Rotating point around another

[vinegar-tom]

I wrote a simple code to rotate one point around another, based on this formula.

For some reason, each time this formula is run, the point rotates as expected, but also gradually converges on the center point. I initially had it running in love.update, but I changed it to rotate 30 degrees every time the space bar is pressed, so that I could see more clearly what was happening.

Code: Select all

function love.load()
  origin = {}
  origin.x = 200
  origin.y = 200

  point = {}
  point.x = 100
  point.y = 100
end

function love.draw()
  love.graphics.circle("fill", origin.x, origin.y, 4)
  love.graphics.circle("fill", point.x, point.y, 2)
end

function love.keypressed(key)
  if key == "space" then
    rotatePoint()
  end
end

function rotatePoint()
  angle = math.rad(30)
  cos = math.cos(angle)
  sin = math.sin(angle)

  point.x = cos * (point.x - origin.x) - sin * (point.y - origin.y) + origin.x
  point.y = sin * (point.x - origin.x) + cos * (point.y - origin.y) + origin.y
end

Any thoughts?

[monolifed]

https://love2d.org/forums/viewtopic.php ... 04#p226204

Edit: Sorry, this is probably a solution for a different problem
What you are doing wrong is that you modify point.x and point.y.

Edit2:

Code: Select all

local p1, p2
function love.load()
	p1 = {
		ox = 100, oy = 100, -- origin
		rx = 100, ry = 50, -- radii
		tilt = math.rad(30), -- axial tilt (no effect if rx == ry)
		angle = 0, -- start angle in degrees
		color = {1,0,0}
	}
	p1.ct = math.cos(p1.tilt) -- these won't change so we compute here
	p1.st = math.sin(p1.tilt)
	p1.x = 0 -- we defer computation to love.update
	p1.y = 0 -- (or you can call rotatePoint on love.load)

	p2 = {
		ox = 250, oy = 100,
		rx = 100, ry = 100,
		tilt = 0,
		angle = 0,
		color = {0,1,0}
	}
	p2.ct = math.cos(p2.tilt) -- 1.0
	p2.st = math.sin(p2.tilt) -- 0.0
	p2.x = 0
	p2.y = 0
end

function love.draw()
	love.graphics.setColor(p1.color)
	love.graphics.circle("fill", p1.ox, p1.oy, 4)
	love.graphics.circle("fill", p1.x, p1.y, 2)

	love.graphics.setColor(p2.color)
	love.graphics.circle("fill", p2.ox, p2.oy, 4)
	love.graphics.circle("fill", p2.x, p2.y, 2)
end

function rotatePoint(p)
  local a = math.rad(p.angle)
  local x = math.cos(a) * p.rx
  local y = math.sin(a) * p.ry
  local ct = p.ct
  local st = p.st

  p.x =  ct * x + st * y + p.ox
  p.y = -st * x + ct * y + p.oy
end

-- ignore tilt, ct, st, and assume ry == rx
function rotatePointCircular(p)
  local a = math.rad(p.angle)

  p.x = math.cos(a) * p.rx + p.ox
  p.y = math.sin(a) * p.rx + p.oy
end

function love.update(dt)
	p1.angle = (p1.angle + 60 * dt) % 360
	rotatePoint(p1)

	p2.angle = (p2.angle + 60 * dt) % 360
	rotatePoint(p2)
	-- or
	-- rotatePointCircular(p2)
end