Drawing a line between two points using SceneKit

Here's a simple extension in Swift:

extension SCNGeometry {
    class func lineFrom(vector vector1: SCNVector3, toVector vector2: SCNVector3) -> SCNGeometry {
        let indices: [Int32] = [0, 1]

        let source = SCNGeometrySource(vertices: [vector1, vector2])
        let element = SCNGeometryElement(indices: indices, primitiveType: .Line)

        return SCNGeometry(sources: [source], elements: [element])

    }
}

There are lots of ways to do this.

As noted, your custom geometry approach has some disadvantages. You should be able to correct the problem of it being invisible from one side by giving its material the doubleSided property. You still may have issues with it being two-dimensional, though.

You could also modify your custom geometry to include more triangles, so you get a tube shape with three or more sides instead of a flat rectangle. Or just have two points in your geometry source, and use the SCNGeometryPrimitiveTypeLine geometry element type to have Scene Kit draw a line segment between them. (Though you won't get as much flexibility in rendering styles with line drawing as with shaded polygons.)

You can also use the SCNCylinder approach you mentioned (or any of the other built-in primitive shapes). Remember that geometries are defined in their own local (aka Model) coordinate space, which Scene Kit interprets relative to the coordinate space defined by a node. In other words, you can define a cylinder (or box or capsule or plane or whatever) that's 1.0 units wide in all dimensions, then use the rotation/scale/position or transform of the SCNNode containing that geometry to make it long, thin, and stretching between the two points you want. (Also note that since your line is going to be pretty thin, you can reduce the segmentCounts of whichever built-in geometry you're using, because that much detail won't be visible.)

Yet another option is the SCNShape class that lets you create an extruded 3D object from a 2D Bézier path. Working out the right transform to get a plane connecting two arbitrary points sounds like some fun math, but once you do it you could easily connect your points with any shape of line you choose.

New code for a line from (0, 0, 0) to (10, 10, 10) below. I'm not sure if it could be improved further.

SCNVector3 positions[] = {
    SCNVector3Make(0.0, 0.0, 0.0),
    SCNVector3Make(10.0, 10.0, 10.0)
};

int indices[] = {0, 1};

SCNGeometrySource *vertexSource = [SCNGeometrySource geometrySourceWithVertices:positions
                                                                          count:2];

NSData *indexData = [NSData dataWithBytes:indices
                                   length:sizeof(indices)];

SCNGeometryElement *element = [SCNGeometryElement geometryElementWithData:indexData
                                                            primitiveType:SCNGeometryPrimitiveTypeLine
                                                           primitiveCount:1
                                                            bytesPerIndex:sizeof(int)];

SCNGeometry *line = [SCNGeometry geometryWithSources:@[vertexSource]
                                            elements:@[element]];

SCNNode *lineNode = [SCNNode nodeWithGeometry:line];

[root addChildNode:lineNode];