Re: parsing bitmap files (2) - C / C++ / MFC Discussion Boards

Richard MacCutchan14-Mar-20 22:13

14-Mar-20 22:13

No, the second read will start from the byte following the position where the previous read stopped. That is normal file IO operations. It will only re-read the header if you rewind the file after the first operation.

Re: parsing bitmap files (2)

Calin Negru15-Mar-20 3:09

Calin Negru

15-Mar-20 3:09

This is my write attempt.

BITMAPFILEHEADER BFH;
	BITMAPINFOHEADER BIH;
	RGBQUAD Palette;
	RGBQUAD  * Pixels = new RGBQUAD[10];

	cf.Read( &BFH, sizeof( BITMAPFILEHEADER ));
	cf.Read( &BIH, sizeof( BITMAPINFOHEADER ));

	for(int i =0; i<10; i++)
	{
		cf.Read(&Pixels[i],sizeof(RGBQUAD));
	
	}
	
	StringCchPrintfA(message,1024,"b0 is %d", Pixels[0].rgbBlue);
	MessageBox(NULL, message, "Textures.exe", MB_OK);
	StringCchPrintfA(message,1024,"g0 is %d", Pixels[0].rgbGreen);
	MessageBox(NULL, message, "Textures.exe", MB_OK);
	StringCchPrintfA(message,1024,"r0 is %d", Pixels[0].rgbRed);
	MessageBox(NULL, message, "Textures.exe", MB_OK);
	
	StringCchPrintfA(message,1024,"r1 is %d", Pixels[1].rgbRed);
	MessageBox(NULL, message, "Textures.exe", MB_OK);
	StringCchPrintfA(message,1024,"g1 is %d", Pixels[1].rgbGreen);
	MessageBox(NULL, message, "Textures.exe", MB_OK);
	StringCchPrintfA(message,1024,"b1 is %d", Pixels[1].rgbBlue);
	MessageBox(NULL, message, "Textures.exe", MB_OK);


	CFile fileWrite;

	Pixels[0].rgbBlue = 100;
	Pixels[0].rgbGreen = 100;
	Pixels[0].rgbRed = 50;
	// Attempt to create the file.
	if( !fileWrite.Open( pszFilename,
		CFile::modeCreate | CFile::modeWrite ) )
		return( FALSE );

	fileWrite.Write( &BFH, sizeof( BITMAPFILEHEADER ) );
	fileWrite.Write( &BIH, sizeof( BITMAPINFOHEADER ) );
	for(int i =0;i<10; i++)
	{
		fileWrite.Write( &Pixels[i], sizeof( RGBQUAD ) );
	}

when I start my program again I expect the values of the first pixel to be updated but they aren`t

Re: parsing bitmap files (2)

Richard MacCutchan15-Mar-20 3:34

Richard MacCutchan

15-Mar-20 3:34

The code looks valid enough, are you sure the file is getting created correctly?

Re: parsing bitmap files (2)

Calin Negru15-Mar-20 4:05

Calin Negru

15-Mar-20 4:05

you`re right, if I try to create the file under a new name it works.

Re: parsing bitmap files (2)

Richard MacCutchan15-Mar-20 4:24

Richard MacCutchan

15-Mar-20 4:24

I have just tried a similar exercise and the changes are correctly made to the output file.

Re: parsing bitmap files (2)

Calin Negru15-Mar-20 7:02

Calin Negru

15-Mar-20 7:02

I will run into other C related issues, you`re not getting rid of me.

Re: parsing bitmap files (2)

Richard MacCutchan15-Mar-20 7:04

Richard MacCutchan

15-Mar-20 7:04

Don't worry, I have the freedom to ignore any messages that don't interest me.

Re: parsing bitmap files (2)

leon de boer15-Mar-20 0:25

leon de boer

15-Mar-20 0:25

You are confused there are two headers one after another you need to read them both before deciding whether to do step 3 and I have put a few hints what you need to do

1.) Read BITMAPFILEHEADER check for "BM", hold offset to bitmap data
2.) Read BITMAPINFOHEADER calc DWORD line stride width from bitmap width and bitdepth, see if you have a palette if so do step 3 else step 4
3.) Read the index palette if the BMP has one
4.) Jump to file offset from step 1 that is your bitmap data. Now you need to remember each line is DWORD (4 byte aligned) that means if you multiply the bytes per each line pixel * bitmap width it has to equal 4. So lets give you an example if you bitmap is 24Bit so each pixel is 3 bytes. So if your image was one pixel in width the first line is the first 3 bytes then there is a pad of one dummy byte and the second line starts at the 4th byte. So there can be 0 to 3 extra bytes per line because each new line must start on a DWORD boundary. So the amount of data you need to read for a line can be a couple bytes more than the just multiplying the bitmpap width * bytes per pixel.

The dummy bytes are usually zero but some smart people worked out long ago if you make all your images odd so you have the extra bytes you can put a hidden watermark, or play spy and put secret messsages in the dummy bytes Smile | :)

In vino veritas

Re: parsing bitmap files (2)

Calin Negru15-Mar-20 1:59

Calin Negru

15-Mar-20 1:59

Richard I have it working. Now on to saving.
phil, leon thanks guys.

c libraries needed to deal with bitmaps

Calin Negru11-Mar-20 6:59

Calin Negru

11-Mar-20 6:59

I fount a class to read and write bitmaps, it`s old source code that uses MFC. it uses CFile to read and write, when I try to compile I get a

Quote:
WINDOWS.H already included

message. Is there a way to work around the problem?

modified 11-Mar-20 13:07pm.

Re: c libraries needed to deal with bitmaps

Richard MacCutchan11-Mar-20 7:44

Richard MacCutchan

11-Mar-20 7:44

fearless_ wrote:
Is there a way to work around the problem?

Yes, remove the duplicate include statement.

Re: c libraries needed to deal with bitmaps

Calin Negru11-Mar-20 9:49

Calin Negru

11-Mar-20 9:49

thanks, with a simple tweak at headers it worked

Re: c libraries needed to deal with bitmaps

Calin Negru11-Mar-20 21:51

Calin Negru

11-Mar-20 21:51

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).

modified 12-Mar-20 4:05am.

Re: c libraries needed to deal with bitmaps

Richard MacCutchan11-Mar-20 22:35

Richard MacCutchan

11-Mar-20 22:35

Really. And we are supposed to guess what code and where the error occurs?

Re: c libraries needed to deal with bitmaps

Calin Negru11-Mar-20 23:27

Calin Negru

11-Mar-20 23:27

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 );
			}
		}

modified 12-Mar-20 8:41am.

Re: c libraries needed to deal with bitmaps

Richard MacCutchan11-Mar-20 23:36

Richard MacCutchan

11-Mar-20 23:36

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.

Re: c libraries needed to deal with bitmaps

Calin Negru12-Mar-20 0:18

Calin Negru

12-Mar-20 0:18

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.

Re: c libraries needed to deal with bitmaps

Richard MacCutchan12-Mar-20 2:31

Richard MacCutchan

12-Mar-20 2:31

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[^].

Re: c libraries needed to deal with bitmaps

Calin Negru12-Mar-20 4:40

Calin Negru

12-Mar-20 4:40

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

Re: c libraries needed to deal with bitmaps

Richard MacCutchan12-Mar-20 5:10

Richard MacCutchan

12-Mar-20 5:10

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

Re: c libraries needed to deal with bitmaps

Calin Negru13-Mar-20 3:22

Calin Negru

13-Mar-20 3:22

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

modified 13-Mar-20 14:06pm.

Re: c libraries needed to deal with bitmaps

Richard MacCutchan13-Mar-20 4:22

Richard MacCutchan

13-Mar-20 4:22

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>

C++

// 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.

Re: c libraries needed to deal with bitmaps

Calin Negru13-Mar-20 7:50

Calin Negru

13-Mar-20 7:50

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

Re: c libraries needed to deal with bitmaps

Richard MacCutchan13-Mar-20 8:07

Richard MacCutchan

13-Mar-20 8:07

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.

Re: c libraries needed to deal with bitmaps

Calin Negru13-Mar-20 21:07

Calin Negru

13-Mar-20 21:07

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?

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