Rotating a sprite and moving it

[Ellie]

Ok,

I have an object. It has the following variables:

- rotation (in degrees, 0..360)
- xpos
- ypos

I want to move it forwards when the up arrow is pressed.

My brain is dead and I can't work out how to do it. It needs to move along x and y depending on what value rotation is.

Can someone put me out of my misery?

 

[Ellie]

Ok I'll try work it out.

Basically I guess I have...

A right angle triangle. x and y are what I need to find out. I have the angle between h and y, which is rotation; the angle between x and y is a right angle, so I need trig. Oh, I see...

So.

I need h (hyp) to equal 1; I have h and I have angle, so CAH will give me the adjacent, or y.

A

C  H

Hmmmmmn.

So y = cos(theta) * h?

y = cos(rotation) * 1
y = cos(rotation)


To get x use pythagoras,

x^2 = 1 - y^2


Does this sound right?

 

[Ellie]

This is as far as I got, and nope, not right at all.

<div class="c" id="{CB}" style="font-family: monospace;"><ol>            <span style="color: #993333;">float y = cos(player->Sprite->GetRotation());

            <span style="color: #993333;">float x = sqrt(<span style="color: #cc66cc;">1 - (y * y));

 

            <span style="color: #b1b100;">if (player->Sprite->GetRotation() > <span style="color: #cc66cc;">180)

            { x *= <span style="color: #cc66cc;">-1; }

            <span style="color: #b1b100;">if ((player->Sprite->GetRotation() > <span style="color: #cc66cc;">90) && (player->Sprite->GetRotation() < <span style="color: #cc66cc;">270))

            { y *= <span style="color: #cc66cc;">-1; }

 

            player->xpos += x;

            player->ypos += y;

 

[zeus81]

radians = degrees * pi / 180.0
x = distance * cos(radians)
y = distance * sin(radians)

Depending on where is the 0 angle, x and y may be inverted and/or multiplied by -1.

 

[Ellie]

That's perfect, thanks.

 

[Morphex]

Actually rotation is something like this:

Rotating a point from position (o.x, o.y) to position (oR.x, oR.y) through an angle "theta" about pivot point (oP.x, oP.y)

oR.x = oP.x + (o.x - oP.x) * cos(theta) - (o.y - oP.y) * sin(theta)

oR.y = oP.y + (o.x - oP.x) * sin(theta) + (o.y - oP.y) * cos(theta)

Thats the complete rotation for 2D around a point, you can find all you need about the rotation from that formulas, otherwise there might be some trouble in some cases.