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DPChallenge Forums >> Hardware and Software >> How does Photoshop determine what a "Highlight" is
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10/20/2007 11:02:22 PM · #1
I'm rephrasing the same question I asked earlier due to very low responses.

When you select Highlights, Shadows, or Midtones, from Color Select or Burn/Dodge:

How does Photoshop determine which pixels correspond to Highlights and the rest?

Thanks,
Paul
10/20/2007 11:06:54 PM · #2
Most likely it uses a curve, such that pixels with high luminosity (perhaps relative luminosity?? You could do a few experiments to see) are highly affected, and pixels with a medium luminosity are slightly affected.

The key is, though, that some pixels are affected a lot, and others some, and others just a bit. This makes the effect smooth.

In fact, you could draw a pure black to pure white gradient and apply Color Balance with R/G/B on shadows/mid/highlights just to see where i positions it's cutoffs. If you use blue on the highlights, you'll notice the blue effect falling off toward the gray middle.

Hope this comes close to helping. =D

Edit: Oh, you'd like to know whether it is customizable... Well, maybe not so helpful. =P

Message edited by author 2007-10-20 23:08:01.
10/20/2007 11:07:17 PM · #3
i'd like to know the same thing, actually. i was editing pictures today and i noticed that when i went to "change lighting" (or something of the sort) darkening the highlights changed the most bizare parts of the pic. it's probably based on the range of light and dark pixels and which pixels fall into which percentile, but that's just my guess...
10/20/2007 11:15:13 PM · #4
Thanks, thats a good idea. However, based on what I see in the levels, it appears to be more than just a threshold cutoff. It looks as though there is a threshold difference between one pixel and another.

Originally posted by smurfguy:

Most likely it uses a curve, such that pixels with high luminosity (perhaps relative luminosity?? You could do a few experiments to see) are highly affected, and pixels with a medium luminosity are slightly affected.

The key is, though, that some pixels are affected a lot, and others some, and others just a bit. This makes the effect smooth.

In fact, you could draw a pure black to pure white gradient and apply Color Balance with R/G/B on shadows/mid/highlights just to see where i positions it's cutoffs. If you use blue on the highlights, you'll notice the blue effect falling off toward the gray middle.

Hope this comes close to helping. =D

Edit: Oh, you'd like to know whether it is customizable... Well, maybe not so helpful. =P
10/21/2007 12:28:38 AM · #5
Any other ideas?
10/21/2007 12:42:29 AM · #6
Perhaps it reads R, G and B levels seperately? Just throwing ideas out there. Is shadows/highlight available in CMYK?

Edit: typos

oops, you were asking about color select etc. Still, it might be relevant.

Message edited by author 2007-10-21 00:44:30.
10/21/2007 02:33:46 AM · #7
It took me a while, but I think I figured it out:
RGB > Lab
Highlights = 170-255 (upper 1/3)
Midtones = 85-170 (mid 1/3)
Shadows = 0-85 (lower 1/3)

These are absolutes in Lab space. RGB space has some overlap.
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