Cross Product and Angle Incorrect?

Development Character & Animation
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1

I’m slowly going crazy because of this weird problem I’m having. I’m trying to program my AI and the expected results are not coming up. So, I ran a quick test that should always return 90 degrees. I get the cross product of my characters forward rotation and get the yaw angle between the forward and the cross product. This should always return 90 degrees on a level plane but I’m not getting that.

//Get the left vector for the actor
FVector Lateral = FVector::CrossProduct(Bot->GetActorRotation().Vector().GetSafeNormal(), FVector(0, 0, 1));

//Offset the point by the actors location since it's a unit vector we don't want it at the origin 
FVector TurnExtent = Lateral + Bot->GetActorLocation();

//calculate the angle and output it to the log.
float angle = (Bot->GetActorLocation() - TurnExtent).Rotation().Yaw;
UE_LOG(LogTemp, Log, TEXT("angle: %f"), angle);

Results

LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 110.746773
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 113.297768
LogTemp: angle: 110.716896

This seems very strange to me so I tried some straight trig:

float angle = ((atan2(TurnExtent.X - Bot->GetActorLocation().X, TurnExtent.Y - Bot->GetActorLocation().Y)) * 180 / PI);

But it got me similar (but different results)

LogTemp: angle: 169.221909
LogTemp: angle: 168.195251
LogTemp: angle: 167.994247
LogTemp: angle: 167.793625
LogTemp: angle: 167.651367
LogTemp: angle: 167.651367
LogTemp: angle: 167.536423
LogTemp: angle: 167.536041

Am I getting the characters rotation incorrectly?

2

I think your math is just off.

 //calculate the angle and output it to the log.
 float angle = (Bot->GetActorLocation() - TurnExtent).Rotation().Yaw;

That is calculating the angle between the Bot’s position (which is probably interpreted as a direction from the origin), and TurnExtent which is a position + direction (again, interpreted as a direction from the origin). I’d simplify all this to unit vectors.

// This line assumes there is no Pitch/Roll on the actor.
const FVector& leftVector = Bot->getActorRotation().Vector().SafeNormal();
const FVector up(0.0f, 0.0f, 1.0f);
const FVector& crossVector = FVector::CrossProduct(leftVector, up);

// We can simply use the dot product to grab the angle.
float angle = FMath::Acos(FVector::DotProduct(leftVector, crossVector));
3

Worked like a charm. I was making it out to be way more complicated than it had to be because I was feeding it incorrect values. Sometimes going back to basic trig throws me for a loop :stuck_out_tongue:

Thanks Again.