I was going to send this message just to Ed Hamrick, but then I thought
people on this list might have comments.
I was looking forward to Ed's implementation of grain removal in Vuescan,
but unfortunately it softens the image too much so I rarely use it. I use a
Minolta Scan Elite that has an infrared channel so cleaning without grain
removal (Clean set to Light) does not soften the image.
Here are two cropped sections of just the sky from a 2820 dpi scan of a Fuji
ISO 100 color negative. One has been sharpened with unsharp mask and one
has not. They both have minimal JPEG compression. As you can see, it is
not just the grainy appearance but the multi-colored character of the grain
that makes it very ugly.
I have few grain problems with slide film, but color negative film is
another story. I often get grain aliasing in the sky but other areas of the
image rarely have much of a grain aliasing problem. I'm not sure why this
is so, but it has been my pretty consistent experience. Even stranger, even
other highlight areas don't exhibit the problem. It is mostly just the sky
-- blue, cloudy, or overcast doesn't matter. My guess is it takes a
combination of a fairly light area plus some amount of blue. I have tried
exposing the sky from -1 to +3 stops to see if that makes a difference in
scanning. It doesn't seem to help with the sky grain aliasing problem
though. I have also tried from 1 to 8 multi-scan passes in Vuescan, but
that also does not improve the sky grain aliasing.
I wonder if Vuescan could have a grain removal option that would only work
on the highlight areas of the image or something like that? Reducing the
ugly grain aliasing from the sky without softening the rest of the image
might get a lot closer to solving the problem, as I see it.
By the way, before anyone suggests selecting the sky and doing a Gaussian
blur, blurring only the blue channel, sharpening only the luminance channel,
using PSP's Edge Preserving Smooth filter, or some other technique I want
you to know that I know about them and have used them from time to time.
Sometimes the result is satisfactory and sometimes it isn't. Sometimes it
is very difficult to get a good selection of the sky because of tree
branches, leaves, and other numerous small details. Since Ed has said that
the grain removal algorithm in Vuescan examines the individual grains in
some way to do it's work it probably can do a better job anyway. If there
was an option to restrict it's work to only the highlights or highlights
with some blue or something else that might help with the sky and not soften
the whole image then that would make it much more useful.
Maybe Ed or someone else has a better idea about how the Vuescan grain
removal option could be expanded and in a practical sense work even better.
If you do then please give us your input.
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