Understanding the sculpt map format

Understanding the sculpt map format is one of the most important things a sculptie maker can know. This knowledge makes choosing the basic sculpt type easier and also allows many advanced techniques to be used in the modelling process. It will also help you to understand what the Primstar scripts are doing to make things quicker for you.

Don't worry if you don't understand all of this the first time you read it. I'll be mentioning the concepts here in different contexts throughout the manual. Come back and read it again after finishing the manual and creating a few sculpties and it should be a lot easier to follow.

All sculpties are a single continuous surface

While sculpties can be made to appear in world as a number of pieces, in reality they are one continuous surface.

It is useful to think of the surface as a profile translated along a path. The profile and path can change across the surface, so it's not strictly accurate, but it will help to keep this idea in mind when modelling.

When adding or removing detail from the mesh, you'll get better results working on an entire profile or path than trying to extrude or cut single faces. You can even start from a single quad face and as long as you extrude the entire edge of the profile or path when adding detail, the final mesh will convert easily and accurately to a sculptie.

Loop cuts are the easiest way to add detail into the middle of the surface as this will add a full profile or path line in a single step.

All sculpties are a plane

It might seem odd if you've seen Sphere, Cylinder and Torus sculpt types, but all sculpties are really just a plane. The difference is in how the edges of the plane are handled. The following picture shows how the seam on a cylinder places the same profile in two places on the sculpt map.

It's like a square of paper wrapped around a tube so it's ends touch. Cut the seam and flatten out the paper and you are back to the planar surface.

The torus shapes also have a circular profile. When that is rotated around the circular path, the donut shape is formed. This also makes a seam on the path where it is closed together.

The Torus X shape has the path and profile reversed. This only becomes relevant if you intend breaking the path seam to turn the torus into a cylinder.

The sphere's profile is half a circle, so when that is rotated around the circular path, a sphere is formed. In Primstar, we use a cylinder style mesh for the sphere to allow the use of subdivision. The ends of the cylinder are scaled to 0 to form the poles of the sphere.

If you set any sculptie baked in Primstar to a Planar type in SL, it should look just the same as when set to it's proper type. It makes you wonder why we have the other types.

Sculpt map ratios

The texture pipeline in Second Life means that only power of two images are supported for sculpt maps. By this I mean that both U and V sizes of the sculpt map are one of these sizes: 8, 16, 32, 64, 128, 256 or 512. Current issues with lossload uploads mean that only width and height combinations that give 4096 or 8192 pixels are supported. These are all the currently supported ratio sizes in Second Life:

1:1 64 x 64

1:2 64 x 128

1:4 32 x 128

1:8 32 x 256

1:16 16 x 256

1:32 16 x 512

1:64 8 x 512

Obviously, you can switch the U and V sizes to use the opposite ratios. Primstar also supports smaller map sizes which limit the maximum number of faces, but these can't be used with Second Life until the lossless upload issues are resolved.

Sculptie Level of Detail

Second Life can display each sculptie at four different levels of detail (LOD). In Primstar, we refer to each of these levels with a number from 0 (lowest detail) to 3 (highest detail). Each LOD has a maximum number of faces that can be displayed:

LOD3: 1024

LOD2: 256

LOD1: 64

LOD0: 36

The distribution of those faces on the surface is directly related to the ratio of the sculpt map. So if we look at the 1:1 ratio at LOD3, we can work out that it is 32 x 32 faces, which gives the 1024 total faces.

If we were using a 2:1 ratio, then we get 46 x 22 faces for a total of 1012 faces. We can't use the exact 2:1 ratio of 46 x 23 faces as that's 1058 faces, more than is allowed at LOD3.

At the opposite end of the LOD range, sculpties have a minimum face count for the path and profile of 4 faces. This means that anything other than the 1:1 ratio map ends up as 4 x 9 faces at LOD0 to make the most of the 36 available faces. This generally means we ignore LOD0 during the modelling and work at LOD1 or higher.

If all this math has confused you, don't worry as Primstar will do the calculations for you. When you add a sculpt mesh, you can adjust the settings for the modelling mesh and see the sculpt map size that will be used, along with the face counts for each LOD level.

Sculptie Mesh Resolution

The sculpt map is a representation of the 3D surface as an image. This is achieved by converting the X, Y and Z co-ordinates of the mesh into the RGB components of the image. As the sculpt maps supported by Second Life use 1 byte per channel, this limits the mesh to 256 positions in each direction. This means you need to avoid aliasing problems with your modelling. Small faces are much more likely to suffer from aliasing issues as there are less angles available with fewer RGB steps.

A popular way to work on precision sculpties is to scale the object to 2.55 and offset by 1.275 on all 3 axis. This aligns the mesh with Blender's grid from 0.0 to 2.55, so you can model with snapping. Be careful though as the final mesh size should still be 2.55 x 2.55 x 2.55 for the bake to preserve the grid.