Re: Attempting to access 1 bit bitmap raw pixel data

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1) Per the docs the bits are located as follows:
char* pBits = (char*)((PBYTE)infoHeader + infoHeader.bfOffBits);

2) You need to read the color table.
When you unpack the byte, you get an index
into the color table that represents the color.
The colors are not mandated to be black and white.
And they don't have to be packed as index[0] = black
and index[1] = white.

3) Your buffer needs to hold WORD_ALIGNED_BYTES(width) * height bytes
For your 2x2 bitmap that's 8 bytes.

4) You loop through each scanline
for ( int i = 0; i < height; i++ ) // 2 scan lines
{
// Find the byte on which this scanline begins
DWORD ByteIndex = (height - i - 1) * WORD_ALIGNED_BYTES(width);
// loop through one WORD aligned scanline
for ( int j = 0; j < WORD_ALIGNED_BYTES(width); j++ ) // rowstride
= 4bytes
for ( int k = 0; k < 8; k++ ) // packed as 8 pixels per byte
{
// locate the bit
BYTE BitNumber = (BYTE)( 7 - (k % 8) );
// get the color index
int Index = (pBits[j] << BitNumber) & 0x80 ? 1 : 0;
// look up the color in the color table
COLORREF color = RGB(pbmi->bmiColors[Index].rgbRed,
pbmi->bmiColors[Index].rgbGreen,
pbmi->bmiColors[Index].rgbBlue);
}
}

<SteveKlett@xxxxxxxxx> wrote in message
news:1126113135.191887.289410@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> OK, making progress, but still not there. This could be the last ?
> though(I hope).
>
> After looping through and checking each pixel of the 4 pixel bitmap, I
> expected to see only 1 of the 4 pixels to be different(my bitmap looks
> like this:
> [X][ ]
> [ ][ ]
> Where the X is a black pixel)
>
> But instead, 2 seem to be black. Anyway, I'm stuck(again) and hoped to
> paste code so that someone might point out my error.
>
> <code>
> void GetBitmapBitsUglyVersion(CString& filename)
> {
> #define WORD_ALIGNED_BYTES(width)((((width-1)>>5)+1)<<2)
>
>
> CFile file(filename, CFile::modeRead);
> unsigned long long length = file.GetLength();
>
> char* pBuffer = new char[length+1];
> char* pDataPtr = pBuffer;
>
> file.Read(pBuffer, length);
> file.Close();
>
> PBITMAPFILEHEADER fileHeader = (PBITMAPFILEHEADER)pDataPtr;
> pDataPtr += sizeof(BITMAPFILEHEADER);
>
> PBITMAPINFOHEADER infoHeader = (PBITMAPINFOHEADER)pDataPtr;
> unsigned int width = infoHeader->biWidth;
> unsigned int height = infoHeader->biHeight;
> unsigned int bitCount = infoHeader->biBitCount;
> unsigned short numPlanes = infoHeader->biPlanes;
>
> pDataPtr+= sizeof(BITMAPINFOHEADER);
> pDataPtr+=(numPlanes+1)*sizeof(RGBQUAD); // skip the color table
> (it's B&W)
>
> char* pBits = pDataPtr;
>
>
> char buffer[128];
>
> for(int i=0; i < width; i++)
> {
> for(int j=0; j < height; j++)
> {
> // Find the byte on which this scanline begins
> DWORD ByteIndex = (height - j - 1) * WORD_ALIGNED_BYTES(width);
> // Find the byte containing this pixel
> ByteIndex += (i >> 3);
> // Which byte is it?
> BYTE BitNumber = (BYTE)( 7 - (i % 8) );
> // get the color index
> int Index = (pBits[ByteIndex] << BitNumber) & 0x80 ? 1 : 0;
> if(Index)
> {
> sprintf(buffer, "x:%d, y:%d is high\r\n", i, j);
> OutputDebugString(buffer);
> }
> }
> }
>
> if(pBuffer)
> delete[] pBuffer;
> }
> </code>
>
> Hopefully someone will see the problem and hopefully it will be a basic
> one ;)
>
> Thanks for the help!
> -Steve
>


.

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