static size_t EncodeImage(Image *image,const size_t bytes_per_line,
  const unsigned char *pixels,unsigned char *compressed_pixels)
{
  MagickBooleanType
    status;

  register const unsigned char
    *p;

  register ssize_t
    i,
    x;

  register unsigned char
    *q;

  ssize_t
    y;

  /*
    Runlength encode pixels.
  */
  assert(image != (Image *) NULL);
  assert(image->signature == MagickCoreSignature);
  if (image->debug != MagickFalse)
    (void) LogMagickEvent(TraceEvent,GetMagickModule(),"%s",image->filename);
  assert(pixels != (const unsigned char *) NULL);
  assert(compressed_pixels != (unsigned char *) NULL);
  p=pixels;
  q=compressed_pixels;
  i=0;
  for (y=0; y < (ssize_t) image->rows; y++)
  {
    for (x=0; x < (ssize_t) bytes_per_line; x+=i)
    {
      /*
        Determine runlength.
      */
      for (i=1; ((x+i) < (ssize_t) bytes_per_line); i++)
        if ((i == 255) || (*(p+i) != *p))
          break;
      *q++=(unsigned char) i;
      *q++=(*p);
      p+=i;
    }
    /*
      End of line.
    */
    *q++=(unsigned char) 0x00;
    *q++=(unsigned char) 0x00;
    status=SetImageProgress(image,SaveImageTag,(MagickOffsetType) y,
      image->rows);
    if (status == MagickFalse)
      break;
  }
  /*
    End of bitmap.
  */
  *q++=(unsigned char) 0x00;
  *q++=(unsigned char) 0x01;
  return((size_t) (q-compressed_pixels));
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.