c libraries needed to deal with bitmaps

Victor Nijegorodov

fearless_ wrote:

Would you mind helping me further? I loaded the bitmap, however when I check the values from the array into which the bitmap is loaded they show up as 0 (null).

1. How did you load the bitmap? 2. How did you "check the values from the array into which the bitmap is loaded"? 3. Did you try to open this bitmap with some graphic editor/viewer?

Calin Negru

here is my code

StringCchPrintfA(pszFilename,1024,"hello.bmp");
// Attempt to open the Dib file for reading.
if( !cf.Open( pszFilename, CFile::modeRead ) )
{
StringCchPrintfA(message,1024,"error reading file");
MessageBox(NULL, message, "Textures.exe", MB_OK);
return( FALSE );
}

// Get the size of the file and store
// in a local variable. Subtract the
// size of the BITMAPFILEHEADER structure
// since we won't keep that in memory.
DWORD dwDibSize= cf.GetLength() - sizeof( BITMAPFILEHEADER );
int bsize =0;
//dwDibSize =
bsize=	cf.GetLength() - sizeof( BITMAPFILEHEADER );
StringCchPrintfA(message,1024,"bitmap size  %d",bsize);
MessageBox(NULL, message, "Textures.exe", MB\_OK);

// Attempt to allocate the Dib memory.
unsigned char \*pDib;
pDib = new unsigned char \[dwDibSize\];
if( pDib == NULL )
{
		StringCchPrintfA(message,1024,"error allocating array ");
			MessageBox(NULL, message, "Textures.exe", MB\_OK);
			return( FALSE );
}
	

BITMAPFILEHEADER BFH;

// Read in the Dib header and data.
try{

	// Did we read in the entire BITMAPFILEHEADER?
	if( cf.Read( &BFH, sizeof( BITMAPFILEHEADER ) )
		!= sizeof( BITMAPFILEHEADER ) ||

		// Is the type 'MB'?
		BFH.bfType != 'MB' ||

		// Did we read in the remaining data?
		cf.Read( pDib, dwDibSize ) != dwDibSize )
		{
			StringCchPrintfA(message,1024,"error processing the file ");
			MessageBox(NULL, message, "Textures.exe", MB\_OK);
		// Delete the memory if we had any
		// errors and return FALSE.
		delete \[\] pDib;
		return( FALSE );
		}
	}

Lost User

And what is the problem and where does it occur? And please use proper <pre> tags so your code is formatted properly. The code button at the top of the edit box provides proper formatting for most languages.

Calin Negru

At the end of code posted above I try to do this:

StringCchPrintfA(message,1024,"dib at 34 %d",pDib[34]);
MessageBox(NULL, message, "Textures.exe", MB_OK);

I expect pDib[34] to be 255 since the bitmap is 10 white pixels ( in a row) 40 bytes in total.

Calin Negru

Richard I appreciate your willingness to help

Calin Negru

Victor thanks for offering to help, take a look at my reply to Richard

Lost User

I think you are missing something in your code. You read the BITMAPFILEHEADER but then assume that the remaining data is the actual bitmap, whereas the BITMAPFILEHEADER is followed by another header as described at BITMAPFILEHEADER (wingdi.h) - Win32 apps | Microsoft Docs[^].

Calin Negru

yeah I was a bit in a hurry, I was expecting to find pixels right after the first header.

Lost User

It is all explained at Bitmap Header Types - Win32 apps | Microsoft Docs[^]. But take special care of the different header types that may exist.

leon de boer

You are messing around with stuff you do not have to do on Windows and there is no advantage in doing what you are doing other than learning. You can load a bitmap with one line of code using the API LoadBitmap() LoadBitmapA function (winuser.h) - Win32 apps | Microsoft Docs[^] One line of code will load it for you and it doesn't care what the bitmap format is.

HBITMAP MyBmp = LoadBitmap(0, "Yourbitmapname.bmp");

Once you have a HBITMAP (handle to a bitmap) in Windows you can basically do everything with it. If you want the details from the bitmap once you have it loaded you just ask windows to extract them for you So with my handle above I can extract the header with 2 lines of code the 3rd line is just to throw the details out (it assumes you are doing a console app).

BITMAP bm;
GetObject(MyBmp, sizeof(bm), &bm);
printf("Bitmap is wth: %u ht: %u bitdepth: %u\r\n", bm.bmWidth, bm.bmHeight, bm.bmBitsPixel);

So I can load the bitmap and extract the header with 3 lines in absolute safety. If you are messing around with the actual header etc in Windows you are doing it all wrong :-) The only time you might play with a BMP file header is writing a BMP in a specific format. That is why there is no need for libraries on Windows to handle bitmaps. Now JPEGS are a little trickier it's about 20 lines of code to get windows to load one of those most of the issue the filename has to be in UNICODE. Let me know if you need to know how to do it.

In vino veritas

Victor Nijegorodov

leon de boer wrote:

One line of code will load it for you and it doesn't care what the bitmap format is.

HBITMAP MyBmp = LoadBitmap(0, "Yourbitmapname.bmp");

You probaly meant the [LoadImageA function (winuser.h) - Win32 apps | Microsoft Docs](https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-loadimagea) function that can load the bitmap from the file. LoadBitmap only loads the bitmap from resources.

leon de boer

Yep sorry needed more coffee last night, clearly :-)

In vino veritas

Calin Negru

I was only using a part of the bitmap processing code I was using as reference. I don`t need the palette part so I trimmed out a bit too much.

m_pDib = pDib;
m_dwDibSize = dwDibSize;

// Pointer our BITMAPINFOHEADER and RGBQUAD
// variables to the correct place in the Dib data.
m\_pBIH = (BITMAPINFOHEADER \*) m\_pDib;
m\_pPalette =
	(RGBQUAD \*) &m\_pDib\[sizeof(BITMAPINFOHEADER)\];

//	get image width and height
m\_width = m\_pBIH->biWidth;
m\_height = m\_pBIH->biHeight;

// Calculate the number of palette entries.
m\_nPaletteEntries = 1 << m\_pBIH->biBitCount;
if( m\_pBIH->biBitCount > 8 )
	m\_nPaletteEntries = 0;
else if( m\_pBIH->biClrUsed != 0 )
	m\_nPaletteEntries = m\_pBIH->biClrUsed;

// Point m\_pDibBits to the actual Dib bits data.
m\_pDibBits =
	&m\_pDib\[sizeof(BITMAPINFOHEADER)+
		m\_nPaletteEntries\*sizeof(RGBQUAD)\];

// If we have a valid palette, delete it.
if( m\_Palette.GetSafeHandle() != NULL )
	m\_Palette.DeleteObject();

I know I must go past the bitmapinfoheader to get to the pixels

Lost User

Please use proper <pre> tags around code blocks (and proper indentation), not Quoted text. The following code sample is preceded by <pre lang="c++"> and followed by </pre>

// Calculate the number of palette entries.
m_nPaletteEntries = 1 << m_pBIH->biBitCount;
if( m_pBIH->biBitCount > 8 )
m_nPaletteEntries = 0;
else if( m_pBIH->biClrUsed != 0 )
m_nPaletteEntries = m_pBIH->biClrUsed;

Much more readable.

Calin Negru

I`m having trouble understanding how this code works. The problem is I`m not a c expert.

cf.Read( &BFH, sizeof( BITMAPFILEHEADER )

For instance how does the portion of data read from my file fits into a structure. It`s not a structure of identical items BITMAPFILEHEADER is a mix of WORDs and DWORDS

Lost User

The sizeof operator gives the number of bytes in the object referred to. So in your case it will be the size of a BITMAPFILEHEADER, and the read operation will read the correct number of bytes into the structure. When complete each field of the structure should be correctly aligned*. *There are conditions where this may not be true, so beware if you have a structure containing single byte elements. You may ignore that for the moment.

Calin Negru

thanks that makes sense. In the code above do I need m_nPaletteEntries at all? Is used to calculate the offset at which individual pixels are stored?

Lost User

You need to examine the different items in the info table as described at BITMAPINFOHEADER structure (Windows) | Microsoft Docs[^]. The values of the items related to colours will tell you whether a colour table is included or not.

Calin Negru

I have to take a short break. Your insight (last post included) is priceless Richard, thanks

`DreamLand Page` is my projects facebook page.

Lost User

Sorry to disappoint you, but my 'insight' is actually just a matter of reading the documentation.