How to Configure ColorSpace Allocation

The allocation / allocation vars are utilized using during GPU 3dlut / shader text generation. (Processor::getGpuShaderText, Processor::getGpuLut3D).

If, in the course of GPU processing, a 3D lut is required, the “allocation / allocation vars” direct how OCIO should sample the colorspace, with the intent being to maintain maximum fidelity and minimize clamping.

Currently support allocations / variables:

ALLOCATION_UNIFORM::

2 vars: [min, max]

ALLOCATION_LG2::

2 vars: [lg2min, lg2max] 3 vars: [lg2min, lg2max, linear_offset]

So say you have an srgb image (such as an 8-bit tif), where you know the data ranges between 0.0 - 1.0 (after converting to float). If you wanted to apply a 3d lut to this data, there is no danger in samplingthat space uniformly and clamping data outside (0,1). So for this colorspace we would tag it:

allocation: uniform
allocationvars: [0.0, 1.0]

These are the defaults, so the tagging could also be skipped.

But what if you were actually first processing the data, where occasionally small undershoot and overshoot values were encountered? If you wanted OCIO to preserve this overshoot / undershoot pixel information, you would do so by modifying the allocation vars.

allocation: uniform
allocationvars: [-0.125, 1.125]

This would mean that any image data originally within [-0.125, 1.125] will be preserved during GPU processing. (Protip: Data outside this range may actually be preserved in some circumstances - such as if a 3d lut is not needed - but it’s not required to be preserved).

So why not leave this at huge values (such as [-1000.0, 1000.0]) all the time? Well, there’s a cost to supporting this larger dynamic range, and that cost is reduced precision within the 3D luts sample space. So in general you’re best served by using sensible allocations (the smallest you can get away with, but no smaller).

Now in the case of high-dynamic range color spaces (such as float linear), a uniform sampling is not sufficient because the max value we need to preserve is so high.

Say you were using a 32x32x32 3d lookup table (a common size). Middle gray is at 0.18, and specular values are very much above that. Say the max value we wanted to preserve in our coding space is 256.0, each 3d lut lattice coordinates would represent 8.0 units of linear light! That means the vast majority of the perceptually significant portions of the space wouldnt be sampled at all!

unform allocation from 0-256: 0 8.0 16.0 … 240.0 256.0

So another allocation is defined, lg2

- !<ColorSpace>
  name: linear
  description: |
      Scene-linear, high dynamic range. Used for rendering and compositing.
  allocation: lg2
  allocationvars: [-8, 8]

In this case, we’re saying that the appropriate ways to sample the 3d lut are logarithmically, from 2^-8 stops to 2^8 stops.

Sample locations: 2^-8: 0.0039 2^-7: 0.0078 2^-6: 0.0156 … 2^0: 1.0 … 2^6: 64.0 2^7: 128.0 2^8: 256.0

Which gives us a much better perceptual sampling of the space.

The one downside of this approach is that it can’t represent 0.0, which is why we optionally allow a 3d allocation var, a black point offset. If you need to preserve 0.0 values, and you have a high dynamic range space, you can specify a small offset.

Example:

allocation: lg2
allocationvars: [-8, 8, 0.00390625]

The [-15.0, 6.0] values in spi-vfx come from the fact that all of the linearizations provided in that profile span the region from 2^-15 stops, to 2^6 stops. One could probably change that black point to a higher number (such as -8), but if you raised it too much you would start seeing black values be clipped. Conversely, on the high end one could raise it a bit but if you raised it too far the precision would suffer around gray, and if you lowered it further you’d start to see highlight clipping.