How do I get the byte representation of a BITMAPFILEHEADER and BITMAPINFOHEADER? - c++

Question

I'm trying to take a screenshot of my computer screen using code, and then store the BYTEs of that image into an array rather than save the image right away using the following function:
void WINAPI CaptureScreenIntoByteArray(
BYTE*& screen_bytes,
DWORD& screen_bytes_size)
{
BITMAPFILEHEADER bfHeader;
BITMAPINFOHEADER biHeader;
BITMAPINFO bInfo;
HGDIOBJ hTempBitmap;
HBITMAP hBitmap;
BITMAP bAllDesktops;
HDC hDC, hMemDC;
LONG lWidth, lHeight;
BYTE* sb = NULL;
ZeroMemory(&bfHeader, sizeof(BITMAPFILEHEADER));
ZeroMemory(&biHeader, sizeof(BITMAPFILEHEADER));
ZeroMemory(&bInfo, sizeof(BITMAPINFO));
ZeroMemory(&bAllDesktops, sizeof(BITMAP));
hDC = GetDC(NULL);
hTempBitmap = GetCurrentObject(hDC, OBJ_BITMAP);
GetObjectW(hTempBitmap, sizeof(BITMAP), &bAllDesktops);
lWidth = bAllDesktops.bmWidth;
lHeight = bAllDesktops.bmHeight;
DeleteObject(hTempBitmap);
bfHeader.bfType = (WORD)('B' | ('M' << 8));
bfHeader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
biHeader.biSize = sizeof(BITMAPINFOHEADER);
biHeader.biBitCount = 24;
biHeader.biCompression = BI_RGB;
biHeader.biPlanes = 1;
biHeader.biWidth = lWidth;
biHeader.biHeight = lHeight;
bInfo.bmiHeader = biHeader;
screen_bytes_size = (((24 * lWidth + 31) & ~31) / 8) * lHeight;
hMemDC = CreateCompatibleDC(hDC);
hBitmap = CreateDIBSection(hDC, &bInfo, DIB_RGB_COLORS, (VOID**)&sb, NULL, 0);
SelectObject(hMemDC, hBitmap);
int x = GetSystemMetrics(SM_XVIRTUALSCREEN);
int y = GetSystemMetrics(SM_YVIRTUALSCREEN);
BitBlt(hMemDC, 0, 0, lWidth, lHeight, hDC, x, y, SRCCOPY);
// Need to also copy bfHeader & biHeader bytes somehow...
screen_bytes = new BYTE[screen_bytes_size];
std::copy(sb, sb + screen_bytes_size, screen_bytes);
DeleteDC(hMemDC);
ReleaseDC(NULL, hDC);
DeleteObject(hBitmap);
}
My specific issue is that I need to have the bfHeader and biHeader saved into the byte array also (so that later on I can save the image as a file)... So this is the problem area...
// Need to also copy bfHeader & biHeader bytes somehow...
screen_bytes = new BYTE[screen_bytes_size];
std::copy(sb, sb + screen_bytes_size, screen_bytes);
How can I also copy the bfHeader and biHeader bytes into the array?
If needed, this is how you call the function...
BYTE* screen_bytes = NULL;
DWORD screen_bytes_size = 0;
CaptureScreenIntoByteArray(screen_bytes, screen_bytes_size);

Answer 1

You need to make the array big enough for the two structures as well.
screen_bytes = new BYTE[sizeof(BITMAPFILEHEADER) + sizeof (BITMAPINFOHEADER) + screen_bytes_size];
Then you can copy the two structures to the front of the array:
std::copy(&bfHeader, &bfHeader + 1, (BITMAPFILEHEADER*)screen_bytes);
std::copy(&biHeader, &biHeader + 1, (BITMAPINFOHEADER*)(screen_bytes + sizeof(BITMAPFILEHEADER)));
And copy the image data after the two structures.
std::copy(sb, sb + screen_bytes_size, screen_bytes + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER));