I have set up a scene in SceneKit and have issued a hit-test to select an item. However, I want to be able to move that item along a plane in my scene. I continue to receive mouse drag events, but don't know how to transform those 2D coordinates into 3D coordinate in the scene.
My case is very simple. The camera is located at 0, 0, 50 and pointed at 0, 0, 0. I just want to drag my object along the z-plane with a z-value of 0.
The hit-test works like a charm, but how do I translate the mouse point from a drag event into a new position in the scene for the 3D object I am dragging?
You don't need to use invisible geometry — Scene Kit can do all the coordinate conversions you need without having to hit test invisible objects. Basically you need to do the same thing you would in a 2D drawing app for moving an object: find the offset between the mouseDown: location and the object position, then for each mouseMoved:, add that offset to the new mouse location to set the object's new position.
Here's an approach you could use...
Hit-test the initial click location as you're already doing. This gets you an SCNHitTestResult object identifying the node you want to move, right?
Check the worldCoordinates property of that hit test result. If the node you want to move is a child of the scene's rootNode, these is the vector you want for finding the offset. (Otherwise you'll need to convert it to the coordinate system of the parent of the node you want to move — see convertPosition:toNode: or convertPosition:fromNode:.)
You're going to need a reference depth for this point so you can compare mouseMoved: locations to it. Use projectPoint: to convert the vector you got in step 2 (a point in the 3D scene) back to screen space — this gets you a 3D vector whose x- and y-coordinates are a screen-space point and whose z-coordinate tells you the depth of that point relative to the clipping planes (0.0 is on the near plane, 1.0 is on the far plane). Hold onto this z-coordinate for use during mouseMoved:.
Subtract the position of the node you want to move from the mouse location vector you got in step 2. This gets you the offset of the mouse click from the object's position. Hold onto this vector — you'll need it until dragging ends.
On mouseMoved:, construct a new 3D vector from the screen coordinates of the new mouse location and the depth value you got in step 3. Then, convert this vector into scene coordinates using unprojectPoint: — this is the mouse location in your scene's 3D space (equivalent to the one you got from the hit test, but without needing to "hit" scene geometry).
Add the offset you got in step 3 to the new location you got in step 5 - this is the new position to move the node to. (Note: for live dragging to look right, you should make sure this position change isn't animated. By default the duration of the current SCNTransaction is zero, so you don't need to worry about this unless you've changed it already.)
(This is sort of off the top of my head, so you should probably double-check the relevant docs and headers. And you might be able to simplify this a bit with some math.)