Get a HBITMAP from an 24bit BMP file loaded into memory - winapi

Question

I have a 24bit BMP file loaded into RAM and I'm trying to create a HBITMAP for this image file. I have found some examples around which I've been experimenting with, but can't seem to make work. Basically, I need a HBITMAP for the file, so that I can unload the file and just keep the HBITMAP which I can dispose of later with DeleteObject(). Since this bitmap is loaded very early on in my application, there is no application Window and therefore no HDC. This is what I have so far:-
HBITMAP cBitmap; // This should be where my bitmap handle ends up.
mem; // This is a void* pointer to the loaded BMP file
tagBITMAPFILEHEADER bfh = *(tagBITMAPFILEHEADER*)mem;
tagBITMAPINFOHEADER bih = *(tagBITMAPINFOHEADER*)(mem + sizeof(tagBITMAPFILEHEADER));
RGBQUAD rgb = *(RGBQUAD*)(mem + sizeof(tagBITMAPFILEHEADER) + sizeof(tagBITMAPINFOHEADER));
BITMAPINFO bi;
bi.bmiColors[0] = rgb;
bi.bmiHeader = bih;
UINT8* pixels = mem + bfh.bfOffBits;
void* ppv;
HBITMAP hBitmap = CreateDIBSection(NULL, &bi, DIB_RGB_COLORS, &ppv, NULL, 0);
SetDIBits(NULL, hBitmap, 0, bih.biHeight, pixels, &bi, DIB_RGB_COLORS);
GetObject(hBitmap, sizeof(BITMAP), &cBitmap);
For some reason that just baffles me, cBitmap ends up being NULL. And another thing that puzzles me... DIB_RGB_COLORS means that the BITMAPINFO has a pointer to literal RGB values, but how does that work with a 24bit image, which doesn't have a palette?

Answer 1

If you are loading a file from disk, it's probably easiest to use LoadImage() with the LR_LOADFROMFILE flag.
To create a HBITMAP from file data that has already been loaded into memory, you can do something similar to the following:
HBITMAP ConvertDibToHBitmap(void* bmpData)
{
HBITMAP hBitmap = NULL;
BOOL success = FALSE;
// NOTE: Assumes the BITMAPFILEHEADER is present (not normally the case for
// an in-memory DIB)
LPBITMAPFILEHEADER bfh = (LPBITMAPFILEHEADER) bmpData;
LPBITMAPINFOHEADER bih = (LPBITMAPINFOHEADER) (bfh + 1);
void *pixels = (char*) (bih + 1); // NOTE: Assumes no color table (i.e., bpp >= 24)
HDC hdc = GetDC(NULL);
if (hdc != NULL) {
hBitmap = CreateCompatibleBitmap(hdc, bih->biWidth, bih->biHeight);
if (hBitmap != NULL) {
HDC hdcMem = CreateCompatibleDC(hdc);
if (hdcMem != NULL) {
HBITMAP hOldBitmap = (HBITMAP)SelectObject(hdcMem, hBitmap);
if (StretchDIBits(hdcMem, 0, 0, bih->biWidth, bih->biHeight,
0, 0, bih->biWidth, bih->biHeight, pixels,
(LPBITMAPINFO) bih, DIB_RGB_COLORS, SRCCOPY) > 0)
success = TRUE;
SelectObject(hdcMem, hOldBitmap);
DeleteDC(hdcMem);
}
}
ReleaseDC(NULL, hdc);
}
if (!success && hBitmap != NULL) {
DeleteObject(hBitmap);
hBitmap = NULL;
}
return hBitmap;
}

Answer 2

Um, that's not how GetObject works. The usage is
GetObject(handle, sizeof(object), &object);
In your case, the expectation is that you call it as
BITMAP bitmap;
GetObject(hBitmap, sizeof(BITMAP), &bitmap);
You are not passing a pointer to a BITMAP structure, so the behavior is undefined.
You don't need GetObject at all. You already have your HBITMAP. It's in the hBitmap variable.
cBitmap = hBitmap;