On 19 Apr 2004 09:40:47 -0400, westin*
(Stephen H. Westin) wrote:
(Don) writes:
A better approach is by preventing the need for such a kludge (which only adds data but no information), scan and gamma process in 16-b/ch mode (if the actual data has between 8 and 16-bits real information) before reducing to 8-b/ch.
The problem is I only get 8 bits from the scanner.
But is it linear? Linear 8-bit isn’t very useful. If you can only get 8 bits, it’s probably better to get gamma-corrected info from the scanner.
Nikon LS-30 here, so it’s 10 bits internal, but only 8 bits external. And what I’m getting from the scanner is pretty bad:
http://members.aol.com/tempdon100164833/nikon/Blue.jpg http://members.aol.com/tempdon100164833/nikon/Green.jpg http://members.aol.com/tempdon100164833/nikon/Red.jpg I tried scanning at gamma 1.00 and applying corrections (to convert to
2.20) in Photoshop and that produces an image with a totally different
histogram. Apparently PS (version 6 here) must use a different gamma algorithm!? Still, lots of gaps in the dark areas, of course.
I guess the only other alternative is to work in linear gamma and do the conversion as the last step.
Maybe. The problem, of course, is that where you stretch contrast in the darker areas, there just isn’t enough information to fill in the histogram.
Bingo! That’s the problem. I’m also trying to contrast mask to extend the dynamic range of the scanner (pesky Kodachromes) and that’s pretty much impossible with a "Swiss cheese" histogram.
If you’re seeing waves instead of isolated spikes,
Photoshop is dithering to reduce banding effects, which is A Good Thing.
!!! That answers my question above why NikonScan histogram looks different than Photoshop’s when doing the same thing (gamma conversion from 1.00 to 2.20)!
Perhaps you could try upsampling the image by 4x or so with Bicubic interpolation, which would introduce some intermediate levels, then downsample (all in 16-bit mode). This will add more levels in a way that might be reasonable.
My intention was to, indeed, use 16-bit for editing but I wanted to figure out this histogram problem first. However, now that you mention it, I’ll give it a try.
BTW, a while back I stumbled across something similar myself. I inadvertently turned on interpolation in NikonScan (long story, it’s an NS bug, has to do with cropping) and the result was a smooth histogram until I discovered that the image was interpolated (resulting in softening of the image).
Don.