What's the algorithm of PS's Snap Neutural Midtones

[steven_woody]

in PS, you bring up level or curvel diagram and press the 'options' button, then

you will get a dialog which used to help balance colors. basically, you specify

the follow three parameters:

 

1, the algorithm ( enhance monochromatic contrast, enhance per channel contrast,

find dark & light colors );

 

2, set black & white points.

 

3, set midtones in (r,g,b)

 

my question is that what's the underlying algorithm used here? in the other

works, what PS will exactlly do in the following two cases:

 

a, use 'find dark & light colors' and set B/W points to (b%,w%) and specity a

midtone as (nr,ng,nb)

 

b, use 'enhance per channel contrast' and set B/W points to (b%,w%) and specity

a midtone as (nr,ng,nb)

 

and, i specially interest how PS use the (nr,ng,nb)

 

thank you very much.

 

-

woody

[Craig_Cooper11664875449]

Using "find dark and light colours" with "snap to mid tones" is the same as using "auto colour" with the benefit of being able to change the cut-off value for your black and white points.

[steven_woody]

Craig,

 

if so, my question changes to how the auto colour algorithm works. and, rather than using 'find dark and light colours', when using 'enhance per channel contrast', how the whole thing works.

 

thanks.

 

-

woody

[Craig_Cooper11664875449]

1) Enhance Monochrome Contrast is the same as Auto Contrast<br>

2) Enhance Per Channel Contrast is the same as Auto Levels<br>

3) Find Dark and Light Colours is the same as Auto Colour

<p>

The Auto Colour command adjusts the contrast and colour of an image by searching the actual image rather than the channel histograms for shadows, midtones, and highlights. It neutralises the midtones and clips the white and black pixels based on the values set in the Auto Correction Options dialog box. The Auto Correction Options dialog box lets you automatically adjust the overall tonal range of an image, specify clipping percentages, and assign colour values to shadows, midtones, and highlights.

<p>

Have a look here <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00F3id">Photoshop and Autocolor</a>

[mendel_leisk]

Just to nail one combo: I believe if you tick/untick "snap neutral mid-tones" with "enhance monochromatic contrast" there is no change.

 

I'd also be interested to learn how PS establishes neutral. I know in some instances it can make some pretty radical and counterproductive shifts in the color balance, with either "auto levels" or "find dark and light colors", usually more so with "snap neutral midtones" ticked. Typically it's on images without a good grey tone.