Technique: Local Auto Levels

Started Oct 7, 2007 | Discussions
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thejaybird Senior Member • Posts: 2,498
Technique: Local Auto Levels

In a previous thread:
http://forums.dpreview.com/forums/read.asp?forum=1031&message=25006158

I promised to explain a technique I've been using on my images. I've put together a tutorial presentation to demonstrate it in detail, and decided to post it here for the benefit of others who might have an interest.

http://docs.google.com/TeamPresent?fs=true&docid=df7tj2cm_11ccskpm

There's also an action available at ActionCentral under the name "Local Contrast":

http://www.atncentral.com/download.htm#Local_Contrast

Best Regards,
Jay

CityLights Forum Pro • Posts: 12,545
Very nicely presented. Thank you! (NT)
Pam R Veteran Member • Posts: 3,421
Re: Technique: Local Auto Levels

Hi Jay,

I won't pretend to understand how this works...my eyes glazed over when the math came up on your presentation ; ) I only know it works fantasically. Thank you sooooo much for sharing this excellent action! Your presentation was very well done also. I just have to look it over a couple more times for it to sink into my thick head : )

Thanks a ton!!

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Pam
http://www.pbase.com/pam_r
'art is working on something 'til you like it...then leaving it that way'

Kent C Forum Pro • Posts: 25,803
Re: Technique: Local Auto Levels

Thanks! Great technique and thanks for explaining the math involved.

One possible monkey wrench in the works would be the different settings of people in the 'options' dialog in levels, since that is how 'auto' is defined. I'm guessing you have yours set to "enhance per channel contrast" with "snap neutral midtones" unchecked. At least I think that's what the adobe default is. I have mine set to find 'dark & light colors' with 'snap neutrals' checked as I've found only autocolor is useful in the past - plus the creativepro article on it
--
Kent

http://www.pbase.com/kentc
tutorial archive:
http://porg.4t.com/KentC.html

Pictus
Pictus Veteran Member • Posts: 5,654
Thanks! Very efficient!! NT

Thanks! Very efficient!! NT

Kent C Forum Pro • Posts: 25,803
Re: Technique: Local Auto Levels

The layer setup in the action looks considerably different than the tutorial. I'm guessing 'legacy'
--
Kent

http://www.pbase.com/kentc
tutorial archive:
http://porg.4t.com/KentC.html

qiv Regular Member • Posts: 302
Does this work with colour images too? (nt)

no text

Pat Senior Member • Posts: 1,608
Re: Technique: Local Auto Levels

Thank you very much for this interesting action. Can you please explain a little about the different pixel size options and a suggestion as to when to use which? For example, 200px vs. 1 px?

I tried it on a few pictures, and the larger pixel size option (200px) seemed to have the effect of changing the overall contrast of a picture, while the smaller size (7px) didn't but had a nice sharpening effect bringing out fine details.

Also, is there any way to download/capture your slide presentation so it's on my hard drive?

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Pat

thejaybird OP Senior Member • Posts: 2,498
Re: Technique: Local Auto Levels

Kent C wrote:

Thanks! Great technique and thanks for explaining the math involved.

One possible monkey wrench in the works would be the different
settings of people in the 'options' dialog in levels, since that is
how 'auto' is defined. I'm guessing you have yours set to "enhance
per channel contrast" with "snap neutral midtones" unchecked. At
least I think that's what the adobe default is. I have mine set to
find 'dark & light colors' with 'snap neutrals' checked as I've found
only autocolor is useful in the past - plus the creativepro article
on it
--
Kent

That's exactly right, my implementation of Local Auto Levels is equivalent to "Enhance Per Channel Contrast" (with shadow and highlight clipping set to 0%).

Unfortunately, to implement "Find Dark & Light Colors" would require different filters than those offered by standard Photoshop. The technique hinges on the Minimum and Maximum filters, and these filters have no option for controlling what should be considered the minimum or maximum--it is just a per-channel selection of the darkest or lightest value.

thejaybird OP Senior Member • Posts: 2,498
Re: Technique: Local Auto Levels

Kent C wrote:

The layer setup in the action looks considerably different than the
tutorial. I'm guessing 'legacy'
--
Kent

http://www.pbase.com/kentc
tutorial archive:
http://porg.4t.com/KentC.html

That's correct, the action is optimized for speed. Several steps have been merged and/or refactored to keep the layer count and operation count low.

thejaybird OP Senior Member • Posts: 2,498
Yes.

Keep in mind that because it is a per-channel operation, it will muck with your colors (just like Auto Levels with "Enhance Per Channel Contrast" checked). To preserve your original colors, you can paste the result onto your original image and change the blending mode to "Luminosity".

thejaybird OP Senior Member • Posts: 2,498
Re: Technique: Local Auto Levels

Pat wrote:

Thank you very much for this interesting action. Can you please
explain a little about the different pixel size options and a
suggestion as to when to use which? For example, 200px vs. 1 px?

The pixel radius controls the size of the detail that will be enhanced. It's very similar to USM or High Pass techniques--the choice of radius is highly dependent on the content and resolution of the image.

For an 8mp image, I would probably start with a radius of 100 and go from there. I usually run it two or three times at different radii and combine the results with masking.

I tried it on a few pictures, and the larger pixel size option
(200px) seemed to have the effect of changing the overall contrast of
a picture, while the smaller size (7px) didn't but had a nice
sharpening effect bringing out fine details.

That's exactly right--the smaller the radius, the more it will resemble a sharpening technique (sharpening in general is just an enhancement of contrast in fine details--i.e. "micro contrast"). A very large radius produces something closer to what regular, non-local "Auto Levels" would give you.

Also, is there any way to download/capture your slide presentation so
it's on my hard drive?

E-mail me at the address given in the "About" action, and I will send you a zip of the presentation. Unfortunately, I don't think there is any way to download it from Google.

carl english
carl english Veteran Member • Posts: 8,140
Re: Technique: Local Auto Levels

I quite sure if you turn the sound up you will hear a round of applause, you certainly deserve one.

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Kent C Forum Pro • Posts: 25,803
Thanks....

That info should help if people have different options settings and are wondering why they get different results
--
Kent

http://www.pbase.com/kentc
tutorial archive:
http://porg.4t.com/KentC.html

Kent C Forum Pro • Posts: 25,803
Thanks...nt
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Ron Whitaker Regular Member • Posts: 346
thanks nt
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Ron_M Senior Member • Posts: 2,886
Re: Technique: Local Auto Levels

Doesn't view very well in Firefox. It runs right off the monitor with no way to scroll and then unable to read it all.
Works OK in IE.

thejaybird wrote:

In a previous thread:
http://forums.dpreview.com/forums/read.asp?forum=1031&message=25006158
I promised to explain a technique I've been using on my images. I've
put together a tutorial presentation to demonstrate it in detail, and
decided to post it here for the benefit of others who might have an
interest.

http://docs.google.com/TeamPresent?fs=true&docid=df7tj2cm_11ccskpm

Best Regards,
Jay

-- hide signature --

Ron_M

thejaybird OP Senior Member • Posts: 2,498
Re: Technique: Local Auto Levels

Ron_M wrote:

Doesn't view very well in Firefox. It runs right off the monitor
with no way to scroll and then unable to read it all.
Works OK in IE.

Hmm... I haven't had any trouble viewing it in Firefox myself... be sure to send a bug report to Google if it doesn't work for you

lylejk
lylejk Forum Pro • Posts: 28,566
Some issues seeing the tutorial too Jay.

Using IE7 and all I see is text (no images). Don't have a Google account and that might be the reason why. Oh well. I downloaded the action and look forward to playing with it when that mood hits me.

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rspino Contributing Member • Posts: 728
Thanks

Just bookmarking this great thread.
--
Roberto (equipment in my profile)
http://spino.smugmug.com

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