Frederick Ferdinand Schafer Painting Catalog
Conventions: Image scanning and post-scan processing procedures
Scanning procedurePrints are scanned on an Epson V350 scanner (since 2007), an HP 3500 scanner (2004-2007), or a Hewlett-Packard IIcx scanner (before 2004). The scanner is configured to "millions of colors" (24 bits per pixel) mode, using a resolution of 400 dpi, with neutral brightness and contrast settings and gamma of 2.2 (the IIcx scanner used a gamma of 1.0), and written in uncompressed TIFF format. These settings are intended to minimize image processing within the scanner and thereby capture as much information of the original print as possible. The IICX scanner creates output in the Macintosh RGB color space, and the other scanners create output in the sRGB color space. None of the scanners provides an ICC color profile with the image.
Transparencies and negatives are scanned by a Kodak (vendor 4220) FilmScanner 4045, and written in Kodak ProPhotoCD format. When the original is in 4x5 format, it is (usually) scanned at six levels; when in 35 mm format, it is scanned at 5 levels. For some kinds of original film material, the scanning system generates a custom ICC color profile that characterizes the scanner.
Minimal post-scan processingAs quickly as possible after acquisition, scanned images are placed on-line with minimal processing, using Photoshop 7.0 (version 5.5 from January 2000 through November 2002, version 3.0.5 before January 2000), as follows:
Standard post-scan processingWhen time is available, images are more carefully processed. The primary additional step is to remove any keystone distortion and to adjust the ratio of the dimensions to match the original. In addition, in the case of PhotoCD's, work begins with the highest available resolution PCD image, acquired with the help of the appropriate ICC color profile. Finally, if other available information suggests that the color is incorrect, more detailed color adjustment may be done. Here is the standard procedure, using the Photoshop version mentioned above.
Of the 700-odd painting photographs collected so far, perhaps 5 provide an excellent representation of the color of the original painting. That is, the painting was correctly lit, the film was appropriate for the lighting, dyes in the negative or print have not faded with time, the scanner did not introduce any casts, a color management system followed the image through to post-scan processing, and nothing else went wrong along the way.
The others don't look like the original. To produce an image with colors closer to the original, many of the images need at least some color adjustment. This adjustment is by nature subjective, so one should consider the colors seen in these images as representative, not as authoritative. (The only consolation here is that the unretouched images were even less representative.)
Adjustment is done using a gamma 2.2 monitor with 6500 K white point that is calibrated by eye to make a scanned image of the Kodak Q60 color calibration target look as similar to the original as possible. Color balance is done whenever information is available that suggests that the image does not represent the actual appearance of the painting. In the case of older, faded transparencies and mislit photographs that show, e.g., yellow sky, pink snow, or green boulders, the need is often obvious.
In most cases, color and brightness adjustment is accomplished by simply stretching the Lab color mode luminosity to cover the full range from 0 to 100%. If this leaves a significant cast, the stretching is undone and the next thing tried is to switch to RGB mode and separately stretch the Red, Green, and Blue luminosities so that each covers a 100% range. If, as in the example at the left, there is at least one region in the painting that is completely black (one of the trees in the canyon below) and one region that is completely white (a rock on the trail at the lower left), this procedure automatically eliminates most color casts introduced since the painting left the easel--errors in scanning, errors in the photograph printing shop, shortcomings in the film, fading of film dyes, color introduced by the lighting, and even color casts contributed by aging varnish. When this technique is applied to an image of a painting that is to be cleaned it often produces a result that predicts what the painting will look like after cleaning.
This technique does not always produce a satisfactory result, particularly when a painting has no completely white region. In such cases, color adjustment is done by eye. Whichever method is used, the result probably does not exactly match the appearance of the painting, but it is usually much closer than the unadjusted image. Two important principles are that color balance adjustments are applied uniformly to the entire image, and are linear over the brightness range.
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|Feb 19, 2017, 09:57 MST|