Calculate motion between keyframes in straight line — no acceleration or easing. Produces mechanical, robotic movement; rarely used alone without easing curves.
In the motion graphics and animation workflow, you need a basic form to interpolate between two positions, rotations, or color values. Linear interpolation is that basic form — the simplest mathematical bridge from Keyframe A to Keyframe B. The computer calculates the path between them as a straight line, in uniform steps. No acceleration, no deceleration. The same amount of movement frame by frame.
Practically speaking: If your 3D object is at position X=0 in Frame 10 and at X=100 in Frame 20, then in Frame 15 it will be exactly at X=50. Period. This uniformity immediately feels mechanical and artificial on set or later in review. The eye recognizes this instantly — the natural acceleration curve that organic movements have is missing. That's why in practice, you almost never use linear interpolation as the final stage, but only as a basis. You set the linear keyframes, then you put an Ease-In/Out curve over them (see: Easing, Motion Curves). This is the sequence of operations in every VFX suite — whether in After Effects, Maya, or Nuke.
Scenario: You are animating a camera move over a 3D landscape. Interpolated linearly, it looks like a robot arm dragging itself across the ground at a constant speed. With a custom ease curve applied, it becomes fluid, organic, cinema-ready. For fast cut transitions where violence and abruptness are desired, linear interpolation can even be used intentionally — but this is the exception, not the rule.
The mathematics behind it is lean: for each frame between A and B, a factor (from 0 to 1) is calculated that increases linearly. F(t) = A + (B − A) × t. Most DCCs (Digital Content Creation Tools) allow you to set the interpolation mode per keyframe. You'll find this in the Graph Editor or Curve Editor, usually as a dropdown or right-click menu. Understand this mechanism, and you'll save hours in fine-tuning animation and motion.