ISub clips negative colors at zero

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Old 07 July 2005   #1
ISub clips negative colors at zero

Must ISub clip negative colors at zero internally?

I understand clipping to 0 at output, but in the middle of my tree?

I'm trying to do something with image differences controlling displacements and I can't use ISubA -- I need the negative numbers to be offsets in the other direction.

(I'm trying to do this http://www.digitalartform.com/archi..._now_or_sm.html in shake)
 
Old 07 July 2005   #2
Hm... How negative to they go? Could you just add a constant to all of them to make them all positive, use the node, then subtract the constant right afterwards? Or do you need to split it into positive and negative values and negate one of them so they both are positive when they are operated on?

-jeremy
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Jeremy Birn
Author, Digital Lighting & Rendering, 3rd Edition
 
Old 07 July 2005   #3
What bit-depth are you working at?

In 8-but and 16-bit, there is no such thing as less than 0 or greater than 1 - if you convert to float before doing your ISub, you should find that you get negative values.
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Hugh Macdonald
nvizible
 
Old 07 July 2005   #4
Thanks!

I didn't realize I had to explicitly cast to float with a byte node; I assumed everything was promoted to float internally as a matter of course.

Finished product:

http://www.digitalartform.com/archi...p_now_or_u.html
 
Old 07 July 2005   #5
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