Last month, I wrote about using reference cards to correct the white balance in digital photos by taking a test snapshot (see Figure 1) [2]. The black, white, and gray plastic cards, which are available from any good photography equipment dealer, should not generate any color values in a digital image. This provides three calibration points for low, medium, and high light intensity, which the GIMP photo editing tool can then reference to correct your snapshots.

Figure 1: Discovering the color values for the three cards in the picture – the script reads the RGB values along the horizontal line at the center to obtain the graph in

Figure 2: The values in the unfiltered image fluctuate too strongly to identify the cards reliably.

Perl Magic

How can a simple Perl script find out which pixel values the three cards create, even though their position in the image is not known, without using artificial intelligence?

If the photographer manages to spread the cards in the center of the image as shown in Figure 1, the script can follow an imaginary horizontal line and identify the cards on the basis of pixel values along the x axis.

The light intensity measured along this line remains constant for a fairly substantial distance, as long as the line lies within one reference card.

Once the line touches the background, the pixel values will start to fluctuate significantly.

Listing 1, graphdraw, uses the CPAN Imager module to create the graphs shown in Figure 2.

The graphs represent the red, green, and blue components of the color values along the horizontal line drawn in Figure 1 on a coordinate system in which the x axis matches the x coordinates in the image and the y value represents the color component value with a range of 0 through 255.

The CPAN Imager module's read() (line 12) is a multi-talented beast that can identify, read, and convert any popular image format to its own internal Imager format for editing.

Errors

If something goes wrong, the Imager methods return false values. For more details about an error, cautious programmers tend to call the errstr() method to return a cleartext description of the issue. The getpixel() method (line 30) examines the RGB values of a pixel in the image at a location defined by its x and y coordinates and returns an Imager::Color object, which contains the pixel's RGB values.

A call to rgba() (line 35) returns these values along with the value for the alpha channel. Here, you are just interested in the first three RGB values.

The script calls shift in line 41 to extract them one by one.

Image View

The Imager::Plot module represents boring numbers as graphs in an attractive coordinate system without too much hassle with respect to scaling, axis labeling, or graphical layout, and it returns image files in all popular formats, allowing the user to check them later with an image viewer or web browser. The new() constructor (line 45) accepts the dimensions for the resulting image and the path to an installed True Type font, which it then uses for axis labeling.

The script collects the required coordinates in a hash of hashes, to which $data points. It stores all the x coordinates in $data->{x} and all red values in $data->{red}; the green and blue values are stored in the same manner. The AddDataSet() method (line 55) then adds the data separately for each of the three graphs, each of which are drawn in a different color.

On completion, a new Imager object is created in line 71; later, it will create the resulting graphics file. Before this happens, the box() method colors the image background white, then Render() draws the coordinate system, the labels, and the three graphs in one fell swoop.

Finally, the write() method saves the output file on disk in PNG format.