Hi Alastair,
One of my friends has asked me whether it’s possible to colour an elevation survey by a blended gradient rather than a discrete scrap by scrap basis.
As far as I know, the answer is "no", and I don't think it would actually be possible. It's not like Survex, where each line or 2D polygon can be shaded with a gradient, knowing the exact 3D altitude of each point within the polygon. Therion rendering is not made from polygons, it is made from a series of 2D vectors positioned arbitrarily around 3D points. Imagine that you have a scrap with 2 stations in it. How should each part of the scrap be coloured: The nearest station point within the scrap? Blended between the 2 nearest station points within a scrap? What about when the scrap protrudes beyond the last station? That's for a simple case. Now try 3 stations in a V shape. The point in the middle is equidistant from all of them. How should it blend? Does the wall beside it belong to the lowest station, the highest station or the middle one? Draw a line from the wall to one station. Should the colour change along that path? Imagine a simple passage with three stations. Start at station1. Go up a slope to station 2. Turn hard right and go up a slope to station 3. Scrap ends. At station 1, the right wall bulges out so from above, it looks like it almost touches station 3, but it does not connect. How should the bulge be coloured, given that its closest station within the scrap is #3? As a caver, you would say 3, but it would be a challenge to describe a solution for that to a computer program like Therion.
would it be possible to metapost a gradient
Yes. Assuming you want linear gradients, not complex ones. Caves are complex. And Therion uses a 7 stage blend to get the rainbow effect. So you would have to be able to do much more than just a simple colour-from,colour-to gradient.
I think his solution relied upon the 1 colour for one scrap method
Given the lengths of passage involved ("every passage in Aggy is twice as long as you want it to be" - Tarquin), that is how I would expect it to be done, and relatively easily too. This is why the general advice is to have each scrap cover a maximum of 5 metres of altitude within the cave. I would love for someone to prove me wrong though, because pure gradients within a cave survey sound awesome.