# Project a 2D screen point to 3D world point

Hi,

I saw the solution here and I have problems to understand the solution.
Specifically I don’t understand,

• why setting arbitrary z value is ok. I read about Normalized Device Coordinate (NDC) in the Internet but I couldn’t find an answer.
• the subtraction of the camera position from the ray direction followed by adding the result to the camera position.

Can someone please add a vector diagram that explains what these operations do?

Thanks,
Avner

I found a very useful illustration of the operations here

As I understand,

after the line:
`vec.set( ( event.clientX / window.innerWidth ) * 2 - 1, - ( event.clientY / window.innerHeight ) * 2 + 1, 0.5 );`
vec is the position of a 3D point (pointA) in space along the ray in Normalized Device Coordinates

after the line:
`vec.unproject( camera );`
vec is the position of pointA in 3D space in world coordinates

after the line:
`vec.sub( camera.position ).normalize();`
vec is the normalized ray direction from the camera

after the line:
`var distance = ( targetZ - camera.position.z ) / vec.z;`
distance is the distance/scale by which to travel along the ray, until reaching a point on the ray which lies in plane z=targetZ

after the line:
`pos.copy( camera.position ).add( vec.multiplyScalar( distance ) );`
pos is the position of the 3D point (pointB) which lies on the ray and is at z==targetZ