Extending a scanned trunk using vertex blending

Tree trunks are often close to the camera and difficult to model which makes them an excellent candidate for photogrammetry. This page describes how to seamlessly extend a scanned trunk using vertex blending. The idea is to wrap the end of the scanned mesh around the beginning of a child branch and use vertex based alpha blending to hide the seam. A procedural canopy can be created off of the procedural child resulting in a complete model.

Note: The description of the process that follows assumes you are familiar with the basics of SpeedTree and is more of a guide to the general approach than it is a step-by-step tutorial.

In order to use a 3d scan, you must first import the mesh as a mesh asset. Next, create a material asset that uses the textures associated with the scan. Be sure to associate the mesh asset with the material.

Meshes must be a single model that can have a single material on it. Photogrammetry meshes are typically saved as single meshes with unwrapped UV data that correspond to a texture atlas.

Once you have assets in place you'll use a Mesh generator to make it part of your model. Add a Mesh generator and then assign the material asset to it as pictured below. It will automatically pick up the mesh asset via its association with the material asset.

Note: You can move or rotate a mesh manually by switching to node editing mode, clicking on the mesh in the scene, and using the gizmo to transform the mesh.

Select the Mesh generator and add the “Extend trunk with vertex blending” geometry template. You should see a Stitch generator and a Branch generator added to the trunk as pictured below.

Create a material for the child branch and apply it. This material should match that of the scanned trunk as closely as possible (all maps: color, normal, gloss, etc.). In addition, make sure you tile the map on the child branch such that the bark pattern density matches that of the photogrammetry model. The quality of the blend will be greatly improved by a good match between the material on the photogrammetry model and the material/mapping that it is being blended onto.

The Stitch will be placed near the center of the mesh. It may be in a valid location but probably isn't. Switch to node editing mode, select either the stitch ring (if it's not valid) or the stitch geometry (if it is valid) and move the Stitch to the desired location. It should be placed near the end but not all the way to the end of the trunk as pictured below.

You must leave enough trunk geometry beyond the stitch location to be used in the blending process. The height of the blend region can be controlled in the “Blend” property group on the Stitch generator. Taller stitches require more trunk geometry beyond the stitch.

Now that you have the stitch where you'd like it, it's time to make the blend region as seamless as possible. Use the properties in the “Blend:Opacity” group to control the transparency of the trunk geometry in the stitch region. Vertices near the stitch line should be opaque while vertices near the top of the stitch will be transparent. Use the noise options to jumble up the effect. Use the properties in the “Blend:Geometry” group to shape the trunk geometry in the blend region to the child geometry. When you're done, you should have something like the picture below.

There will typically be a little bit of back and forth between blend settings, child positioning, child radius editing, and stitch positioning to get the stitch just right.

Use traditional SpeedTree techniques to add a procedural branch structure and leaf canopy to the model. Start by adding child branches to the blended branch and go from there. Pictured below are a completed model and the corresponding generator hierarchy.

Note: It's often a good idea to add a few twigs or other decorations to the photogrammetry mesh using Target generators to further blend it in with the procedural geometry.

The following list outlines a few things to keep in mind when using photogrammetry trunk meshes with vertex blending.

• Choose a reasonably smooth portion to place the stitch if you can.

• You can up the “Clearance” property of a stitch to “eat” more of the trunk geometry above the stitch.

• Switch to node editing mode to move the trunk mesh, stitch, and branch until the stitch is as seamless as possible.

• You can up the tesselation of the stitch. Doing so will cost extra polygons but it may make the stitch region blend more effectively.

• A single Stitch generator can create multiple stitches. Use this fact to stitch up several holes on a single mesh without having to have a unique generation editor configuration for each one.