Steve,
Well, that was 4 years ago!
This is probably the wrong forum for this, but - since then Tim Roth
now has code for converting some LUT formats to ICC device link
profiles, not input profiles.
Why device link profiles you may ask:
- Most LUTs in movie production are used to convert from one device
encoding (DCI P3, DPX, Cineon, Rec 709, ...) to another (for device
conversion) or to same (for applying a look). This is how the movie
guys have worked for many years, and it fits the ICC device link
model.
- On the other hand, all input and output ICC profiles convert from a
device encoding to a reference color space or the inverse direction,
not to or from another device encoding. This is how the graphic arts
industry has worked for 15 years, and this is a more robust way to
chain color conversions.
There are challenges with using even the highest quality LUTs and
device links when you do multiple conversions.
- If you use LUTs or ICC device link profiles to convert from DPX to
DCI P3 to Rec 709, versus directly DPX to Rec 709, you may get
different results, as the LUTs | device links lack a reference
towards which to calibrate them.
- If you use input and output ICC profiles to convert from DPX to DCI
P3 to Rec 709, versus directly DPX to Rec 709, you get practically
the same results, because every conversion goes through the reference
color space, (and the inverse transform should be an accurate inverse
of the forward transform, for in-gamut colors)
But you knew all that , right?
On using input profiles:
Yes, it's a good approach for film, and that's how we sometimes
support DPX-encoded camera film.
But what is the goal with your workflow?
For print film simulation, we actually create output profiles.
These can be used in two ways:
1. Converting digital camera input to DPX.
2. previewing the theater look of DPX files.
Here's the transform sequence for these cases:
1. Digital camera RGB -> Digital camera input profile (A) -> ICC ref
space (R1) -> DPX camera film profile (B1) -> DPX RGB
2. DPX RGB -> DPX film print path simulation (B2) -> ICC ref space
(R2) -> ICC Display profile -> Desktop monitor
Profile A can be a pure input profile, or an invertible profile for
alternate use as B2 (which makes it an output profile).
When Digital camera RGB is in OpenEXR format (float), the Digital
camera input profile (A) lacks gamma decoding,
The B1 transform above is an output (profile) transform.
Output profiles can be used in the inverse direction for input
transforms (A) or more often for preview transforms (B2).
In some cases we combine B1 and B2 into a single (output) profile.
Profiles B1 and B2 can also be two separate profiles.
The R1 reference colors are very different from the R2 values in this
case. R1 encodes scene colorimetry, and R2 is theater colorimetry,
both @ 5000K.
R1 is ideal for photo-realistic compositing, but you need R2 for previewing.
You can combine sequences 1 and 2 into a single transform sequence.
This is how we do soft-proofing in Photoshop.
Digital camera RGB -> A -> R1 -> B1 -> DPX RGB -> B2
-> R2 -> Display profile -> Desktop monitor
In general, in a LUT the transforms A and B1 have been combined into
one (device link) transform bypassing R1.
In order to use such LUT as an input ICC profile, the ICC profile has
to be a combination of the LUT (dev1->dev2) + B2 (dev2->ICC ref
space).
If dev2 is well known (and not film) it should be fairly easy to
create a dev2->ICC ref space transform.
B2 needs to be made for same color temp or white balance as the temp
or white on the output side of the LUT.
Hopefully this helps.
Lars
