Thatâs not the best way to do it, in case you want to downscale a CLUT, because it is equivalent to use the ânearest-neighborâ interpolation to the 3D CLUT cube for downsizing.
Moreover, you cannot easily generate a .png haldclut for a 8x8x8 or a 12x12x12 CLUT.
What you really want is using an averaged interpolation instead (because you want downsizing), which means you want to average neighboring colors in the 3D CLUT when decreasing the resolution.

I donât know if ImageMagick is able to do that (not sure it does interpolation of 3D images).
With GâMIC, it can be done with something like:

The ImageMagick â-hald-clutâ operation changes each pixel in an image by looking up the colour in the HALDclut image, which is conceptually 3D. It does a 3D interpolation, not a simple nearest-neighbour, to calculate each output pixel. Hence I donât think there is a lack of precision with the method I showed. (But it would take more time than I have today to demonstrate this.)

ImageMagick doesnât have GâMICâs powerful ability to manipulate (including resizing) general 3D images.

There may be some confusion over terminology. In ImageMagick, the level of a HALD clut is the square root of the length of each side of the cube. For example, HALD:8 is a cube with 64x64x64 = 262144 entries, and HALD:12 has 2985984 entries.

ImageMagick canât create HALDcluts of 8x8x8 or 12x12x12. The smallest sizes are 4x4x4, 9x9x9, 16x6x16, 25x25x25, and so on.

A more general clut wouldnât need to be a cube, eg it might have 5x13x7 entries. ImageMagick canât process those.

This is excellent, almost exactly what i was looking for. However is it possible in GâMIC to take an png haldclut in as an input and output it as a cube. Either as a straight through or a shrinking it as it goes.