rotation question

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josephjohn
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rotation question

Post by josephjohn » Sat Dec 16, 2017 8:38 pm

So I have a piece of code to slowly rotate a sprite towards the mouse position, and it works. However, when it has to rotate from negative to positive (3.1 to -3.1) and vice-versa, it does a full rotation around. How can I make sure it rotates in the shortest distance instead of a full rotation?

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xNick1
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Re: rotation question

Post by xNick1 » Sun Dec 17, 2017 5:49 am

Maybe check in the update function the value of your rotation variable.

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If rotation < -3.1 then rotation = -3.1
The same when it's greater than 3.1

Then draw the sprite.

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love.graphics.draw(img, x, y, rotation, width, height, scalex, scaley)
Maybe there are more proper ways but it should work
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zorg
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Re: rotation question

Post by zorg » Sun Dec 17, 2017 6:10 am

What's the 3.1 exactly? Are you using degrees, radians, or something else?
Usually, you'd calculate both turns in one of the above systems, and use the one that's a smaller number (you might need to math.abs them for the comparison, but use the original signs otherwise)
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mr_happy
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Re: rotation question

Post by mr_happy » Sun Dec 17, 2017 7:07 am

Perhaps look into atan2 (https://love2d.org/wiki/General_math)? I think you can then work out which way is the shortest, rather than always going the long way round :D

josephjohn
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Re: rotation question

Post by josephjohn » Sun Dec 17, 2017 3:41 pm

I might have explained it wrong. The problem is this:

I have a sprite, which rotates towards the mouse when I click. I works great when going from 1rad to 2rad, or from 1 rad to -1 rad, but when it goes from -3rad to 3rad, it rotates around the larger angle, so instead of going from -3rad to 3rad by moving the shorter route(about 0.28rad), it moves 6rad.

Example:
Image

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mr_happy
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Re: rotation question

Post by mr_happy » Sun Dec 17, 2017 4:31 pm

As I said before, atan2 will help you fix the issue - here's a good explanation: https://gamedev.stackexchange.com/quest ... f-a-vector

EDIT: I see what your diagram is driving at now, but the way to solve it is still with atan2

josephjohn
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Re: rotation question

Post by josephjohn » Sun Dec 17, 2017 4:33 pm

mr_happy wrote:
Sun Dec 17, 2017 4:31 pm
There's something wrong with your diagram (or understanding, and I don't mean that in a nasty way) - there are 2PI radians (call it 6 and a bit radians) in 360% so "-3 to 3" is almost a complete rotation. As I said before, atan2 will help you fix the issue - here's a good explanation: https://gamedev.stackexchange.com/quest ... f-a-vector
I'm using atan2 to get the angle I want, and when I just set the sprite to instantly turn, it perfectly snaps to face the mouse, but the problem is when i make it slowly rotate, it does a full rotation instead of just moving 1rad or less.

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mr_happy
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Re: rotation question

Post by mr_happy » Sun Dec 17, 2017 4:54 pm

Does it rotate the 'long way round' when your mouse is any quadrant? Sounds to me like you just need to get the signs right depending upon which quadrant the cursor is in. If you look at the output of atan2 using something like this you'll see what's required:

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radius = 100
for i = 0, 359 do
   x = radius * math.cos(math.rad(i))
   y = radius * math.sin(math.rad(i))
   io.write(i, ' : ', math.atan2(x, y), '\t') 
end
I just typed that in, I'm not on a machine with lua installed so I might have made a typo etc :D

josephjohn
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Re: rotation question

Post by josephjohn » Sun Dec 17, 2017 5:06 pm

mr_happy wrote:
Sun Dec 17, 2017 4:54 pm
Does it rotate the 'long way round' when your mouse is any quadrant? Sounds to me like you just need to get the signs right depending upon which quadrant the cursor is in. If you look at the output of atan2 using something like this you'll see what's required:

Code: Select all

radius = 100
for i = 0, 359 do
   x = radius * math.cos(math.rad(i))
   y = radius * math.sin(math.rad(i))
   io.write(i, ' : ', math.atan2(x, y), '\t') 
end
I just typed that in, I'm not on a machine with lua installed so I might have made a typo etc :D

No, it only rotates the long way around when going from about -2 to 2 and vice versa. Basically when going from anything less than -pi/2 to anything more than pi/2 and vice versa. Here is the code:

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function love.load()
    x = 64
    y = 64
    tryX = 0
    tryY = 0
    velX = 0
    velY = 0
    rot = 0
    tryRot = 0
    rotDist = 0
    sprite = love.graphics.newImage("sprites/YOUR SPRITE HERE.png")
end


function love.update(dt)
    if rot < -math.pi or rot > math.pi then
        rot = rot % math.pi
    end

    if self.rot ~= tryRot then
        rotate(dt)
    end
end


function love.draw()
    love.graphics.draw(self.sprite, self.x - 3, self.y - 3, self.rot + math.rad(90), 3, 3, 8, 8)
end


function love.mousepressed(mx, my)
    tryX = mx
    tryY = my
    tryRot = math.atan2(tryY - y, tryX - x)
    rotDist = rot - tryRot
end


function rotate(dt)
    rot = self.rot - rotDist/64
end
I don't know if this code works since i took out a bunch of irrelevant stuff and made it into a main.lua file, but that's the jist

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mr_happy
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Re: rotation question

Post by mr_happy » Sun Dec 17, 2017 5:26 pm

atan2 takes (y, x)

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