Victor Image Processing Library How-to Tips

Improve Scanned Text by Scale to Gray Image Processing

Scanned image of text

Text processed by Scale to Gray technique

In this example the original image containing the text is converted from bilevel to grayscale and the text is gently blurred around the edges. This Scale to Gray technique is also called "antialiasing."

The steps in Scale to Gray are:

• determine how large the image containing the text is
• allocate space for a grayscale version of the image
• convert from bilevel to grayscale
• antialias the text by gently blurring it around the edges
  
  For a fast display of a very large image use the convert1bitto8bitsmooth function to combine conversion and antialias into one step.
  
  Or use matrix convolution with a kernel of

                 1  2  1
                 2  8  2
                 1  2  1
  

  and divisor equal to 20 (that's the sum of the kernel elements).

Scale to gray - the Source Code

int scaletogray(imgdes *srcimg, imgdes *desimg)
{
    imgdes myimage8;
    static char kernel[] = {1, 2, 1, 2, 8, 2, 1, 2, 1};
    int divisor = 20;
    int rcode, width, length;

    // If the source is not bilevel, there's no reason to proceed.
    if(srcimg->bmh->biBitCount != 1)
       return(BAD_BPP);

    // Determine size of source image
    width = srcimg->bmh->biWidth;
    length = srcimg->bmh->biHeight;

    // Allocate space for grayscale image
    rcode = allocimage(&myimage8, width, length, 8);
    if(rcode == NO_ERROR) {
       if(width * length > 1800 * 1800) {  // Set an arbitrary limit for "large image"
          // Convert from bilevel to grayscale and perform 1-D antialias
          rcode = convert1bitto8bitsmooth(srcimg, &myimage8);
          }
       else {  // Smaller images can spend more time for a complete 2-D antialias 
          // Convert from bilevel to grayscale
          convert1bitto8bit(srcimg, &myimage8);
          // Gently blur the edges 
          rcode = matrixconvex(3, kernel, divisor, &myimage8, &myimage8);
          }
       if(rcode == NO_ERROR) {
          // Replace the destination image with the new grayscale image
          freeimage(desimg);
          copyimgdes(&myimage8, desimg);
          }
       else
          freeimage(&myimage8);
       }
    return(rcode);
}

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