What is CAST ?
Artificial Stain Technique, inspired in the interaction between Pyro stain with the variable contrast paper.
This is only an small addition to Alan Ross selective masking, it's just looking at the same from another point of view: We make a mask to effectively deformate the paper curve to suit our desire.
Proportional stain in pyro negatives casts an stronger low contrast yellow filter over denser areas, this effetivelly modifies paper curve for highlights.
Hmmmm.... we can do the same with Ps !!! just we make a grading map to asign a color to each gray level.
... then we print colored image on a tranparency, et voila, we have a contrast mask that exposes each density level with the filter grade we selected, as always we place a mylar diffuser between the negative and mask, in that way inkjet dots in the mask are not seen !
In this case we have most of the image exposed with grade 2 but highlights are exposed with 00, this mostly re-shapes paper curve to what a Silver Chloride paper would deliver.
But wait... we may make our grading map to include all color filters:
Important:
We may blur the original BW scan, or we may blur later the color mask... if we blur the bw image first we should work in 16-bits per channel to avoid banding. We also may not blur the mask at all, diffuser layer under the mask will do it.
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