Enhancing Faint Pictograms in the Field

1. DStretchI developed DStretch in 2005. It is a computer program that enhances digital photos of rock art using a JPL algorithm called decorrelation stretch. The original algorithm does not work well on rock art, but I soon learned how to modify it to improve performance at rock art sites. It proved very quickly to be of great utility to rock art researchers and enthusiasts. At some painted sites the effect is amazing. Details of the paintings become visible for the first time. The meaning of pictogram sites may have been lost, but due to DStretch the artistic skill of the painters can now be appreciated. This was very exciting, but there was a problem: to see the amazing results you had to bring your photos back to the computer to see the enhancements. Having the enhancements available at the site has proved to be very important. It allows for photographs that correctly frame the pictograms. Often part of a pictogram was missed in a photo simply because it could not be seen. More importantly, often a site which seemed to be just a collection of faint stains would turn out to be a complex painting. Interpretation remains difficult, but patterns and motifs can now be recognised. Doing this at the site can lead to better comprehension of the painted s intentions (Fig. 1).2. In the fieldIn this article I will document the efforts I have made to use DStretch to view enhancements while at a rock art site.Putting DStretch onto a laptop is one solution to this problem, but there are issues with this. Laptops typically do not have screens that are easily visible in full sun. It is clumsy to move the images from camera to laptop. ImageJ (the host program for DStretch) does not have sophisticated image browsing capabilities. Despite this, laptops have been used by me and other researchers to make enhancements in the field, but I wanted a better solution.By 2008 a new visualisation mode appeared. A group of enthusiasts called CHDK (Canon Hack Development Kit) determined a way to install their own software in certain Canon cameras. I was able to modify their software to display DStretch enhancements in the LCD display after a button press. The enhancements were not the same as the computer DStretch, but were good enough for the field. One drawback is they can not be saved. At first the cameras to which DStretch could be ported were point and shoot with only medium image quality. Nevertheless, they proved to be extremely useful, notably on a trip to Tanzania 1 made in 2009. I encountered sites with beautiful, complex paintings done in red. Figure 2 shows the Canon PowerShot A720 point and shoot camera I used on that trip at the site Masange A13. The photo in Figure 2 was made with a separate Sony camera that I used for high quality photos. Figure 3 shows a DStretch YRE enhancement of the Sony photograph typical of what was revealed in the A720 LCD.In 2012 CHDK for the Canon G1X camera became available. This camera has excellent image quality, and it has RAW mode. RAW mode is very useful since jpeg compression can destroy the colour information used by DStretch. …