Making sense of Autobillboards

This isn't in the docs yet (edit: sounds like it's coming very soon though!) and is a very useful part of working with forests (as well as general T3D use), so I'm going to discuss it here. I'm also doing this so I have a place to put this information for myself. In this post, the words "billboard" and "imposter" are used interchangeably. This information is current as of T3D Beta 1.

What is "Autobillboarding"?
Autobillboarding is a technique of placing an Autobillboard detail node in your mesh hierarchy, alongside the other detail markers, so that T3D automatically creates multiple billboards for that mesh, generating both diffuse and normal maps. Using autobillboards is an easy way to manage a high polygon count in your environment. Within the User Properties of the autobillboarding marker, the artist will specify the number of billboards to be created around each axis of the mesh, and other useful properties detailed below.

Can't I just do this in the Shape Editor? Why should I do it in the source art file?
As of the T3D 1.1 Beta 1, some of this information can be set in the Shape Editor. However, this will not change the DAE file, and of course, will not change the source art file. Some find this to be a problem in the art pipeline. For this reason, I feel it is currently good practice to use the Shape Editor to find which settings are best, and then apply these to the original source file.

How do I add an autobillboard to my mesh?
Simply create a dummy node similar to a detail marker named BB::detailXX. The trailing number of the next-highest detail marker determines the threshold where the object moves between billboard and lowest LOD:



How do I specify its properties?
Now you need to add User Properties to that BB:: node to tell T3D what kind of billboards to create.:



The following settings allow the artist to specify how the billboards are created. The term "polar billboards" describes the two billboards that face straight up and straight down:

BB:DIM=
Specifies the dimensions of the billboards to be created. This is for a single billboard, not the entire sheet of billboards. In my case, each billboard will be 256x256.

BB:EQUATOR_STEPS=
Number of steps around the equator of the mesh. In my case, there are five steps around the equator that imposters are being created for.

BB:POLAR_STEPS=
Number of longitudinal steps between the polar billboards. Polar steps does not include or calculate imposters created for the top of bottom of the shape... only the billboards in between. The numbering of this may seem unintuitive, but here's ultimately how it works:

• A setting of 0 will give 1 polar step between the polar billboards
• A setting of 1 will give 3 polar steps between the polar billboards
• A setting of 2 will give 5 polar steps between the polar billboards
• A setting of 3 will give 7 polar steps between the polar billboards
• A setting of 4 will give 9 polar steps between the polar billboards

BB:POLAR_ANGLE=
Angle (in degrees) from the equator that acts as the threshold between the polar billboards and the rest of the billboards.

BB:INCLUDE_POLES=
This setting decides whether or not there are polar billboards. With this off, the POLAR ANGLE setting is irrelevent.

• =0 or =False turns this off
• =1 or =True turns this on

Note... as of this writing, a bug is keeping polar billboards from working properly, and is generating too many billboards to the imposter file. I'll post back when it's been fixed.

BB:DL=
Specifies the detail level of which mesh that is sampled to create billboards:

• =0 samples the highest LOD
• =1 samples the second-highest LOD
• =2 samples the third-highest LOD

Once the game begins loading a mesh with an Autobillboard node, T3D generates the imposter files that contain the billboard images. The files are titled meshname.imposter.dds and meshname.imposter_normals.dds.

Be cautious with these numbers. Keep in mind that in the example above, EQUATOR_STEPS=5 + POLAR_STEPS=2 with no polar billboards will give me 25 imposters. Each being 256x256, that will result in imposter files (one for diffuse/transparency maps, and one for normal map) that are both 1024x2048:



You can see that I have a lot of wasted space at the bottom of these textures. I could add another step to my EQUATOR_STEP setting, consequently adding another 5 billboards, without increasing the overall size of the imposter texture. That's a 20% gain in texture for barely any more overhead. A couple more tips:

• When changing the Autobillboard User Properties in your source art, you will have to delete the imposter files on disk in order for T3D to generate new ones upon startup.
• Make sure your bounding box is as tight on your mesh as possible, this will maximize the size of the rendered imposter inside your allotted billboard space, in my case 256x256. A larger bounding box will make the image of the tree smaller, a tighter bounding box will make the image of the tree larger, and have more resolution.

With all this done, your meshes should autobillboard in-game: