Photoshop and autocolor

[bernard_frank]

Can someone explain me how "Autocolor" works in PS7? It seems to operate on much

more than just color balance, but, hard as I try, I can't find exactly on what settings it has

an effect. Sometimes I think it works okay (when in a hurry), but I would like to adjust

some of the effetcs, or try to emulate them.

 

Thanks for your help.

[Ed_Ingold]

Autocolor is the same as "auto" in "Levels" - the min and max points of each color are set to clip (default 0.5%).

[scott_turner]

<p>Bernard,</p>

 

<p>You can find an excellent tutorial on Autocolor by Bruce Fraser <a href="http://www.creativepro.com/story/feature/17164.html">here </a>. It should answer most of your questions and more.</p>

 

<p>Scott</p>

[mendel_leisk]

Auto Color is using the 3rd set of Levels parameters, as summarized in the Photoshop helpfile. Now, what statement of that 3rd set of parameters actually means is a bit of a head scratcher to me:

 

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1. Enhance Monochromatic Contrast clips all channels identically. This preserves the overall color relationship while making highlights appear lighter and shadows appear darker. The Auto Contrast command uses this algorithm.

 

2. Enhance Per Channel Contrast maximizes the tonal range in each channel to produce a more dramatic correction. Because each channel is adjusted individually, Enhance Per Channel Contrast may remove or introduce color casts. The Auto Levels command uses this algorithm.

 

3. Find Dark & Light colors finds the average lightest and darkest pixels in an image and uses them to maximize contrast while minimizing clipping. The Auto Color command uses this algorithm.

 

Select Snap Neutral Midtones if you want Photoshop to find an average nearly neutral color in an image and then adjust the gamma values to make the color neutral. The Auto Color command uses this algorithm.

 

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Some comments:

 

Of the 3, Levels with Find Dark & Light colors is the only option that moves Output Levels away from 0 and 255 values, sort-of damping things.

 

I believe Auto colors uses the current clipping levels you have set as defaults, but not sure. You could test it out, going through Levels using #3 parameters with your default clips, say set to 0/0, then repeat the exercise just using auto colors. And then compare histogram statistics of the 2 results. In any event, I would suggest you use Levels instead of Auto Color, then in Options, select "find dark and light colors" and (maybe) "Snap Neutral Midtones", just so you can see what is happening. A very slight clip (the minimum is .01) can really make dramatic difference in contrast, and clean up color casts.

 

You can also adjust the target colors for Shadows, Midtones and Highlights. The defaults for red/green/blue values are 0/0/0, 128/128/128 and 255/255/255 respectively. There is a tutorial covering this, and Auto Color in general, here:

 

http://www.creativepro.com/story/feature/17164-1.html

 

I don't really understand the mechanics behind the Auto Color command, or it's equivalent in Levels, or the other Levels functions (Auto Contrast or Auto Levels), and would also appreciate any info on the subject. To broaden the question a bit, how does this Photoshop function compare, say to Vuescan's Color Balance "white balance". My hunch is that PS's levels functions are very objective, relatively dumb functions. They are applying very cut-and-dried settings, and can back-fire. Nevertheless, very useful.

[Ed_Ingold]

"2) Enhance Per Channel Contrast maximizes the tonal range in each channel to produce a more dramatic correction. Because each channel is adjusted individually, Enhance Per Channel Contrast may remove or introduce color casts. The Auto Levels command uses this algorithm."

 

The second choice under Levels/Options describes the operation of "Autocolor". You can see the effect by looking at the histograms for each individual color.

[bobmichaels]

All I know is that it's easy and frequently works. And when it doesn't give me what I want, I just undo and adjust manually. But I have no clue HOW it works.

 

Wouldn't it be nice if it worked in a layer and showed you suggested adjustments on the color bars so you could tweak from there.

[scott_turner]

<p><i>Wouldn't it be nice if it worked in a layer and showed you suggested adjustments on the color bars so you could tweak from there.</i></p>

 

<p>Bob, I <i>highly</i> recommend you read the Fraser tutorial that Mendel and I both point to in previous responses. Autocolor <b>can</b> be done as an adjustment layer (either levels or curves) and you can easily tweak colors afterwords on the same level. Again, see the tutorial for an explanation. I'm not much of an Autocolor fan, but used the way Fraser suggests, it can serve as a good starting point for color adjustments.</p>

 

<p>Scott</p>

[scott_turner]

Grrr... Proofreading problems again.

 

"...on the same level." should read, "...on the same layer."

 

Scott

[bobmichaels]

Scott: OK, I'll take your suggestion. I read the Fraser article when it first came out but never tried it as there was too much that didn't make sense then and I didn't have time to sort it out. But I'll go back to it based on your and Mendel's suggestions.

[bernard_frank]

Thank you, all. I have started using "Levels" in a more refined way, ie R/G/B levels plus

luminosity level, and I'm getting there.

[steven_woody]

thanks for your inputs, but no one answered yet how it really works. i've actually read the tutorial you folks mentioned and i did see visual feedbacks of these three algorithm. but to grasp them fully, i think we need to get know details in term of algorithm. for example, what is neutral gray? how PS find it? when got it, what PS will do then?

 

the ultimate answer when we approach, i think we can resolve the following question: for a given (r,g,b) or a set of (R,G,B), when we choice this algorithm and set it these parameters, what the new (r,g,b ) or the new (R,G,B) set we will get?

 

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woody

 

 

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