How do I convert between 2D paper space coordinates world 3D coordinates for AEC models in a forge viewer?

I'm following a blog post by Petr Broz with my published views from a Revit project in BIM 360, but I was wondering how I would convert between 2D coordinates and 3D world coordinates if my published 2D view is actually just a stand-alone floor plan view without being placed on any sheet.

I was trying to get the viewport but I realized the published view doesn't have any viewport in the model. So I cannot actually use those functions in the extension for viewports. Any advise would be greatly appreciated.

Solution

My mistake was manually trying to convert x,y,z coordinates by dividing them with the viewport scale, thus they couldn't align with 3D world coordinates.

By providing the correct model unitscale, I could get the transform with get3DTo2DMatrix().

    viewport = viewportExt.getViewports(viewer.model)[0];
    viewportXform viewport.get3DTo2DMatrix(viewer.model.getUnitScale());
    
    // From foot to millimeters
    x = x* 0.00328084;
    y = y* 0.00328084;
    z = z* 0.00328084;
    var worldPt = new THREE.Vector3(x,y,z);
    worldPt.applyMatrix4(viewportXform);