OpenColorIO

Open Source Color Management

OpenColorIO v1.0.8 documentation

FAQ

License?

New BSD.

You are welcome to include the OpenColorIO in commercial, or open source applications. See the License for further details.

Terminology

  • Transform - a function that alters RGB(A) data (e.g transform an image from scene linear to sRGB)
  • Reference space - a space that connects colorspaces
  • Colorspace - a meaningful space that can be transferred to and from the reference space
  • Display - a virtual or physical display device (e.g an sRGB display device)
  • View - a meaningful view of the reference space on a Display (e.g a film emulation view on an sRGB display device)
  • Role - abstract colorspace naming (e.g specify the “lnh” colorspace as the scene_linear role, or the color-picker UI uses color_picking role)
  • Look - a color transform which applies a creative look (for example a per-shot netural grade to remove color-casts from a sequence of film scans, or a DI look)

What LUT Formats are supported?

Ext Details Notes
3dl Autodesk Apps: Lustre, Flame, etc. Supports shaper LUT + 3D Read + Write Support.
ccc ASC CDL ColorCorrectionCollection Full read support.
cc ASC CDL ColorCorrection Full read support.
csp Cinespace (Rising Sun Research) LUT. Spline-based shaper LUT, with either 1D or 3D LUT. Read + Write Support. Shaper is resampled into simple 1D LUT with 2^16 samples.
cub Truelight format. Shaper Lut + 3D Full read support.
cube Iridas format. Either 1D or 3D Lut. Full read support
hdl Houdini. 1D Lut, 3D lut, 1D shaper Lut Only ‘C’ type is supported. Need to add R G B A RGB RGBA ALL. No support for Sampling tag. Header fields must be in strict order.
look IRIDAS .look Read baked 3D LUT embedded in file. No mask support.
mga/m3d Pandora 3D lut Full read support.
spi1d 1D format. Imageworks native 1D lut format. HDR friendly, supports arbitrary input and output domains Full read support.
spi3d 3D format. Imageworks native 3D lut format. Full read support.
spimtx 3x3 matrix + color offset. Imageworks native color matrix format Full read support.
vf Inventor 3d lut. Read support for 3d lut data and global_transform element

Note

Shaper LUT application in OCIO currently only supports linear interpolation. For very small shaper LUT sizes this may not be sufficient. (CSP shaper luts excluded; they do use spline interpolation at load-time).

Can you query a color space by name (like “Rec709”) and get back XYZ coordinates of its primaries and whitepoint?

Not currently.

OCIO is a color configuration ‘playback’ tool that tries to be as flexible as possible; color information such as this is often only needed / relevant at configuration authoring time. Making primaries / whitepoint required would limit the pipeline OCIO could service. In the strictest sense, we would consider OCIO to be a ‘baked’ representation of color processes, similar to how Alembic files do not store animation rig data, but rather only the baked geometry.

Also, remember that not all colorspaces using in visual effects even have strongly defined color space definitions. For example, scanned film negatives, when linearized with 1d transfer curves (the historical norm in vfx), do not have defined primaries/white point. Each rgb value could of course individually be tied to a specific color, but if you were to do a sweep of the pure ‘red channel’, for example, you’d find that it creates a curves in chromaticity space, not a single point. (This is due to the 1d linearization not attempting to undo the subtractive processes that created the pixels in the first place.

But many color spaces in OCIO do have very strongly defined white points/chromaticities. On the display side, for example, we have very precise information on this.

Perhaps OCIO should include optional metadata to tag outputs? We are looking at this as as a OCIO 1.2 feature.

Can you convert XYZ <-> named color space RGB values?

OCIO includes a MatrixTransform, so the processing capability is there. But there is no convenience function to compute this matrix for you. (We do include other Matrix convenience functions though, so it already has a place to be added. See MatrixTransform in export/OpenColorTransforms.h)

There’s talk of extended OCIO 1.2 to have a plugin api where colorspaces could be dynamically added at runtime (such as after reading exr chromaticity header metadata). This would necessitate adding such a feature.

What are the differences between Nuke’s Vectorfield and OCIOFileTransform?

(All tests done with Nuke 6.3)

Ext Details Notes
3dl Matched Results  
ccc n/a  
cc n/a  
csp Difference Gain error. Believe OCIO is correct, but need to verify.
cub Matched Results Note: Nuke’s .cub exporter is broken (gain error)
cube Matched Results  
hdl n/a  
mga/m3d n/a  
spi1d n/a  
spi3d n/a  
spimtx n/a  
vf Difference Gain error. Believe OCIO is correct, but need to verify.

All gain differences are due to a common ‘gotcha’ when interpolating 3d luts, related to internal index computation. If you have a 32x32x32 3dlut, when sampling values from (0,1) do you internally scale by 31.0 or 32.0? This is typically well-defined for each format, (in this case the answer is usually 31.0) but when incorrectly handled in an application, you occationally see gain errors that differ by this amount. (In the case of a 32-sized 3dlut, 32/31 = ~3% error)

What do ColorSpace::setAllocation() and ColorSpace::setAllocationVars() do?

These hints only come into play during GPU processing, and are used to determine proper colorspace allocation handling for 3D LUTs. See this page How to Configure ColorSpace Allocation for further information.