contrast-stretch action only on one hand of the histogram ?

F
Posted By
Fabrice
Oct 26, 2006
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589
Replies
4
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Closed
Hello,

I would like to create an action that adjusts only the left boundary of the histogram (in the shadows), without moving the right boundary corresponding to the highlights. I tried to use the "auto-levels" feature in PS CS2, by specifing a cut-off threshold of 0.01% for the shadows, and 0.00% for the highlights. The problem is that CS2 still moves the cut-off boundary of the highlights to the first non-populated value at the right of the histogram. For example, with an histogram beginning at 10 and finishing at 230, auto-levels sets the boundaries at 12 or so (which is fine) and at 230 (which is not what I want). I would like auto-levels to set the boundaries at 12 and 255.
How can this be done in an action in PS CS2 ?

Thank you,
Fabrice.

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K
KatWoman
Oct 27, 2006
"Fabrice" wrote in message
Hello,

I would like to create an action that adjusts only the left boundary of the
histogram (in the shadows), without moving the right boundary corresponding
to the highlights. I tried to use the "auto-levels" feature in PS CS2, by specifing a cut-off threshold of 0.01% for the shadows, and 0.00% for the highlights. The problem is that CS2 still moves the cut-off boundary of the
highlights to the first non-populated value at the right of the histogram. For example, with an histogram beginning at 10 and finishing at 230, auto-levels sets the boundaries at 12 or so (which is fine) and at 230 (which is not what I want). I would like auto-levels to set the boundaries at 12 and 255.
How can this be done in an action in PS CS2 ?

Thank you,
Fabrice.

don’t use AUTO
just use LEVELS
make the setting of levels manually
move sliders you want or change numbers in the boxes
F
Fabrice
Oct 27, 2006
For example, with an histogram beginning at 10 and finishing at 230, auto-levels sets the boundaries at 12 or so (which is fine) and at 230 (which is not what I want). I would like auto-levels to set the boundaries at 12 and 255.
How can this be done in an action in PS CS2 ?

don’t use AUTO
just use LEVELS
make the setting of levels manually
move sliders you want or change numbers in the boxes

The problem is that I want to specify a *percentage* of the range to be cut-off, and not a constant value, so that the threshold will be adapted to the images content. I want a "generic" action for different images. That’s why I cannot change numbers in the boxes, since they are absolute values, aren’t they ?

Fabrice.
N
nomail
Oct 28, 2006
Fabrice wrote:

For example, with an histogram beginning at 10 and finishing at 230, auto-levels sets the boundaries at 12 or so (which is fine) and at 230 (which is not what I want). I would like auto-levels to set the boundaries at 12 and 255.
How can this be done in an action in PS CS2 ?

don’t use AUTO
just use LEVELS
make the setting of levels manually
move sliders you want or change numbers in the boxes

The problem is that I want to specify a *percentage* of the range to be cut-off, and not a constant value, so that the threshold will be adapted to the images content. I want a "generic" action for different images. That’s why I cannot change numbers in the boxes, since they are absolute values, aren’t they ?

Yes, absolute values won’t work in an action. The only thing I could think of is a bit complicated, but should work. Let your action do the following:

1. Copy the image to a new layer
2. Set advanced blending mode of this layer so that the darker half is opaque and the lighter half is transparent
3. Use Auto Levels on this layer
4. Flatten


Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl
F
Fabrice
Nov 1, 2006
Yes, absolute values won’t work in an action. The only thing I could think of is a bit complicated, but should work. Let your action do the following:

1. Copy the image to a new layer
2. Set advanced blending mode of this layer so that the darker half is opaque and the lighter half is transparent
3. Use Auto Levels on this layer
4. Flatten

Hi,
Thank you for the suggestion, it is indeed a bit complicated but works. Anyway, I find it quite incredible that PS should not be able to do directly such an
easy thing ! Finally, I have turned to Imagemagick: one single option does the job !

Thank you,
Fabrice.

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