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C++ Win32 API: save screen to bmp (256 colors problem)

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I hope that this is the right forum to ask this question.

I am trying to copy the screen and save it to a bmp file. The code does work, but not on a pc with 256 colors.

I tested the code on my Windows 7 and it works correctly.
When I tried this code on an older pc (Windows 2000) which has only desktop settings with 8-bit color, it failed.
I don't get any errors, but the image is corrupted. (I think that the file is too small: 768 kB for an image of 1024x768)

It is unimportant why, but I will not be able to debug the code on my older pc.
The only way to find out if everything works is to use messageboxes.
I have only tested one thing so far: cClrBits is 8 on my old pc.

Does anyone know why the code isn't working?
Are there perhaps functions in this code that don't work with 256 colors?

Thanks in advance.



// windows.h is included and _T(x) is defined here.
// The function prototype of WriteBMPFile is also here.

//////////////////////////////////////////////////////////////////////////////////////////////////////////////////

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{

int Width = GetSystemMetrics(SM_CXSCREEN);
int Height = GetSystemMetrics(SM_CYSCREEN);

HDC ScreenDC = GetDC(NULL);
HDC hDC = CreateCompatibleDC(ScreenDC);
HBITMAP hBitmap = CreateCompatibleBitmap(ScreenDC, Width, Height);
HBITMAP hOldBitmap = (HBITMAP)SelectObject(hDC, hBitmap);

if ( ! BitBlt(hDC, 0, 0, Width, Height, ScreenDC, 0, 0, SRCCOPY))
MessageBox(NULL, _T("BitBlt failed."), _T("PRINSCREEN PROGRAM: Error"), 0);

SelectObject(hDC, hOldBitmap);
DeleteDC(hDC);
ReleaseDC(NULL, ScreenDC);

WriteBMPFile(hBitmap, _T("SCREENSHOT.BMP"));

return 0;
}

//////////////////////////////////////////////////////////////////////////////////////////////////////////////////

void WriteBMPFile(HBITMAP bitmap, LPTSTR filename)
{
BITMAP bmp;
PBITMAPINFO pbmi;
WORD cClrBits;
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;

// create the bitmapinfo header information

if (!GetObject( bitmap, sizeof(BITMAP), (LPSTR)&bmp))
{
MessageBox(NULL, _T("Could not retrieve bitmap info."), _T("PRINSCREEN PROGRAM: Error"), 0);
return;
}

// 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.
if (cClrBits != 24)
pbmi = (PBITMAPINFO) LocalAlloc(LPTR, sizeof(BITMAPINFOHEADER) + sizeof(RGBQUAD) * (1<< cClrBits));
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.
pbmi->bmiHeader.biSizeImage = (pbmi->bmiHeader.biWidth + 7) / 8 * pbmi->bmiHeader.biHeight * cClrBits;

// Set biClrImportant to 0, indicating that all of the
// device colors are important.
pbmi->bmiHeader.biClrImportant = 0;

// now open file and save the data
pbih = (PBITMAPINFOHEADER) pbmi;
lpBits = (LPBYTE) GlobalAlloc(GMEM_FIXED, pbih->biSizeImage);

if (!lpBits)
{
MessageBox(NULL, _T("Could not allocate memory."), _T("PRINSCREEN PROGRAM: Error"), 0);
return;
}

HDC hDC = CreateCompatibleDC(NULL);

// Retrieve the color table (RGBQUAD array) and the bits
if (!GetDIBits(hDC, HBITMAP(bitmap), 0, (WORD) pbih->biHeight, lpBits, pbmi, DIB_RGB_COLORS)) // I also tested it with DIB_PAL_COLORS
{
MessageBox(NULL, _T("GetDIB error."), _T("PRINSCREEN PROGRAM: Error"), 0);
return;
}

// Create the .BMP file.
hf = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, (DWORD) 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, (HANDLE) NULL);
if (hf == INVALID_HANDLE_VALUE)
{
MessageBox(NULL, _T("Could not create file for writing"), _T("Error"), 0);
return;
}

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))
{
MessageBox(NULL, _T("Could not write in to file (BITMAPFILEHEADER)."), _T("PRINSCREEN PROGRAM: Error"), 0);
return;
}

// Copy the BITMAPINFOHEADER and RGBQUAD array into the file.
if (!WriteFile(hf, (LPVOID) pbih, sizeof(BITMAPINFOHEADER) + pbih->biClrUsed * sizeof (RGBQUAD), (LPDWORD) &dwTmp, ( NULL)))
{
MessageBox(NULL, _T("Could not write in to file (BITMAPINFOHEADER and RGBQUAD)."), _T("PRINSCREEN PROGRAM: Error"), 0);
return;
}

// 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))
{
MessageBox(NULL, _T("Could not write in to file (array of color indices)."), _T("PRINSCREEN PROGRAM: Error"), 0);
return;
}

// Close the .BMP file.
if (!CloseHandle(hf))
{
MessageBox(NULL, _T("Could not close file."), _T("PRINSCREEN PROGRAM: Error"), 0);
return;
}

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

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Do you have any objections to using GDI+? If not, then you may want to use construct a GDI+ Bitmap object for your screenshot and use the Save() member function to save to a file instead. It'd be a lot less code, and it'd allow you to change to saving alternate file types easily.

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I never used GDI+ before, but I don't have any objections to using it.

I will try it and see if it works that way.

Thanks for your help.

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