Addressing pixels in a monochrome DIB (329004)

MORE INFORMATION

• Locate the scanline of the pixel in the surface bits by taking the Y coordinate of the pixel and multiplying it by the DWORD aligned scanline width.
• Locate the byte that contains the pixel in the scanline by taking the X coordinate and dividing it by 8.
• Locate the bit in the byte by taking the X coordinate and moding it by 8 and subtracting it from 7 (to account for the bit ordering).

/*******************************************************************\ 
*                                                                   *
* BOOL TogglePixel(HBITMAP hbm, int x, int y);                      *
*                                                                   *
* Toggles a pixel in a monochrome DIB section                       *
*                                                                   *
* Parameters:                                                       *
*   hbm - handle to a DIB section                                   *
*   x   - X coordinate of the pixel to toggle                       *
*   y   - Y coordinate of the pixel to toggle                       *
*                                                                   *
* Notes:                                                            *
*   This function only works with DIB sections because there is no  *    
*   way to get a pointer to the surface bits of a device dependent  *
*   bitmap.                                                         *
*                                                                   *
\*******************************************************************/ 
BOOL TogglePixel(HBITMAP hbm, int x, int y)
{
    BITMAP  bm;
    LPBYTE  pByte;
    int     iByte, iBit;

    // Retrieve the surface information.
    if (!GetObject(hbm, sizeof(bm), &bm))
        return FALSE;  // Bail, it's not a valid HBITMAP

    // Check the bit depth of the surface.
    if (bm.bmPlanes * bm.bmBitsPixel != 1)
        return FALSE;  // Bail, it's not monochrome 

    // Get a BYTE pointer to the surface bits.
    pByte = (LPBYTE)bm.bmBits;
    
    // Make sure you have a DIB section (per Q187539).
    if (!pByte)
        return FALSE;  // Bail, it's not a DIB section

    // Make sure the coordinates are valid.
    if ((x < 0) || (y < 0) || (x >= bm.bmWidth) || (y >= bm.bmHeight))
        return FALSE;  // Coordinate is not within the surface  
    
    // Get to the scanline you want.
    pByte += ALIGNULONG(bm.bmWidthBytes) * (bm.bmHeight - y - 1);

    // Find the Byte and Bit you need to set.
    iByte = x >> 3;       // Divide by 8 to locate the byte 
    iBit  = 7 - (x & 7);  // Mod by 8 to locate the bit within the byte

    // Adjust the bits pointer to point to the right byte.
    pByte += iByte;

    // Flip the bit.
    *pByte ^= (1 << iBit);

    return TRUE;
}