And Repeat

The photographer still needs to repeat the reference card shot whenever the scene changes. All following photos of the same scene can then be corrected by GIMP and the picfix script.

To make sure this approach works even if the snapshot happens to have a fairly homogeneous background, lines 66 through 68 not only check to see whether the mean value in the buffer is less than 3, but also whether the algorithm is in the middle third of the image, ignoring the left and right thirds.

The script uses normal Perl arrays as ring buffers and uses push() to append new values before checking to see whether the array exceeds the maximum length of the buffer. If this is the case, it deletes the first array element by calling shift(). This shortens the array by one element, and the second element moves up to the first spot.

To calculate the first derivative of the fairly complex pixel function, you can use a simplified numeric approach.

The ring buffer, @intens_ring, stores the intensity values of the last 50 pixels, which were created by adding the red, green, and blue values at the processed x coordinates.

To extract the three values from the four-element list returned by the rgba() method (line 36), the script relies on the array slice technique @components[0,1,2] (line 41). The value of the first derivate – that is, the slope of the graph at this point – is then approximated as the difference between the first and last elements in the ring buffer at constant distance x. Positive or negative slopes are of no interest, so abs() converts them to positive values if needed.

To find out whether the algorithm is currently in one of the flat parts of the graph that are being investigated, or in a more mountainous region, the script sets up a second ring buffer, @diff_ring, which contains the last 50 values determined for the first derivate of the graph (lines 51, 52). The avg() function defined in line 107ff. calculates the mean value of 50 intensity values. If the algorithm is currently in a peaky region, a mean value below a threshold of 3 is all you need to identify a flat part.

Once the script hits a flat area, it takes a mean increase of more than 10 to convince the state machine that it is back in the mountains.

Each time the script identifies a flat area, line 72 stores the RGB values of the first pixel found in this area in the @ctl_points array. Because you are only interested in three flat spots; the last instruction in line 101 scraps any others.

Results

Finally, the Dump() function from the YAML module from CPAN outputs the results (Figure 6) in the form of a YAML file, sample.yml.

Storing the results and passing them in to last month's picfix script as -c sample.yml lets you correct the color casting in the image with the cards, and any number of images you took with the same lighting – but don't forget to hold the cards in the middle of the photo to make sure that the simple algorithm finds them. To find the cards otherwise, you would need a far more sophisticated algorithm.

On the other hand, Perl, with its treasure of modules on CPAN, gives you ample fodder for your experiments.

The Author

Michael Schilli works as a Software Developer at Yahoo!, Sunnyvale, California. He wrote "Perl Power" for Addison-Wesley and can be contacted at mailto:mschilli@perlmeister.com. His homepage is at http://perlmeister.com.