bPixelBits = (BYTE *) malloc (bmpFileHeader.bfSize - bmpFileHeader.bfOffBits);
の部分でbmpFileHeader.bfSize-bmFileHeader.bfOffBitsを引いたバイト数を確保してま
すが、この二つのメンバを引いてる意味を教えてください。
お願いします。

#include <windows.h>

BITMAPFILEHEADER bmpFileHeader;
BITMAPINFO bmpInfo;
BYTE * bPixelBits;

LRESULT CALLBACK WndProc(HWND hWnd , UINT msg , WPARAM wp , LPARAM lp) {

HDC hdc;
PAINTSTRUCT ps;

switch (msg) {
case WM_DESTROY:
free(bPixelBits);
PostQuitMessage(0);
return 0;

case WM_PAINT:
hdc = BeginPaint(hWnd , &ps);
SetDIBitsToDevice(
hdc , 0 , 0 ,
bmpInfo.bmiHeader.biWidth ,
bmpInfo.bmiHeader.biHeight ,
0 , 0 , 0 , bmpInfo.bmiHeader.biHeight ,
bPixelBits , &bmpInfo , DIB_RGB_COLORS
);

EndPaint(hWnd , &ps);
return 0;
}
return DefWindowProc(hWnd , msg , wp , lp);
}

int WINAPI WinMain(HINSTANCE hInstance , HINSTANCE hPrevInstance ,
PSTR lpCmdLine , int nCmdShow) {

HWND 　hWnd;
MSG　 msg;
WNDCLASS　 winc;
HANDLE hFile;
DWORD dwBytes;

hFile = CreateFile(lpCmdLine , GENERIC_READ , 0 , NULL ,
OPEN_EXISTING , FILE_ATTRIBUTE_NORMAL , NULL);
if (hFile == INVALID_HANDLE_VALUE) return 1;

ReadFile(hFile , &bmpFileHeader , sizeof (BITMAPFILEHEADER) ,
&dwBytes , NULL);
if (bmpFileHeader.bfType != 0x4D42) {
MessageBox(NULL , TEXT(This is not a bitmap file) , NULL ,
MB_OK);
return 1;
}

ReadFile(hFile , &bmpInfo , sizeof (BITMAPINFOHEADER) , &dwBytes ,
NULL);
bPixelBits = (BYTE *) malloc (bmpFileHeader.bfSize -
bmpFileHeader.bfOffBits);

ReadFile(hFile ,bPixelBits ,
bmpFileHeader.bfSize - bmpFileHeader.bfOffBits , &dwBytes ,
NULL);
CloseHandle(hFile);

winc.style = CS_HREDRAW | CS_VREDRAW;
winc.lpfnWndProc = WndProc;
winc.cbClsExtra = winc.cbWndExtra = 0;
winc.hInstance = hInstance;
winc.hIcon = LoadIcon(NULL , IDI_APPLICATION);
winc.hCursor = LoadCursor(NULL , IDC_ARROW);
winc.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
winc.lpszMenuName = NULL;
winc.lpszClassName = TEXT(KITTY);

if (!RegisterClass(&winc)) return 1;

hWnd = CreateWindow(
TEXT(KITTY) , TEXT(Kitty on your lap) ,
WS_OVERLAPPEDWINDOW | WS_VISIBLE ,
CW_USEDEFAULT , CW_USEDEFAULT ,
CW_USEDEFAULT , CW_USEDEFAULT ,
NULL , NULL , hInstance , NULL
);

if (hWnd == NULL) return 1;

while (GetMessage(&msg , NULL , 0 , 0 )) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}

return msg.wParam;
}