prevent comb in historgram?

R
Posted By
Roberto
Jul 6, 2007
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659
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5
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Closed
I scanned about a hundred old b/w photos in 8 bit grayscale and then retouched them (remove scratches, change gamma or curve).

Some of the resulting histograms have comb patterns. I guess I should have scanned in 16 bit grayscale and then output the final results in 8 bit grayscale. But it is too much effort to re-scan and re-retouch them.

Is there a way to create the missing histogram values by some sort of intepolation so that if people drop my photos into photoshop, they won’t see a comb histogram?
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Andrew Morton
Jul 6, 2007
peter wrote:
I scanned about a hundred old b/w photos in 8 bit grayscale and then retouched them (remove scratches, change gamma or curve).

It would have been better to adjust the gamma or curve at the scanning stage, if the software allows for that.

Some of the resulting histograms have comb patterns. I guess I should have scanned in 16 bit grayscale and then output the final results in 8 bit grayscale. But it is too much effort to re-scan and re-retouch them.
Is there a way to create the missing histogram values by some sort of intepolation so that if people drop my photos into photoshop, they won’t see a comb histogram?

Blur the images.

But if the images look good, why worry about the histogram?

Andrew
MR
Mike Russell
Jul 6, 2007
peter wrote:
I scanned about a hundred old b/w photos in 8 bit grayscale and then retouched them (remove scratches, change gamma or curve).
Some of the resulting histograms have comb patterns. I guess I should have scanned in 16 bit grayscale and then output the final results in 8 bit grayscale. But it is too much effort to re-scan and re-retouch them.
Is there a way to create the missing histogram values by some sort of intepolation so that if people drop my photos into photoshop, they won’t see a comb histogram?

Add a small amount of noise to areas where you see banding. I recommend that you ignore the histogram as such, but I agree others may not feel this way, even if your image looks excellent.
—
Mike Russell – www.curvemeister.com
R
Roberto
Jul 6, 2007
"Andrew Morton" wrote in message
It would have been better to adjust the gamma or curve at the scanning stage, if the software allows for that.

Good point. However, it seems faster to do it afterwards because I’m scanning 3 to 5 photos per page (scanning an album) and each photo requires different gamma. So I would have to scan each photo individually to adjust gamma at scan time. Plus, sometimes I don’t like the gamma/curve I first decided on, which means I would have have to re-scan or suffer the comb histogram anyway.

Blur the images.

But if the images look good, why worry about the histogram?

So it is not immediately obvious that I retouched the image 🙂 This may not be essential for family photos, but would be a good skill/habit to have if someday I have to retouch a photo stealthily.
GH
Gernot Hoffmann
Jul 6, 2007
Gaps in a histogram are quite normal,
if the darkest pixels are mapped near to 0
and the lightest pixels are mapped near to 255.

Such a level adjustment improves any image
considerably.

The comb appearance is really mixed up by
printing – a totally new interpretation of under-
lying RGB values by the rasterizer.

Some years ago this funny ‘gap-in-histogram-
story’ was brought up by somebody, and he did’t
ever got his feets back to the ground.

Best regards –Gernot Hoffmann
TN
Tom Nelson
Jul 9, 2007
The gaps are caused by rounding error as the values are remapped by a Curves, Levels or other tonal/color adjustment. They have to be pretty bad to show up as banding or posterization so I wouldn’t worry about them unless you can see them in the image.

In the future, scanning in 16 bit will help, of course. So will getting the tones as perfect as possible in the scan.

Tom Nelson
Tom Nelson Photography

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