Luminosity masks

S
Posted By
simplicity
Apr 12, 2005
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545
Replies
9
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Closed
A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based on a brightness value above 128. Is there a way to make such a selection based on a specified value other than 128? Thanks.
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luminosity mask

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R
RSD99
Apr 12, 2005
(1) Copy the image to a new layer

(2) Use the ‘Threshold command on that new layer.

wrote in message
A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based on a brightness value above 128. Is there a way to make such a selection based on a specified value other than 128? Thanks.
JP
Jean Pierre Daviau
Apr 22, 2005
"RSD99" a
HT
Henrik Tived
Apr 23, 2005
Sorry if this have beeen mentioned before, but have you tried to invert that selection ?

🙂

Henrik

| wrote in message | > A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based
on
| > a brightness value above 128. Is there a way to make such a selection | > based on a specified value other than 128? Thanks.
P
patrick
Apr 23, 2005
I’m with the OP! The ability to adjust the luminosity level in the selection would be a terrific tool!
Anyone?
.. . . . patrick

"tived" wrote in message
Sorry if this have beeen mentioned before, but have you tried to invert that
selection ?

🙂

Henrik

| wrote in message | > A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based
on
| > a brightness value above 128. Is there a way to make such a selection | > based on a specified value other than 128? Thanks.

S
simplicity
Apr 23, 2005
Inverting will only create an opposite selection, which is still based on the brightness value of 128, and won’t solve my problem.

tived wrote:
Sorry if this have beeen mentioned before, but have you tried to invert that selection ?

🙂

Henrik

| wrote in message | > A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based
on
| > a brightness value above 128. Is there a way to make such a selection | > based on a specified value other than 128? Thanks.
S
simplicity
Apr 23, 2005
Thank you for understanding. It occurs to me that if the tool knows how to select at 128, it ought to know how to select at 64, 32, etc. All we need is to understand how the tools select at 128, and take it from there. Better yet, Adobe can build the selection option in.

patrick wrote:
I’m with the OP! The ability to adjust the luminosity level in the selection would be a terrific tool!
Anyone?
. . . . patrick

"tived" wrote in message
Sorry if this have beeen mentioned before, but have you tried to invert that
selection ?

🙂

Henrik

| wrote in message | > A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based
on
| > a brightness value above 128. Is there a way to make such a selection | > based on a specified value other than 128? Thanks.

T
Tacit
Apr 23, 2005
In article , wrote:

Thank you for understanding. It occurs to me that if the tool knows how to select at 128, it ought to know how to select at 64, 32, etc. All we need is to understand how the tools select at 128, and take it from there. Better yet, Adobe can build the selection option in.

Actually, it doesn’t "select at 128" per se. It draws the dotted "marching ants" marquee at 128, because that’s the point where something is 50% selected.

What it does is make a selection based on luminosity. Something that’s completely light is completely selected. Something that’s completely dark is completely unselected. Something that’s 50% light is 50% selected; something that’s 30% light is 70% selected; and so on. The selection is made smoothly and continuously; it does not center around any value. All selections draw their dotted lines at the 50% mark; you believe, because of the way the "marching ants" marquee looks, that everything above 128 is selected and everything below 128 is not, but that is not actually what has happened.

To make a selection around any arbitrary luminosity level:

1. Select the luminosity of the image (Control-Alt-~).
2. Enter Quick Mask mode.
3. Image->Adjust->Levels
4. Move the hilight (white) and shadow (black) triangles together, and set them both on the leuminosity level you wish to select around.
5. Exit Quick Mask mode.

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J
jenelisepasceci
Apr 25, 2005
wrote:

A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based on a brightness value above 128. Is there a way to make such a selection based on a specified value other than 128? Thanks.

Create a luminosity mask and save it as an alpha channel. You will find that the selection is not made based on a value of 128 but is rather a representation of the L channel in LAB mode. The value of 128 is used to display the "marching ants", but has no further meaning. A luminosity mask is made based on, guess, the luminosity. This means that each pixel is affected by the mask based on it’s luminosity, the lighter, the more. If you want to create a mask based on any threshold luminosity value, save the luminosity mask as an alpha channel and apply the threshold command on this channel or, if you want a smooth transition, change the appearance of the alpha channel with the levels, or better, curves command.

Peter
S
stralka
Apr 25, 2005
(Peter Wollenberg) wrote in message news:…
wrote:
A luminosity mask ((cntl + alt + ~ on PCs) creates a selection based on

Since it looks like you’re coming from Germany: how would you create a luminosity mask with a German keyboard? The tilde is reached by the +-key combined with AltGr; the combination Ctrl+Alt+AltGr++ doesn’t seem to work…

Robert

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