Manuscripts fidelity in the digital libraries era: the contrast enhancement evaluation conundrum

Digitization of ancient manuscripts spurs inter-disciplinary research but the quality may be limited due to manuscript preservation state or scan hardware limitations. Contrast Enhancement improves the subjective quality of images for end users. Estimating the performance of contrast enhancement is challenging since ancient manuscript are composite, i.e. they contain drawings and calligraphic elements, and the contrast enhancement metric should capture fidelity, color quality and recovery of faded text. After applying different global and local contrast enhancement techniques to a set of 15𝑡ℎ century manuscripts, where the text and pictorial representations were partially compromised due to conservation problems, we assessed the quality of the enhanced manuscripts by performance metrics, and compared them with human supervised ranking of the enhanced manuscript in terms of either color and text quality. By comparison of the metrics with the supervised ranking results, we identify the most accurate performance metric, namely the metric based on brightness preservation. Future work will address evaluation of the enhancement for artificial intelligence based segmentation, dating, visual search, text recognition purposes.