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Generating Skin with Frequency Separation

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  #11  
Old 08-30-2010, 04:45 PM
kav kav is offline
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Re: Generating Skin with Frequency Seperation

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Originally Posted by bakerser View Post
True for 8bpc images (rounding 'protection'); not so for 16bpc images, where the image will suffer gradual creep one direction or the other. My caution was targeted towards the latter category, as I'd understood Der W's comments to be in regard to the Curves dialog's resolution -- if I was wrong there, please disregard with my apologies.
I haven't tested that, but on 16 bit images they use totally different numbers on this technique, and the 128 offset isn't put in. I haven't ever seen rounding creep personally. I am tempted to test this now. i'm not sure what 16 or 32 bit numbers photoshop uses for a middle grey approximation. Obviously 128 is still entered on the user end which is an 8 bit value. I'm getting 0-65535 as 16 bit numbers via simple math of 2^16 with the value for 0 taken out. If it's 15 bits that would mean 0-32767. I don't know exactly how these are distributed in 16 vs. 8. If it's an even distribution 32768/256 = 128, giving each 8 bit number 128 values that point to it from a 16 bit number if they're using this method of distribution which I'm not sure is the case. Since it's a midtone value it should be close enough for approximation. By this math showing a midtone creep via a blending mode that is considered to use middle grey as a neutral value, you'd need a lot of layers for any bit flipping in 16 bit numbers to move one of the three 8 bit numbers a full point. It's not done with a 128 offset at 16 anyway but via using a scaling factor of 2 and an inversion to approximate. I haven't been able to get any values to creep so far. I'll try doing this the separation thing + copy merged 10 times and see if I can produce a numerical shift on a few different points across the image. I could see this being more of a consideration something other than working on photographs for the purpose of enhance their look.
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  #12  
Old 08-30-2010, 04:59 PM
bakerser bakerser is offline
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Re: Generating Skin with Frequency Separation

To address a few things:
- PS absolutely does not offer 65k values. Whether this is a good thing or not is much debated - I happen to think that it is. In reality it offers 32769 values (0-32768), allowing a perfect neutral value at 16384. You can verify this by filling a 16bpc layer with 50% gray and reading it with the eyedropper.
- The problem with 8bit curves is exactly what you're getting at - with LL-family separations. Indeed, if I were just looking at a normal layer and adjusting it by 1/32k, it wouldn't be that big a deal for most all applications. But when I attempt to apply a curve (which interpolates values mind you - even our 'lock' points are not inviolable), that 1/2 point(+) distance from true 50% gray which the curve maps out becomes a full point move after the LL doubling is brought into play. This forces an undesirable move in the result where we shouldn't have to have one.
- There are of course lots of ways to deal with the above - it's just a minor limitation in truly fine work, and an inconvenience for the rest.
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  #13  
Old 08-30-2010, 08:07 PM
bakerser bakerser is offline
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Re: Generating Skin with Frequency Separation

A brief introduction to visual frequencies for those not yet accustomed: here.

Part Two

Part Three

Last edited by bakerser; 09-04-2010 at 01:12 PM.
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  #14  
Old 09-02-2010, 01:39 PM
russpears russpears is offline
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Re: Generating Skin with Frequency Separation

What is the HF Layer, LL etc.
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  #15  
Old 09-02-2010, 03:32 PM
bakerser bakerser is offline
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Re: Generating Skin with Frequency Separation

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Originally Posted by russpears View Post
What is the HF Layer, LL etc.
HF = "High Frequency"
LF = "Low Frequency"
LL = "Linear Light"

Let me know if there are others which we should have been defining earlier .
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  #16  
Old 10-09-2011, 01:30 AM
russpears russpears is offline
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Re: Generating Skin with Frequency Separation

What is a gradient "helping" layer used for? And how is it made?
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  #17  
Old 10-12-2011, 10:14 PM
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ray12 ray12 is offline
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Re: Generating Skin with Frequency Separation

If your base skin background is 127 or 128 you will not likely notice a very substantial effect. That kind of difference would only change the brightness level by a very little amount. It doesnt affect color itself since it is a neutral gray color...just the luminance. That mid color totally drops out with some blend modes...so it is not a big issue. Besides, the skin pore textures are painted in very slightly and are blended in...so again...very little effect.

Ray12
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