Black image when creating a Bitmap from a selection of the screen in C++ - 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:
You should not delete hSrcDC, since you didn't create it. Use ReleaseDC, which is the companion to GetDC.
Consider using CAPTUREBLT in your BitBlt call.