Using retroReveal as a Complement to DStretch for Enhancing Red Ochre Pictographs

Abstract

The web-based program retro­Reveal has been used primarily for providing improved visibility of documents with faint text, including stamps, currency, music, and so forth. It has yet to be used to its full potential by archaeologists interested in rock art. The plugin DStretch, used on the ImageJ platform, has been the standard for enhancement of faint red ochre rock art images. We introduce retroReveal as a supplement to photographic investigation through comparison of images from four rock art sites in Alberta, Canada. Processing photographs with the two techniques typically yields comparable results, but often with slight differences. In a few cases, retroReveal makes certain features more apparent than is the case with DStretch; in other instances, the opposite is true. Other positive and negative aspects of the two techniques are discussed. Experiments with black pictographs indicate that retroReveal does not perform satisfactorily with these images. Overall, our results indicate that retroReveal should be added to the toolkit for illuminating painted rock art images.