Example of new dimscale for jit.gl.slab and jit.gl.imageunit?

I got the impression from the 1.6.2 release notes that the new dimscale feature of jit.gl.slab and jit.gl.imageunit might offer an answer to an issue I've been trying to resolve: high-quality downsampling/interpolation of hi-res images of approx. 6000x4000 (about what you get from a Nikon 5000ED slide scanner at highest res). Alas, I've tried dimscale with objects both without success. I did start to get results from jit.gl.imageunit (Lanczos), but it stopped working at increased sizes (I'm assuming the break point is that magic number 4096?).

Can you please provide an example in one or both of these objects that downsamples a large jit.matrix of the size in question to a video-res matrix with high-quality interp (Lanczos being a good example).

If it helps, I've posted a zip containing a family of test files: the largest file I'm working with, as well as Photoshop downsamples of it that serve both as test files and references for what a decent downsample looks like at various sizes. (Ignore stretched aspect ratios.):

www.spirittreasures.com/guest/scaling_test_JPEGs.zip

Thanks so much!

Jeff