"Jules" wrote in message
Well, I would like to thank all of you for your kind answers and suggestions. I'll try to translate all of them in the french version of photoshop and use them.
Those answers look acute and appropriate to my problem.
Looking back over this thread, I see that my contribution was not really relevant to Jules's original question. In honor of the Christmas holiday, I would like to make another attempt.
1) duplicate the image to a new layer.
2) run Filter>Other>Maximize with the radius set to 7.
3) run Filter>Blur>Gaussian blur with the radius set to 35. This creates an image that more or less reflects the overall brightness of the paper, minus the black lines.
3) set the mode of the new layer to "Linear Dodge". This removes most of the variation in the paper color.
4) If you want to recreate the original line art, Add a Levels adjustment layer above the Linear Dodge layer. Set the black triangle to 30 and the white triangle to 209. There is no advantage to using curves in this situation, and Levels has the advantage that the histogram clearly indicates the paper and ink pixel values..
The result after step 4 will be very close to a clean black on white image of the original. If you want to retain some of the texture of the original paper, reduce the transparency of the Levels layer.
There are many tiny yellow patches on the white paper. These are due to a dark blue channel in the original image, as often happens under relatively dim tungsten light. One way to remove the patches is to replace the blue channel with the green one.
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Mike Russell
www.curvemeister.com