it's always a good idea to check all of the return values.

Other things I've noticed:

1. You should not delete hSrcDC, since you didn't create it. Use ReleaseDC, which is the companion to GetDC.
2. Consider using CAPTUREBLT in your BitBlt call.