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For a camera movement in three.js I need to calculate point C so to move the camera from point A to a certain distance dist to point B.

For a camera movement in three.js I need to calculate point C so to move the camera from point A to a certain distance dist to point B.

• javascript
• math
• 3d
• three.js

Share Improve this question Follow asked Mar 15, 2018 at 23:39 piaaaacpiaaaac 6122 silver badges1010 bronze badges

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3 Answers 3

Sorted by: Reset to default 6

three.js has methods to do that very easily.

Assuming a, b, and c are instances of THREE.Vector3(),

a.set( 2, 1, 4 );
b.set( 9, 4, 2 );

c.subVectors( a, b ).setLength( dist ).add( b );

three.js r.91

So you need to calculate the coordinates of point C, given that it lies on the line between B and A at the given distance from B? This is pretty straightforward using the following steps:

• Calculate the vector from B to A (this will just be A - B).
• Normalize that vector (make it's length 1).
• Multiply that by the distance you want.
• Add that vector to the point B.

So a short javascript example:

const A = [2, 1, 4];
const B = [9, 4, 2];

const dist = 3;

function addVectors(v1, v2) {
    return v1.map((x, i) => x + v2[i]);
}

function scalarMultiply(v, c) {
    return v.map(x => x*c);
}

function getLength(v) {
    return Math.hypot(...v);
}

const vecB2A = addVectors(A, scalarMultiply(B, -1)); // Step 1
const normB2A = scalarMultiply(vecB2A, 1/getLength(vecB2A)); // Step 2
const distB2A = scalarMultiply(normB2A, dist); // Step 3
const C = addVectors(B, distB2A); // Final step

console.log(C);

Point C is equal to point B minus 'dist' times a unit vector whose direction is AB. So it is quite easy:

vector v from A to B is equal (xB-xA, yB-yA, zB-zA) / distance(AB)

Then C = B - d*v where d is the distance from B you want C to be.

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