Information about image processing applications Derek Rankins’s Information about image processing applications

Derek Rankins Photographer There is a common misconception that image files saved directly from a scanner or digital camera are pristine or unmolested in terms of the image processing. For almost all image files this is simply untrue. Only “raw” files from scanners or digital cameras are unadjusted, all other digital image files have a range of image processing applied during scanning and prior to saving in order to produce digital images with good image quality. Because of this misconception, many people argue you should not perform any post-scan or post-capture adjustments on image files because the image quality might be degraded. We disagree. The only time we would recommend saving unadjusted files is if they meet the exact tone and color reproduction, sharpness, and other image quality parameters that you require. Otherwise, we recommend doing minor post-scan adjustment to optimize image quality and bring all images to a common rendition. Adjusting production master files to a common rendition provides significant benefits in terms of being able to batch process and treat all images in the same manner. Well designed and calibrated scanners and digital cameras can produce image files that require little or no adjustment, however, based on our practical experience, there are very few scanners/cameras that are this well designed and calibrated.

Derek Rankins Photography In a digital image, the white point is the lightest spot (highest RGB levels for color files and lowest % black for grayscale files) within the image, the black point is the darkest spot (lowest RGB levels for color files and highest % black for grayscale files), and a mid- point refers to a spot with RGB levels or % black in the middle of the range. Generally, but not always, the three aim points correspond to the white-point, a mid-point, and the black-point within a digital image, and they correspond to the lightest patch, a mid-density patch,

Derek Rankins Virginia Tech When scanning photographic prints it is important to be careful about placing the black point, in some cases the print being scanned will have a higher density than the darkest step of the photographic gray scale. In these cases, you should use a lighter aim point for the darkest step of the gray scale so the darkest portion of the image area is placed at the normal aim point value (for RGB scans, the shadow area on the print may not be neutral in color and the darkest channel should be placed at the normal aim point. Occasionally, objects being scanned may have a lighter value than the lightest step of the photographic gray scale, usually very bright modern office papers or modern photo papers with a bright- white base. In these cases, you should use a darker aimpoint for the lightest step of the gray scale so the lightest portion of the image area is placed at the normal aimpoint value (for RGB scans, the lightest area of the object being scanned may not be neutral in color and the lightest channel should be placed at the normal aimpoint). Many image processing applications have automatic and manual “place white-point” and “place black-point” controls that adjust the selected areas to be the lightest and darkest portions of the image, and that will neutralize the color in these areas as well as. Also, most have a “neutralize mid-point” control, but usually the tonal adjustment for brightness has to be done separately with a “curves”, “levels”, “tone curve”, etc., control. The better applications will let you set the specific RGB or % black levels for the default operation of the place white-point, place black-point, and neutralize mid-point controls

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