Sections & Spans

I think it is 16 by 16 is the maximum but can I do something like 8 sections and 20 spans on a sphere?

Technically, there really is no minimum or maximum. The sculpty exporter works by sampling the physical surface of the model, not by reporting isoparm or CV positions. The reason for sticking with 16x16 is simply to make sure you don't have so much surface complexity that the exporter won't be able to capture it all in a sculpt map. With only 32x32 faces to work with on an in-world (square) sculpty, trying to reproduce more than 16x16 spans is a risk.

When you go into oblong sculpties, it's best to stick with the following logic. If you halve one axis, double the other. So, if you're doing 8 sections, I'd go with 32 spans. And make sure you do likewise on your sculpt map dimensions. If you cut one axis down from 64 to 32 pixels, double the other from 64 to 128. Make sense?

Now, would 20 spans actually hurt anything? Probably not. But I like to keep things clean. With 32, you'll get a better sense of where the actual polygons will be on the in-world version than you would with 20. This will make things like LOD culling and texture placement far more predictable.

If you use a 128 x 64 sculpt map you will get the following LODs:

LOD 3: 46 x 22
LOD 2: 23 x 11
LOD 1: 12 x 5
LOD 0: 9 x 4

So 23 x 11 is the equivalent of the 16 x 16 for this size of map.

The other available ratios on oversized maps don't have nice clean switches between any of the LOD levels, so for fine control you really need to model at the highest LOD.

Map Size: 256 x 32

LOD 3: 93 x 11
LOD 2: 51 x 5
LOD 1: 16 x 4
LOD 0: 9 x 4

Map Size: 512 x 16

LOD 3: 204 x 5
LOD 2: 64 x 4
LOD 1: 16 x 4
LOD 0: 9 x 4

Domino, those numbers are surprising me. I'll take your word for it that they're accurate, since I've never known you to be incorrect about anything technical.

But if that's the case, then it seems I've been mistaken myself for quite some time now. I'd thought oblongs stuck with powers of two for their highest LOD meshes, meaning 64x16 quads, 128x8 quads, and 256x4 quads, all maintaining a nice neat total of 1024. I also thought the maps should simply be double those numbers, 128x32px, 256x16px, and 1024x8px, respectively, just as the standard 64x64 map doubles the standard 32x32 mesh.

Back when oblongs were first proposed to Qarl (then referred to as "rope prims", the idea was to take the same number of polygons that made up existing sculpties, and simply redistribute them rectangularly instead of squarely, in nice, predictable, power of two fashion. But from what you're saying, that's not actually what was implemented.

Remember that sculpty dev meeting you asked me to attend back when oblongs were still in the works? I thought we'd established right then and there that the above scheme was how it was going to be. What changed between then and now?

It's all down to how the ratio is calculated. https://jira.secondlife.com/browse/VWR-9384 was the patch that moved some ratios to the 8192 pixel maps.

Basically the important things are the ratio of the map and the total faces for the requested LOD. The formula favors the ratio, the power of two stuff occurs naturally with certain map sizes.

For a 128 x 64 map and it's 2:1 ratio we get 46 x 22 (1012 quad faces) which is the closest we can get to 1024 faces. With smaller map sizes, the limit for maximum faces (number of pixels / 2) kicks in and forces things to power of two ratios.

So a 64 x 32 map gives 32 x 16 faces. But 32 x 16 is only 512 faces.. We've sacrificed a lot of faces to get to a 2:1 power of two ratio. This was one of Qarl's concerns with VWR-9384 until I pointed out the behaviour of the 128 x 64 map size.

Sticking with the half one direction, double the other technique (which is good advice for most cases): 128 x 32 gives a 4:1 ratio so we get 64 x 16 faces (the full 1024).

I've coded all this into Primstar, so when you add a sculpt mesh it's easy to see what the final map size and LODs will be.



At the moment, due to the lossless upload problems http://jira.secondlife.com/browse/VWR-8551 only 4096 and 8192 pixels maps will work correctly. So 64 x 64 or 128 x 64 plus their ratio variations (half one side, double the other) are the only ones that work.

I've published the first draft of my overview of the sculpt map format.

http://dominodesigns.info/manuals/primstar/understanding_sculpt_maps

I've probably explained things a little differently than most people would, and probably in less detail than some would like. There's a couple of pictures missing and it could do with a better description for "profile translated along a path".

But all the key information is there including a full list of the different sculptie ratios. Obviously the description is Primstar orientated, but just swap path and profile for section and span to pretend you are reading about Maya nurbs

Thanks, Domino. That's great information.

I had never known that several of the ratios you mentioned were possible. I'd been using just 64x64, 32x128, 16x256, and 8x1024 all this time.

I noticed you did not have 8x1024 on your list, so I did a little playing around, and I came up with some interesting results:

1. An 8x1024 sculpt map yields a 4x256 quad mesh, if my count is right. This is exactly the same as using an 8x512 map. So I can cut my map size in half on this kind of mesh, which is good to know.

2. An 8x8 map yields a 4x4 mesh. Interesting. Good for gemstones and such.

That's all I've got so far. I'm so glad for this thread. Always fun to learn new things.

1) Yeah, 8 x 1024 and 8 x 512 should give the same result.

2) Hopefully sometime, the lossless bug will be fixed, so all the other sizes open up. They just repeat the ratios, but let us control the maximum number of faces used. On the smaller sizes just divide the width and height by two to get the number of LOD3 faces. 4 x 4 is obviously the 1:1 ratio on an 8 x 8 map. At a total of 16 faces, it's less than the LOD0 36 faces and would look the same at all 4 LOD levels.