Reversal film is a type of film that produces a positive image on a transparent celluloid base. Given its low processing cost, during the last four decades of the 20th century, reversal films have been very popular in many parts of the world, being used for both educational and recreative purposes. The Romanian Animafilm studios published throughout the decades an impressive collection of such films on various subjects, mostly animated stories, but also with historical or educational topics, and many of them are archived in many educational and cultural institutions. They are today a part of the 20th century legacy, in the same time containing images of sights, monuments, panoramas that can no longer be seen in the real world, and their reversal film images are sometimes the only thing preserving their heritage.
Today, the existing film rolls are suffering from time decay, the obvious wear and tear from being projected so many times or simply stored, but also specific reversal-film preservation issues. Reversal film is a sensitive medium subject to various aging-related degradation process (e.g. dye fading), causing serious degradation and color loss to the film slides. Usually, color film restoration is a manual task performed by experts involving chemical processes. However, the focus today on film preservation implies firstly the digitization of the original film stock, which, in this way, can be better preserved (less wear and tear from being projected) since its content can be accessed, analyzed and processed digitally.
In Digital color restoration for the preservation of reversal film heritage, by Octavian-Mihai Machidon, Mihai Ivanovici, published in Journal of Cultural Heritage, Vol. 33, 2018, pages 181-190, https://doi.org/10.1016/j.culher.2018.01.021, the authors investigate the possibilities of reversal film image digitization, color enhancement and digital restoration for the purpose of preserving its heritage and also increasing its content availability in the digital era. Some techniques for reversal film digitization are reported, presenting results obtained after digitizing the same film slide using various methods and devices. Further on, the authors proposed an unsupervised, pipelined approach for the digital color restoration of animated reversal films. This approach consists of two sequential tasks, first removing the color cast from the film slides and secondly performing automated color correction, using for calibration known color references based on those of the film stock itself. These approaches, both the color cast removal and color correction, strongly depend on the choice of this ideal color chart. The proposed algorithms for performing the two tasks are detailed in the paper. The color cast removal uses the information provided by the color chart of the reversal film and it is based on the optimum mixing of the image colors with the complementary average color. The color correction is based on a linear transformation of the color space assuming a linear model for color appearance. Several results – sets of digitally restored reversal film slides – are presented, showing a qualitative comparison between the images. Two objective metrics were used for assessing the results of the proposed algorithms and they indicate a substantial improvement in color recovery and a significant increase of the restored image color content.