Legacy ImageMagick Discussions Archive

Hello,
I would like to use the contrast-stretch operator on black&white photos coming from film scans (35mm Ilford XP2 film and Nikon Coolscan V at 4000 dpi)

I found out that, due to B&W grain, my "ideal" contrast-strech operator works better on a smaller size version of the same file:
1. resize scan to a much smaller size
2. apply contrast stretch to the smaller file

My explanation to this is that, on the smaller file, the B&W film grain (which, in the bigger file, is more similar to black or white pixels) gets averaged to grey pixels and the histogram technique of the contrast-stretch operator deals better with this.

My question is this: how could I apply the "downsample and contrast-stretch" technique to the original, bigger file?
Is it possible to extract some information from the contrast-stretch operation on the smaller file (say, the black and white points which were chosen by contrast-stretch) and use this information on the bigger file?

Thanks,
Federico

I don't have experience of scanned films, but used to do B&W film photography.

I expect that data from a smaller size could be useful for processing the original. But it wouldn't be used directly.

Think of the full-size scan as black and white only, with varying sizes of grain, with no (or very few) gray pixels. Data from the small resize tells you that the image needs to be lighter or darker or whatever. Then the question is: what operation on the full-size has that effect? I don't know, but it might involve changing the size of the grains, or introducing gray between them, or something else.