Setting the palette on a BITMAPINFO struct

Hi,
I'm trying to code an 8-bit image loader/displayer, and I'm trying to fill out the bmiColors of the BITMAPINFO struct for use with the CreateDIBitmap function. However, the bmiColors array has only 1 entry instead of 256, so how do I get my RGBQUAD array of 256 entries to be "attached" to it?

> so how do I get my RGBQUAD array of 256 entries to be "attached" to it?

The array is represented by an area of memory immediately after the header, thus:

LPBITMAPINFO lpbi = (LPBITMAPINFO)malloc(sizeof(BITMAPINFOHEADER) + 256 * sizeof(RGBQUAD));

You can then fill in the bmiColors array as if it were 256 entries long.

My code currently looks like this:-

Code:

HBITMAP ReadFrame(unsigned char *Image, unsigned char Palette[3][256], HPALETTE hPalReturn)
{
	int i;
	LPBITMAPINFO lpbmpinfo = (LPBITMAPINFO)malloc(sizeof(BITMAPINFOHEADER) + 256 * sizeof(RGBQUAD));
	HBITMAP hBitmap;
	HDC hScreenDC;
	HPALETTE hPal, hOldPal;

	for(i=0;i<256;i++)
	{
		lpbmpinfo->bmiColors[i].rgbRed = Palette[0][i];
		lpbmpinfo->bmiColors[i].rgbGreen = Palette[1][i];
		lpbmpinfo->bmiColors[i].rgbBlue = Palette[2][i];
		lpbmpinfo->bmiColors[i].rgbReserved = 0;
	}
	lpbmpinfo->bmiHeader.biSize = sizeof(lpbmpinfo->bmiHeader);
	lpbmpinfo->bmiHeader.biBitCount = 8;
	lpbmpinfo->bmiHeader.biCompression = BI_RGB;
	lpbmpinfo->bmiHeader.biHeight = -240;
	lpbmpinfo->bmiHeader.biPlanes = 1;
	lpbmpinfo->bmiHeader.biWidth = 320;
	hScreenDC = GetDC(NULL);
	hPal = CreateDIBPalette(lpbmpinfo);
	hOldPal = SelectPalette(hScreenDC, hPal, FALSE);
	RealizePalette(hScreenDC);
	hBitmap = CreateDIBitmap(hScreenDC, &lpbmpinfo->bmiHeader, CBM_INIT, Image, lpbmpinfo, DIB_RGB_COLORS);
	SelectPalette(hScreenDC, hOldPal, FALSE);
	ReleaseDC(NULL, hScreenDC);
	hPalReturn = hPal;

	return hBitmap;
}

CreateDIBPalette simply makes a GDI palette object from the palette stored in lpbmpinfo.

However, when I run this, CreateDIBitmap does not appear to work and an Access Violation error occurs. I'm porting this code from VB, and it works there (although not without modification to the BITMAPINFO struct to have 256 entries (as there is no pointer support)). I'm stumped...

Well, I note that you're not clearing the BITMAPINFOHEADER to zero, so things like biSizeImage will be unknown. Either use calloc or memset.

Other than that, check to make sure the pointer to the bitmap data is valid and points to the right quantity of data.

I ZeroMemory'ed the lpbmpinfo->bmiHeader struct after, to no avail.
However, when I commented out the "hPal = CreateDIBPalette(lpbmpinfo);" line, it worked fine. Looks like I have a problem somewhere here:-

Code:

HPALETTE CreateDIBPalette(LPBITMAPINFO lpbmi)
{
    int i;
	LOGPALETTE pal;
    
    pal.palVersion = 0x300;
    pal.palNumEntries = 256;
    for (i=0;i<256;i++)
	{
        pal.palPalEntry[i].peRed = lpbmi->bmiColors[i].rgbRed;
        pal.palPalEntry[i].peGreen = lpbmi->bmiColors[i].rgbGreen;
        pal.palPalEntry[i].peBlue = lpbmi->bmiColors[i].rgbBlue;
        pal.palPalEntry[i].peFlags = PC_NOCOLLAPSE;
	}
    return CreatePalette(&pal);
}

Oh, silly me. I assumed you'd used a debugger and therefore knew it was CreateDIBitmap that crashed.

LOGPALETTE uses the same technique for a variable-length palette:

Code:

LPLOGPALETTE lpPalette = (LPLOGPALETTE)malloc(sizeof(LOGPALETTE) + 256 * sizeof(PALETTEENTRY));

Bah, that function was left VB-style and I didn't even notice...
It works now, thanx

P.S.: I did use a debugger, but skirted round the parts I thought were right. I suppose learning how to debug properly is half the battle for solid code :P