computing an average in photoshop

"blushark" wrote in message

hi,

how would i go around computing an average value of several photos in photoshop?

i have few identical photos with unique noise in each one, and computing an
average of those photos would decrease that noise. some nikon film scanners
use this method under name "multisampling", because random noise statistically cancels out over several images.

Add each image as a new layer with decreasing transparency . Layer 2 will be 50 percent transparency, layer three will be 33 percent, layer four 25 percent, etc.

is there perhaps another application that could do this on 16 bit/ch images?

16 bit layer support was first available with Photoshop CS. If you have PS v7 or earlier, check out the PanoTools PTAverage utility. It’s free, and although the user interface is largely absent, the image quality is excellent.
http://www.path.unimelb.edu.au/~dersch/

PTGui provides an excellent interface for PanoTools, though it is not free, and is oriented toward creating panoramas and mosaics, not averaging images. —

Mike Russell
www.curvemeister.com/forum/

Add each image as a new layer with decreasing transparency . Layer 2 will be 50 percent transparency, layer three will be 33 percent, layer four 25 percent, etc.

i’ve tried guessing opacities, but only now understood how to calculate them exactly. if i’m going to cancel out noise, it’s important that every image gets into equation with equal weight.

is there perhaps another application that could do this on 16 bit/ch images?

16 bit layer support was first available with Photoshop CS. If you have PS v7 or earlier, check out the PanoTools PTAverage utility. It’s free, and although the user interface is largely absent, the image quality is excellent.

that program would be ideal for my purpose, but after giving it a few test runs, it turns out it messes up colors working with 16bpc TIFFs. (i tried it with greyscale images and got color results!) that’s a shame because it’s exactly what i was looking for.

thanks a lot for your help Mike!

"blushark" wrote in message

Add each image as a new layer with decreasing transparency . Layer 2 will
be 50 percent transparency, layer three will be 33 percent, layer four 25 percent, etc.

i’ve tried guessing opacities, but only now understood how to calculate them exactly. if i’m going to cancel out noise, it’s important that every image gets into equation with equal weight.

The above weighting factors give equal weight to each layer.

[re using the PanoTools PTAverage utility.]

that program would be ideal for my purpose, but after giving it a few test runs, it turns out it messes up colors working with 16bpc TIFFs. (i tried it with greyscale images and got color results!) that’s a shame because it’s exactly what i was looking for.

Try converting your grayscale image to RGB before averaging.

Good luck.
—

Mike Russell
www.curvemeister.com/forum/

"blushark" wrote in message

[…] because random noise statistically cancels out over several images.

Then you don’t want the average but the differences.

Here is a tutorial you may find helpful.

http://www.cambridgeincolour.com/tutorials/noise-reduction.h tm

—
Don Leman

"blushark" wrote in message

hi,

how would i go around computing an average value of several photos in photoshop?

[…] because random noise statistically cancels out over several images.

Then you don’t want the average but the differences.

no, i need the average to get rid of the superimposed noise which is statistically random and will cancel out (given a sample large enough) and to keep just the constant value which is actually on the film i’m scanning.

Try converting your grayscale image to RGB before averaging.

that’s what i did. (i had a computer generated image to test the software) later i tried actually scanning a real sample and the results were catastrophical. this time there were no visible color shifts but the noise was worse than in what came out of the scanner.

luckily, photoshop did the trick wonderfully, even though it’s a bit of manual work. i might make a script once i thoroughly test the procedure.

thank you very much for your help! 🙂

Here is a tutorial you may find helpful.

thanks, if only i had that few days ago 🙂

Hi Don,

Here is a tutorial you may find helpful.

http://www.cambridgeincolour.com/tutorials/noise-reduction.h tm

Lot of good tutorials on that site. Thanks for posting the link:)

—
Arnor Baldvinsson
San Antonio, Texas