Initialize divider to sum of the kernel elements in Java

Painting QR in Java Initialize divider to sum of the kernel elements

Remember that the current pixel has to be placed at the center of the kernel. Only a matrix with an odd number of rows and columns will have a center element.

This subroutine checks whether a file name has been passed in, and if so, it localizes the *DATA glob (to make sure the file handle gets closed on exiting the subroutine) and reopens the DATA file handle to the file name passed in. If no file name was passed in, DATA is left in its original state, which will cause it to be opened to the source file, just after the __DATA__ token.

The file is read in, line by line, skipping any lines that are empty or that start with a hash mark. This allows the file to contain comments and to be more human readable. The elements of the matrix are stored in a global array (@kernel) of array references, in which each element in the array constitutes a row. Once the file has been read, the resulting matrix is checked to make sure that all the rows are of the same length, and that the number of rows and the number of columns are odd. This last check will also fail when a matrix is empty. If the divider hasn t been set as a command-line option, it is initialized here to the sum of all the kernel elements. Finally, the x and y offset are set. The average_color() subroutine is defined as follows:

sub average_color { my ($im, $x, $y) = @_; my @sum_rgb; for my $row_i (0 .. $#kernel) { for my $col_i (0 .. $#{$kernel[0]}) { my @rgb = get_color_safe($im, $x + $row_i - $x_offset, $y + $col_i - $y_offset); for my $ci (0 .. $#rgb) { $sum_rgb[$ci] += $kernel[$row_i][$col_i] * $rgb[$ci]; } } } return map { $_ = int($_/$divider) + $bias; $_ = 0 if $_ < 0; $_ = 255 if $_ > 255; $_ } @sum_rgb; }

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This is where all the real work is carried out. This subroutine accepts an Image::Magick object and the coordinates of the pixel which is at the center of the window. It then iterates over all the elements of the kernel, calculating the appropriate x and y CONVOLUTION 221

coordinates of the pixel that are covered by that particular cell, and getting the values for the three color components. We can t merely use the calculated x and y coordinates directly, because of edge effects. The get_color_safe() function is used to implement one of the methods discussed earlier (see figure 12.2). For each color component, we add the value of the current pixel, multiplied by the value of the kernel element, to the relevant element of @sum_rgb. Thus, at the end of the outermost loop, the array @sum_rgb contains three elements, each representing one color component, and each value will be the sum of the respective color components of the pixels covered by the kernel, multiplied by the numbers in the corresponding elements of the kernel. Before these values are returned, they are first checked to make sure they fall in the valid range between 0 and 255, and then divided by $divider and increased with $bias. We will now return to the edge effects. When calculating the weighted average of one of the pixels close to the edge of the image, the kernel sticks out over the edge. There are several ways to deal with that (see figure 12.2). This program implements only one of them: it is assumed that pixels outside the boundaries of the image have the same value as the edge pixels.

sub get_color_safe { my ($im, $x, $y) = @_; $x $y $x $y = = = = 0 if $x 0 if $y $width $height < < 0; 0; 1 if $x >= $width; 1 if $y >= $height;

Edge conditions Discard opacity component (3)

return (split /,/, $im->Get("pixel[$x,$y]"))[0, 1, 2]; }

This subroutine will simply return the red, green, and blue values of the pixel at the given coordinates, except when those coordinates fall outside the boundaries of the image, in which case it will return the RGB values for the pixel on the closest edge of the image. This leaves only the definition of the color_hex() method, which is used only to translate a triplet of red, green, and blue values to a string of the shape #3a3a2b. This might be a familiar notation to anyone who has ever worked with colors in HTML. Each pair of hexadecimal digits after the hash mark encodes one of the red, green or blue color values, respectively.

sub color_hex { return sprintf("#%02x%02x%02x", @_); }