In this lesson, you will learn how to define custom cost functions to weight specific animation attributes, ensuring your character selects the most natural poses during rapid direction changes.
Understanding Cost Functions
When using Motion Matching, the system constantly scans your Animation Database to find the "best" pose that matches the character’s current trajectory. However, the default matching criteria often prioritize raw geometric similarity. This can result in "foot sliding" or jarring transitions when a player makes a sudden turn.
A Cost Function is essentially a mathematical scoring system. It tells the engine, "I don't just want the pose that looks like the current one; I want the pose that fulfills these specific requirements." By assigning weights to different attributes—like velocity, orientation, or past motion—we can penalize poses that would look unnatural, forcing the system to pick a more fluid alternative.
Applying Weights to Trajectory
If you want your character to handle rapid direction changes gracefully, you can increase the cost weight of the Trajectory Query. By emphasizing the "future path" in your cost function, you tell the database that matching the movement direction is more important than matching the exact current frame of animation.
In the Pose Search Database asset, you can adjust these settings within the Search Settings section:
// Conceptual Logic for a Weight Multiplier
float CalculateCost(FAnimationPose& Pose, FTrajectory& DesiredTrajectory)
{
float TrajectoryCost = CompareTrajectory(Pose, DesiredTrajectory) * TrajectoryWeight;
float VelocityCost = CompareVelocity(Pose, CurrentVelocity) * VelocityWeight;
return TrajectoryCost + VelocityCost;
}
By increasing the TrajectoryWeight, you instruct the engine to prioritize poses that align with where the player is about to go. If a character is sprinting forward and the player suddenly jerks the stick backward, a higher trajectory weight forces the database to look for a "pivot" animation rather than just playing the next frame of a forward sprint, which would look like the character is sliding.
Ultimately, custom cost functions act as a filter, ensuring that the mathematical "best fit" aligns with your artistic goals for character movement.
Key Takeaway: Custom cost functions allow you to mathematically prioritize movement logic—like trajectory alignment—over simple pose similarity, creating stable, natural-looking transitions during high-speed gameplay.