How to fix wrong unprojected positions?

woopdeedoo

I'm drawing lines in 2D to represent selections/bounding boxes in 3D, because I can't use GLES3 and there's no line thickness in 3D in GLES2. I unproject the vertices, and go from there. There is one problem, though: when a vertex is behind the camera, the unprojection comes out wrong.

I'm using CanvasItem.draw_polyline(), so if I abort the line drawing when a point is behind the camera, the selection lines will disappear. If I don't, then I get this:

Is there some way to fix this?

It seems to me that all the lines that are wrong (the ones converging at the center of the screen) all follow the exact opposite direction that they would if they were ok. Maybe there is some way I could rotate them 180º? I tried rotating the unprojected vector of the end-point (the one behind the camera) but the results weren't any good.

Calinou

Camera has a is_position_behind() method you can use to check whether a point in 3D space is behind the camera.

cybereality

Maybe try clipping the line at the near plane? You should be able to clamp it so it is just on the near plane (or slightly in front) with some simple math.

woopdeedoo

@Calinou said: Camera has a is_position_behind() method you can use to check whether a point in 3D space is behind the camera.

Sorry, I wasn't clear at all, but I'm indeed using that to be able to tell when to draw lines or when to abort drawing. The thing is when I abort drawing, it's still not ideal because then the entire polyline disappears. So I'm trying to see if there's a solution to the wrong unprojections that might be better.

@cybereality said: Maybe try clipping the line at the near plane? You should be able to clamp it so it is just on the near plane (or slightly in front) with some simple math.

I suppose you mean finding a point between the two 3D points that is still slightly in front of the camera, and then unprojecting that point?

That might work. I'm not sure I know how to do it, but maybe I have an idea. I'm gonna give that a try.

cybereality

It's this function clip polygon you want to use. https://docs.godotengine.org/en/stable/classes/class_geometry.html#class-geometry-method-clip-polygon

woopdeedoo

Wow that just made it all so simple! I was pulling my hairs trying to process each vertex. :)

So basically this code seems to work perfectly:

func unprojected_polyline(verts):
	var points := []
	var clipped_verts = Geometry.clip_polygon(verts, cam.get_frustum()[0])
	for v in clipped_verts:
		points.append( cam.unproject_position(v) )
	return points

Thank you.

cybereality

Awesome, it worked!

woopdeedoo

Unfortunately, I ended up realizing that 2D lines are always drawn on top of all the 3D geometry, which is not what I want in some cases.

So I've been experimenting with Calinou's suggestion of using 3D meshes, but that's also not without its problems.