Allow 3D Raycasts to hit back faces on all collision shape types

[tucono]

Describe the project you are working on

A 3D fps game with a heavy focus on stealth and perception

Describe the problem or limitation you are having in your project

As part of my observation system, I need to determine both what objects are in the way of line-of-sight between two points, as well as how long those objects are in the line-of-sight path. While obtaining a list of objects is relatively trivial (continuous addition of objects to raycast exception lists until none are left), obtaining the lengths of the objects along the line is not as simple.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

The raycast hit_back_faces option currently works only on concave polygon shapes with back face enabled. However, it would be extremely useful if we could cast a ray from inside an object or area, and have it directly provide the collision point to the surface. This should hopefully be a single raycast call.

Currently, to find object lengths, I use a raycast with hit_from_inside on and iteratively step forward with small increments until outside the object, then point the raycast back at the object to find the external surface. Depending on scene density and the number of observers, this can result in a significant number of raycasts.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Build a raycast query with the following specifications:

• query.from : A point inside the area/object
• query.to : A point outside the area/object
• query.hit_from_inside = true
• query.hit_back_faces = true : Allow hitting the surface from inside the area/object. If false, it should return the same results as a current raycast (zero-ized vectors if hit_from_inside is true)

The result position would then contain the intersection point on the object/area surface, and the collision normal would be inverted to point inwards towards the object

How it works in code:
The simplest and most non-intrusive method is to modify the GodotPhysicsDirectSpaceState3D::intersect_ray function to do the following:

• If an internal hit is detected AND hit_back_faces AND hit_from_inside are true:
  • Check if endpoint is inside shape. If so, return a positive collision with zero vector for position & normal (current behavior)
  • Otherwise - swap to and from points of raycast and perform intersection check. Then invert the normal to show it is pointing internally.

I've got a version of this here

Notes:
For a collision object with multiple collision shapes, this will return the closest surface point for all shapes.

For intersecting collision objects, the raycast may be internal to one however it will report the other as the collision since it is closer to the raycast origin. This seems reasonable to me as turning on hit_back_faces is an indication that the user views the collision shapes as surfaces rather than volumes. (To that end, would it be better to require hit_from_inside=false to produce this behavior as a way to further enforce the volume vs surface mentality?)

If this enhancement will not be used often, can it be worked around with a few lines of script?

Technically yes, but with potentially significant performance impacts that can only be mitigated with reduced fidelity.

[tucono]

On reviewing the source code, nothing is standing out to me immediately in the collision shapes themselves that would preclude an internal raycast from providing surface results. However, this is all currently bypassed in the godot space 3D file ln 154 which either skips the shape entirely or simply returns a zeroized result. I'm curious if we simply modify the check to allow the raycast to continue if 'hit_back_faces=true` whether we would obtain my desired result.

I may try and build it this weekend and see what happens

[tucono]

If anyone else is following, it appears built-in shapes for godot were built with an assumption that all intersect_segment calls are expected to be from outside-in (origin outside the shape) so simply modifying the aforementioned if statement is not the solution. I've forked the engine and hope to make the basic shapes (box, cylinder, sphere) work. I have not yet tested complex shapes but am hoping FaceShape3D works out of the box here