Trying to create a 3DLUT with Gradient Fill and the result is not the same

[davel45848139]

I'll do a test, on top of the background layer I'll add two Gradient Fill layers, one I'll put Overlay as Blending Mode and the other one I'll put Soft Light with 50% opacity ... Then Export / Color Lookup Tables / Save .... And then when I use it, it's just not the same look ... Anyone knows why? Can we use Gradient Fill for LUTs ?

Thanks

[Jeff Arola]

Probably something adobe needs to look at and or comment on.

Anyway, what version of photoshop and operating system are you using?

I see what your talking about with photoshop cc 2017.1.1 on mac osSierra 10.12.4.

What is the order of the layers or which gradient fill layer has the overlay and which one has the Softlight 50% opacity.

Have you tried different gradients for the fill layers?

[davescm · Accepted solution]

Hi

A Look up table (LUT) tells Photoshop how to transform the Red/Green/Blue pixels from an initial value to a new RGB value. The same LUT is used to transform each pixel regardless of its physical position. The table only needs the input RGB value for that pixel to "look up" the output RGB value.

A gradient does not transform every pixel just based on the RGB value - it changes the transformation according to where the pixel is on the image - hence the gradient.

So a gradient will not work as a LUT - the LUT does not have any "position" property.

Dave

[davescm]

To add to my last reply - attached is the spec for LUTs which explains how they are constructed.

http://wwwimages.adobe.com/content/dam/Adobe/en/products/speedgrade/cc/pdfs/cube-lut-specification-1...

Dave

[davel45848139]

Ok thanks, the Gradient Map Adjustment Layer appears to be working fine however... so it appears that only Adjustment Layers will work when creating LUTs

[davescm]

Hi

Remember, a gradient map does not apply a different adjustment by position - it replaces dark to light pixels with the appropriate color from the gradient. In that respect it works in a very similar way to a LUT and it's result can be saved as a LUT.

Dave