I brought this up on the CLF Spec Rev VWG #7 call and it was suggested that I post it on ACESCentral.
For the LUT1D node I’ve been thinking about the awesomeness of the “halfDomain” 16f indexed LUTs and they bring a lot to the table, especially for conversions from linear. However, they are currently fixed in size (at 2^16 elements), and so quite large (262K bytes) and on a GPU with limited memory, they could start to add up and compete with other large textures. By comparison, for our dozen currently shipping LUTs that take linear input (pre-shaped with a log shaper) we use a mixture of 8K, 16K, 32K, and 64K element LUTs.
Therefore, It would be nice to have the same option for halfDomain LUTs. So in addition to 64K elements, I recommend also supporting 32K, 16K, 8K, and 4K element halfDomain LUT1D nodes. In terms of implementation, one would right-shift off the lower bits of the 16f index by 1 to 4 (or more) places to index the smaller ranges (they are all powers of 2). I’m happy to elaborate on the technical details if they are not clear. As with any LUT1D lookup, the output values would be interpolated (just like you already need to do with 32f input values anyway).