🔮 blim - Bean's Unprofessional View Transform

Also see: flim

Introduction

blim is an experimental OpenColorIO view transform intended to produce relatively vibrant and contrasty colors out of the box, while avoiding clipping and unwanted hue skews. blim is by no means production-ready, nor is it based on film emulation.

You can watch this random video of mine and read its description for more context. If you're really confused by all of this, or you want to learn more, I highly recommend the blogs mentioned in Useful Links.

Eye Candy

• See comparisons between blim and other view transforms in the releases section.

• You can find links to collections of OpenEXR image files for testing in Useful Links.

• Below are some example images gone through blim v0.4.1.

Scripts

The code is structured in the following way:

Script	Role	Uses
main.py	Generates a 3D LUT for blim	blim.py
blim.py	Transforms a given linear 3D LUT table	utils.py
utils.py	Contains helper functions	-

Here are the external libraries required to run the scripts:

• Colour

• NumPy

• Joblib

Using the LUT

First, a few notes:

• blim's 3D LUT is designed to be used in an OpenColorIO environment.

• blim only supports the sRGB display format as of now.

If main.py runs successfully, you should see a file named blim.spi3d in the same directory. Alternatively, you can look up the latest LUT - no pun intended - in the releases section, which may be outdated.

The LUT's expected input and output formats are mentioned in the LUT comments at end of the file, but they can also be seen in the code.

Here's an example of the LUT comments (note that this might not match the latest version):

# -------------------------------------------------
# 
# blim v0.3.2 - Bean's Unprofessional View Transform
# 
# LUT input is expected to be in Linear BT.709 I-D65 and gone through an AllocationTransform like the following:
# !<AllocationTransform> {allocation: lg2, vars: [-12, 5, 0.000244140625]}
# 
# Output will be in sRGB 2.2.
# 
# Repo:
# https://github.com/bean-mhm/blim
# 
# Read more:
# https://opencolorio.readthedocs.io/en/latest/guides/authoring/authoring.html#how-to-configure-colorspace-allocation
# 
# -------------------------------------------------

Here's an example of how you can add blim to an OCIO config:

colorspaces:
  - !<ColorSpace>
    name: blim
    family: Image Formation
    equalitygroup: ""
    bitdepth: unknown
    description: blim - Bean's Unprofessional View Transform
    isdata: false
    allocation: uniform
    from_scene_reference: !<GroupTransform>
      children:
        - !<ColorSpaceTransform> {src: Linear CIE-XYZ I-E, dst: Linear BT.709 I-D65}
        - !<AllocationTransform> {allocation: lg2, vars: [-12, 5, 0.000244140625]}
        - !<FileTransform> {src: blim.spi3d, interpolation: linear}
  ...

Paying attention to the transforms, you will notice a ColorSpaceTransform from CIE-XYZ I-E to Linear BT.709 I-D65. This is because the example OCIO config has its reference color space (the reference role) set to CIE-XYZ I-E. If your config already uses Linear BT.709 I-D65 as its reference this is not needed. If your config uses another color space as its reference, you should manually do a conversion to Linear BT.709 I-D65. You can get the conversion matrices using the Colour library.

Next, we have an AllocationTransform, which can be directly copied from the LUT comments. The AllocationTransform here literally just takes the log2 of the tristimulus (RGB) values and maps them from a specified range (the first two values after vars) to the [0, 1] range. The third value in vars is the offset applied to the values before mapping. This is done to keep the blacks.

Finally, a FileTransform references the 3D LUT.

Here's an example of how you can add blim as a view transform in an OCIO config:

displays:
  sRGB:
    - !<View> {name: blim, colorspace: blim}
    ...

... refers to the other view transforms in the config. ... is generally used as a placeholder for the other parts of the code. I can't believe I had to mention this, but a friend was actually confused by it.

Useful Links

• flim, Bean's Filmic Transform
• The Hitchhiker's Guide to Digital Colour - Troy Sobotka
• CG Cinematography - Christophe Brejon
• AgX Config Generator - Troy Sobotka
• RealBloom, Physically Accurate Bloom Simulation - Me
• Test Image Collection 1 - Troy Sobotka
• Test Image Collection 2 - Troy Sobotka
• PolyHaven, HDRIs