How do you use vectors, dot product and cross product in game development?

What the interviewer wants to hear

• Vectors represent positions, directions and velocities
• Normalising gives a direction of length 1
• Dot product: angle between vectors, front/behind, view cones, lighting
• Cross product: perpendicular vector, left/right test, surface normals
• Distance and magnitude for range checks

Model answer

Vectors are everywhere in games. A position is a point, a velocity is a direction with speed, and the difference between two positions gives the direction and distance from one object to another. Normalising a vector gives a pure direction of length 1. The dot product of two unit vectors gives the cosine of the angle between them, so I use it to check whether an enemy is in front of the player, to build a view cone, or for lighting, where light intensity depends on the angle to the surface normal. The cross product gives a vector perpendicular to two others. I use it to find surface normals, to build a coordinate frame for a camera, and to check whether a target is to the left or right of the player.

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