Unproject concept misunderstanding

Questions
1

Hey guys!
It seems I misunderstand unproject concept.
Lets say I have a camera at position (20, 0 ,0).
Then I’m creating new vector and unproject it from camera NDC:
THREE.Vector3().unproject(camera));
In this case I’m expecting this vector after unprojecting in world coordinates to have the same coordinates as the camera - (20, 0, 0).
But in reality it returns (18.000199980002, 1.1021943644760852e-15, 7.806019937778277e-16).
While Y and Z looks fine why X become 18, it is not looking like precision error?

Here is a codepen:

2

not sure about the math but i used these answers to fit a plane perfectly onto the screen viewport, maybe it can help you

1 Like
3

thanks, I’ll check it!

4

What version of 3JS are you using ?
If recent try this:

camera.position.set(20, 0 ,0);
camera.updateMatrixWorld();
new THREE.Vector3().unproject(camera);

2 Likes
5

I’m using the latest(134) but I also checked a few more versions and it works the same way there. I’ve found an interesting thing based on your advice:
when I unproject the camera inside the init function - it returns the vector (20, 0, -1.9998000199979997). (no clue why z is not zero here also)
but when I unproject camera inside animate function - it returns (18.000199980002, -1.2245243467126813e-16, -4.4404480536952556e-16) (I see some pattern here, since 20 - 1.9998000199979997 is equal 18.000199980002, but this computations and differences still make no sense to me)

Here is an example of this behavior:

6

I’ve found out that the camera near plane plays the crucial role in this calculations, so basically I was thinking of NDC as a basis with a center in the camera position but it ends up being more tricky thing

1 Like
7

Great Find,
I always set Camera Near to small float close to zero. With Camera Near set to 0.0001, the formula return the correct coords. As to why 1 returns wrong data is beyond my paygrade.
I would consider this a bug!