如何沿旋转方向移动物体？

Question

I wanted to ask what would be the right way to move an object in the direction it is rotated?

Right now I have:

    local ang = body:getAngle() * 180 / 3.14      /// get body's rotation in degrees
    local x, y = body:getPosition();      /// get current position
    ang = ang%360

    x = x + math.sin(ang) 
    y = y + math.cos(ang)

    print(ang)

    body:setPosition(x,y)

Yet the body is moving very weird. Any ideas what I'm doing wrong?

Thanks

Answer 1

You need the angle in radians and use the cosine function for the x value and the sine function for the y value. A function (untested) for this in lua would look something like this:

function moveAlongAngle(body, angleInRadians, dt, speedVector)
    local x, y = body:getPosition()
    x = x + math.cos(angleInRadians) * dt * speedVector.x
    y = y + math.sin(angleInRadians) * dt * speedVector.y
    body:setPosition(x,y)
end

This is because you convert the angle from polar to cartesian coordinates: http://en.wikipedia.org/wiki/Polar_coordinate_system#Converting_between_polar_and_Cartesian_coordinates

Answer 2

you are mixing radians and degrees. first you transform your angle into radians, but afterwards you try to normalize the degrees using modulo. you dont need to normalize, since sin and cos are periodic functions.

the math.sin and math.cos actually expect the angle in radians and not in degrees, so you need to devide by 180 and multiply by pi.

this is of course assuming your initial variable ang is in degrees.