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Accessing DIBs at the pixel level using RWxxxx functions

The RWxxxx function group in Leonardo provides you with an easy way to read, write and work pixel-by-pixel with DIB data. Leonardo is particularly well-suited to, and recommended for, working with 48 bit and CMYK image formats.


Opening a DIB for read/write operations

In order to process DIB data at the pixel level at high speeds, RWOpenDIB caches supplementary information about the DIB. This requires that a HDIBRW cache handle must first be assigned from Leonardo as follows:

Read/write of individual pixels

The calling application can now address the pixels in the DIB. The following functions are available for access to DIBs at the pixel level:

Read/write of entire DIB lines

Even faster access to individual pixels can be achieved by working with a whole DIB line, or by working with parts of the line as array of RGBQUAD16 structures. For this purpose the application needs to allocate an array of RGBQUAD16 structures.
#define BLOCK_SIZE	256

RGBQUAD16            argbq[BLOCK_SIZE];
UINT                 x, i;

DIBInfo(hDib, &bmi, sizeof(bmi));

for (x=0; x<bmi.biWidth; ++x)
   RWGetLineRGB16(hRW, x, y, BLOCK_SIZE, argbq);

   for (i=0; i<BLOCK_SIZE; ++i)
      argbq[i].rgbq16Red   = 65535 - argbq[i].rgbq16Red
      argbq[i].rgbq16Green = 65535 - argbq[i].rgbq16Green
      argbq[i].rgbq16Blue  = 65535 - argbq[i].rgbq16Blue

   RWSetLineRGB16(hRW, x, y, BLOCK_SIZE, argbq);

Closing the DIB

When no further access to the DIB is required, the calling application must free the DIB access handle obtained by the call to RWOpenDIB by calling the function RWClose

Performance considerations

Routines in the RWxxxx function group are optimized for speed, but they can't achieve the same performance as direct memory access by the application.

Especially slow functions are setting of pixels by RGB values if the DIB has a color palette (biBitCount=4 or biBitCount=8 or biBitCount=1 with colors not beeing black and white). In this case, the function RWSetPixelRGB16 first has to determine the best color palette index for the RGB value, a very time-consuming process. Once the index value for a RGB-Color has been determined, it is cached in a 256 KByte memory block.

For two especially important cases Leonardo contains optimized code to speed up processing: