NULL pointer for bmBits after converting a bit array to HBITMAP

Question

I'm trying to convert a pixel array to a HBITMAP as instructed in this post: How to convert an Array of pixels to HBITMAP.

Basically, the array is converted to a HBITMAPand then verified by copied to the clipboard. However, when accessing the bit value from the BITMAP construct by its bmBits member, it returns NULL.

I might have something missing here. If a HBITMAP is created successfully, why we still get a NULL pointer to its bit values?

uint8 width = 160;
uint8 height = 120;

uint8* pixels = new uint8[160 * 120 * 4];
for (int i = 0; i < 160 * 120 * 4; i++){
    pixels[i] = (i % 4 == 1) * 255; // testing pixels
}

BITMAPINFOHEADER bmih;
bmih.biSize = sizeof(BITMAPINFOHEADER);
bmih.biWidth = width;
bmih.biHeight = -1 * height;
bmih.biPlanes = 1;
bmih.biBitCount = 32;
bmih.biCompression = BI_RGB;
bmih.biSizeImage = 0;
bmih.biXPelsPerMeter = 10;
bmih.biYPelsPerMeter = 10;
bmih.biClrUsed = 0;
bmih.biClrImportant = 0;

BITMAPINFO dbmi;
ZeroMemory(&dbmi, sizeof(dbmi));
dbmi.bmiHeader = bmih;
dbmi.bmiColors->rgbBlue = 0;
dbmi.bmiColors->rgbGreen = 0;
dbmi.bmiColors->rgbRed = 0;
dbmi.bmiColors->rgbReserved = 0;

HDC hdc = ::GetDC(NULL);

HBITMAP hbmp = CreateDIBitmap(hdc, &bmih, CBM_INIT, pixels, &dbmi, DIB_RGB_COLORS);
if (hbmp == NULL) {
    ::MessageBox(NULL, L"Could not load the desired image", L"Error", MB_OK);
    return NULL;
}

::ReleaseDC(NULL, hdc);

// a little test if everything is OK
OpenClipboard(NULL);
EmptyClipboard();
SetClipboardData(CF_BITMAP, hbmp); // I can verify the image by pasting
CloseClipboard();

// verify the bitmap
BITMAP bitmap;
::GetObject(hbmp, sizeof(BITMAP), &bitmap);
uint8* lpbits = (uint8*)bitmap.bmBits;

assert(lpbits != NULL); // Why this assertion failed??

// cleanup
// DeleteObject(hbmp);

call GetLastError to see why the call failed - at least you will get a clue — pm100, Aug 29 '17 at 00:14
@pm100: most GDI functions don't use `GetLastError()` for error reporting, and that is certainly the case for the GDI functions used in this example (only the clipboard functions do). — Remy Lebeau, Aug 29 '17 at 00:27
@RemyLebeau Fair enough. So no way to get anything other than 'well that didnt work'. However if some underlying thing failed (file error) then GetLastError will return something (although it could be a red herring) — pm100, Aug 29 '17 at 00:30
Despite its name, `CreateDIBitmap()` actually creates a DDB instead of a DIB. The reference tells that it once had a flag `CBM_CREATDIB` which is no longer supported. — zett42, Aug 29 '17 at 07:10

Remy Lebeau · Accepted Answer · 2017-08-29T00:26:38.850

The answer is in the GetObject() documentation:

If hgdiobj is a handle to a bitmap created by calling CreateDIBSection, and the specified buffer is large enough, the GetObject function returns a DIBSECTION structure. In addition, the bmBits member of the BITMAP structure contained within the DIBSECTION will contain a pointer to the bitmap's bit values.

If hgdiobj is a handle to a bitmap created by any other means, GetObject returns only the width, height, and color format information of the bitmap. You can obtain the bitmap's bit values by calling the GetDIBits() or GetBitmapBits() function.

This means that bmBits is populated only when querying a DIB bitmap for a DIBSECTION structure, which contains a BITMAP amongst other things.

You are querying a DDB bitmap for a BITMAP structure, so bmBits does not get populated, and you have to retrieve the pixel bits separately.

NULL pointer for bmBits after converting a bit array to HBITMAP

1 Answers1