CODEWITHSUNDEEP
javascriptmath2dgame-physicsvector-graphics
Daniel Kaplan
Daniel Kaplan

Reputation: 67310

Why isn't my homing missile algorithm working?

I've taken code that's heavily inspired by this answer but my projectile is not homing in the way I expect. The initial projectile direction is often perpendicular to the target. At which point, it does seem to home in on his direction, but if it "passes" him, it seems to get stuck in place like it's frozen at a point but then seems to follow the movements the target makes without moving at its intended speed. I've commented a line of code that I'm concerned about. He's using V3 and V4 in his algorithm which I suspect is a typo on his part but I'm not sure. If anyone can help me with what I'm doing wrong here, I'd be very grateful. 

normalizedDirectionToTarget = root.vector.normalize(target.pos.x - attack.x, target.pos.y - attack.y) #V4
V3 = root.vector.normalize(attack.velocity.x, attack.velocity.y)
normalizedVelocity = root.vector.normalize(attack.velocity.x, attack.velocity.y)

angleInRadians = Math.acos(normalizedDirectionToTarget.x * V3.x + normalizedDirectionToTarget.y * V3.y)
maximumTurnRate = 50 #in degrees
maximumTurnRateRadians = maximumTurnRate * (Math.PI / 180)
signOfAngle = if angleInRadians >= 0 then 1 else (-1)
angleInRadians = signOfAngle * _.min([Math.abs(angleInRadians), maximumTurnRateRadians])
speed = 3
attack.velocity = root.vector.normalize(normalizedDirectionToTarget.x + Math.sin(angleInRadians), normalizedDirectionToTarget.y + Math.cos(angleInRadians)) #I'm very concerned this is the source of my bug
attack.velocity.x = attack.velocity.x * speed
attack.velocity.y = attack.velocity.y * speed
attack.x = attack.x + attack.velocity.x
attack.y = attack.y + attack.velocity.y

Edit: Code that Works

normalizedDirectionToTarget = root.vector.normalize(target.pos.x - attack.x, target.pos.y - attack.y) #V4
normalizedVelocity = root.vector.normalize(attack.velocity.x, attack.velocity.y)
angleInRadians = Math.acos(normalizedDirectionToTarget.x * normalizedVelocity.x + normalizedDirectionToTarget.y * normalizedVelocity.y)
maximumTurnRate = .3 #in degrees
maximumTurnRateRadians = maximumTurnRate * (Math.PI / 180)
crossProduct = normalizedDirectionToTarget.x * normalizedVelocity.y - normalizedDirectionToTarget.y * normalizedVelocity.x
signOfAngle = if crossProduct >= 0 then -1 else 1
angleInRadians = signOfAngle * _.min([angleInRadians, maximumTurnRateRadians])
speed = 1.5
xPrime = attack.velocity.x * Math.cos(angleInRadians) - attack.velocity.y * Math.sin(angleInRadians)
yPrime = attack.velocity.x * Math.sin(angleInRadians) + attack.velocity.y * Math.cos(angleInRadians)
attack.velocity = root.vector.normalize(xPrime, yPrime)
attack.velocity.x *= speed
attack.velocity.y *= speed
attack.x = attack.x + attack.velocity.x
attack.y = attack.y + attack.velocity.y

Upvotes: 1

Views: 873

Answers (1)

Abhishek Bansal
Abhishek Bansal

Reputation: 12715

According to me, if you have a vector (x,y) and you want to rotate it by angle 'theta' about the origin, the new vector (x1,y1) becomes:

x1 = x*cos(theta) - y*sin(theta)

y1 = y*cos(theta) + x*sin(theta)

(the above can be derived using polar coordinates)

EDIT: I'm not sure if I understand correctly, but if you know the speed and the absolute value of the final angle (say phi), then why can't you simply do:

Vx = speed*cos( phi )

Vy = speed*sin( phi )

EDIT 2: also, while taking cos-inverse, there can be multiple possiblities for the angleinradians. You may have to check the quadrant in which both vectors lie. Your maximum turning rate is 50 degrees in either direction. Hence, the cosine for that angle shall always be positive. (cosine is negative only for 90 to 270 degrees.

EDIT 3: I think to get information about +ve turn direction or -ve turn direction, cross product is a better idea.

EDIT 4: Vx / Vy should work if you carry out the following:

initialAngleInRadians = Math.atan(normalizedVelocity.y / normalizedVelocity.x)
finalAngleInRadians = initialAngleInRadians + angleInRadians
Vx = speed*cos(finalAngleInRadians)
Vy = speed*sin(finalAngleInRadians)

Upvotes: 1

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