Color Dropout

Changes colors in a 24-bit image based on their proximity to a given RGB value. Colors to drop are specified by an RGB value and a radius; values within radius/2 are converted completely to the target color, while colors between radius/2 and radius are blended in decreasing amounts with the target color. Any number of colors may be chosen, with no impact on processing time.

Figure 1 RGB colorspace cross-section before and after a color dropout operation. Settings shown, color 127, 127, 64, radius 50.

Since variance is common in scanned images, it is possible to specify that a given color is to be adapted on an image-by-image basis. For example, if dropping white (255, 255, 255) then setting SCANFIX_COLOR_ADAPT will tell the algorithm to analyze the image, and change the value of the color up to 1/2 the radius in any direction to match the nearest peak RGB value. This is most useful when a small radius is specified. Use of this adaptive feature is slower, however, as it requires analysis of each individual image before processing to find the appropriate peak. A larger radius, if it can safely be used, will provide better performance.

Subcode is set to SF_SUBCODE_COLORDROP (29).
u.SC29.ColorCount The number of colors in the drop list.
u.SC29.DropToColor The RGB value that the selected colors will be changed to
u.SC29.Extract Set to true to extract the specified colors, and drop all other colors to the DropToColor.
u.SC29.ColorList A pointer to an array of RGB values and radii indicating all the regions of color to be used. This list should be allocated before REQ_INIT and freed by the user after REQ_TERM.This list should be allocated and freed by the user.