Monday, May 30, 2011

Evolutionary Police Sketch Software and Genetic Algorithms

The FBI prefers artists for sketching suspects, but  software is rapidly making inroads. Demand for a software solution was created by the many police departments that couldn't afford such an artist.

The first generation of sketch software, such as FACES,  asked victims to select  individual features from a catalogue, as shown at the right.
Recently developed software avoids the catalogue  by using 'evolutionary' techniques. EvoFIT asks users to click on the 6 photos out of 70 that look the most like the assailant. The six chosen faces are then combined using genetic algorithms to create 70 offspring images. The user continues this process until satisfied with the criminal's likeness.

Evolutionary Algorithms

The novelty in the EvoFIT approach is the combination of multiple pictures to produce various offspring, which is done with genetic algorithms. Genetic algorithms imitate the reproductive process by blending characteristics from two patterns or parents ('parents' could be images, a job rotation schedule, chess simulation, or investment strategies) into multiple 'offspring'. This cycle is repeated until a satisfactory answer is found, but the interesting point is that the offspring are usually superior to the parents (this is why evolution works). In this particular instance, 'superior' means they're more like the actual criminal than the parent images.

A Picture is Worth A Thousand Words

In this example, users were asked to 'sketch' Ben Affleck with EvoFIT. The first picture is one of the initial 6 chosen by the user. The second picture is one of the 70 offspring from the first generation, and the third picture is one of the offspring from the second generation of likenesses. 

Note how the nose and brow are consistent over the generations, while the eyes and chin grow more similar to Ben Affleck and the shape becomes more oblong. I also think that the middle picture looks like Ben Affleck's brother Casey Affleck, which further demonstrates the parrallels between the algorithm and real world genetic similarity.

Automated Picture Matching

Witnesses often look through hundreds of criminals' pictures in hopes of catching them. Witnesses grow tired after looking at this number of photos though, and it impedes their ability to recognize the assailant. Furthermore, our perceptions over-emphasize weight gain, hair color, or aging of the assailant compared to when the picture was taken. Accordingly, computers are now used to match sketches to a database of mug shots, because it works instantaneously and it relies on unalterable relationships in facial structure. More specifically, this software examines the location of the nose relative to the mouth and eyes, length of mouth, width of nose, length of nose, location of eyebrow ridges, etc.

Quantification and Benefits: Sketching

  • Greatly improved the accuracy of police sketches.
  • Yields a 45% arrest rate vs. single digits before EvoFIT was used.
  • "victims no longer have to worry about describing their assailant."
  • "The software exploits the fact that the human brain is much better at recognizing a face than describing it."
  • "we can schedule (the interview) around the victim's time rather than the artist's time"
  • Rapid, iterative creation of sketches. Approximately half the time taken with artist sketches.

Quantification and Benefits: Matching

  • 45% match from the sketch to the arrestant. 
  • Instantaneous, so the suspect has less time to flee.
  • Overcomes witness fatigue, because they don't need to look at hundreds of mug shots.
  • Reduces the time required by the witness.
  • Protects the innocent. Would you rather be one of 6 in a lineup or one person in a database of a million?


If you can't identify a suspect, then you can't arrest them. The drastic improvement in victim's facial recall and sketch matching makes the entire criminal justice system work better. While I am sure that some criminals are convicted based on CSI results, I think the ability to identify a suspect is critical to the entire system. For example, if you were a fisherman, would you rather have a fish on the hook 3% or 90% of the time?

"Sketch Software Still Suspect." June 2000.
Interquest Faces Software
"The Darwinian Police Sketch." Popular Science, July 2005.