Question about ExtentEnvelope and NavigatorPolicyFactory

[DJDuque]

Hi, I am slowly playing around with ACTS, and I am currently trying to understand the gen3 geometry building.

As a for-fun exercise I am playing around with some Rust bindings for ACTS (https://github.com/DJDuque/acts-rs), and I think I now have a basic understanding of the geometry building. But there are 2 things (so far) that I know I don't understand:

1. The ExtentEnvelope that you can pass to e.g. LayerBlueprintNode or Blueprint::Config. Are there pictures somewhere (maybe some old slides or docs I missed) to help understand this. And why would this be needed?
2. Same for the NavigationPolicyFactory. Any chance there are some old slides somewhere dedicated to explain this in more detail? I have no idea what these policies actually do, but is the issue with the default "TryAll" policy just a matter of efficiency?

Thanks

[asalzburger]

As for the ExtentEnvelope: this allows you to define some minimum tolerance envelope in certain directions for building the enclosure of geometry structures.

That is important:

• the navigation will run by smallest distance, hence to inter into a cylinder volume you want to approach this surface first, and then resolve all surfaces internal to this volume (given by the navigation policy). Hence, you do not want to already land on a surface at that stage because that would make the navigation ambiguous. The envelope guarantees that, e.g. for cylindrical volumes you'd set the envelope to something small different from 0 (e.g. in AxisR), which then in turn creates a little gap between the volume portal surface and the closest reachable surface in the inside.

[DJDuque]

Hi @asalzburger,

Thanks for the reply. So, if I understand correctly, the ExtentEnvelope is basically telling "make the volume extend at least X past the minimum required to enclose the surfaces". And you can set what this X is per direction.

But this X is a std::array<f64, 2>. What do the 2 numbers mean? For example, in the case of a cylinder volume and ExtentEnvelope{{.r = {a_mm, b_mm}}? Is a telling how much to extend/reduce (depending what makes the volume bigger) the inner radius and b the outer radius? Should these always be positive, or does e.g. the first entry in .r should be negative because we want the inner radius a little bit smaller? Similarly e.g. an ExtentEnvelope{{.x = {a_mm, b_mm}} are a along negative x and b along positive x?

What happens in the case of cylinder volume and ExtentEnvelope{{.x = {a_mm, b_mm}}? Or does e.g. a cylinder volume only look at the .z and .r directions and it ignores the others; while a cuboid volume only looks at .x, .y, and .z and it ignores e.g. .phi?

Thanks again for your reply.

[asalzburger]

The envelope can only increase the size of a volume, not shrink it.

Let's make it concrete:

• you want to build a volume with some detection elements in it
• let's assume you parsed the internal geometrical elements and you get the possible innermost radius is 55 mm, the biggest radial extent you found was 60 mm*
• if you now build a cylindrical volume with ExtentEnvelope{{.r = {2_mm, 4_mm}} envelope, you will end up with a cylinder with innermost radius: 55 mm - 2 mm = 53 mm and correspondingly a outer radius of 64mm (so the 2 numbers always refer to the envelopes at lower range and higher range)
• these numbers might be overruled by some attachment policy in the geometry construction, but that's another story.

[DJDuque]

This is very helpful. Thanks

[paulgessinger]

For the navigation policy: effectively what you're saying is correct:

The try all policy is correct but inefficient, especially with many surfaces.

There is a multi-policy that dispatches to a set of contained policies.

The factory exists because the policies are created with a finalized volume: the surfaces and child volumes are there, and the sizing is final. This allows the policy to create acceleration structures if desired. With the factory pattern, the policies and configuration can supplied beforehand, and then constructs the policies with the final volumes.