Interpolating rotated objects

Question

I've finally got interpolation working to 'smooth' out movement, however it doesn't seem to work for objects that are rotating (around their own center) - when I try to do this, the objects in question 'wobble'.

Note: I'm not talking about interpolating the actual rotation but rather the movement of a rotated object

So, let's say I have an object moving from left to right. Using the code below (with interpolation), it moves nice and smoothly. However, after rotating the object (let's say by 45 degrees, but this could be any angle), it's no longer moving smoothly, it still moves along it's path (left to right in this case) but it's no longer smooth.

Main Game Loop

@Override
public void onDrawFrame(GL10 gl) {

    //Grab time
    newTime = System.currentTimeMillis()*0.001;
    frameTime = newTime - currentTime;
    if ( frameTime > (dt*25))
        frameTime = (dt*25);

    currentTime = newTime;

    accumulator += frameTime;

    //Save game-state and update logic
    while (accumulator >= dt){
        saveGameState();  //Back up positions before updating them
        updateLogic(); //Update logic
        t += dt;
        accumulator -= dt;
    }

    //Get the amount to interpolate by
    interpolation = (float) (accumulator / dt);

    //Interpolate positions and render
    interpolate();      
    render(interpolation);
}

Rotation Code

public void rotate(float x, float y, float angle){

    //Rotate the quad   
    Matrix.setIdentityM(mRotationMatrix, 0);
    Matrix.translateM(mRotationMatrix, 0, centreX, centreY, 0f);
    Matrix.rotateM(mRotationMatrix, 0, angle, 0, 0, 0.1f);
    Matrix.translateM(mRotationMatrix, 0, -centreX, -centreY, 0f);
}

Edit

OK so it appears this is the problem:

• During logic update, sprite is moved and rotates it around it's center (using it's new/current X and Y to calculate it's center).
• Get's to render()
• Render method interpolates position creating new 'throwaway' coordinates at which object should be drawn
• Render method draws sprite at it's 'render' x and y coordinates
• Logic update moves and rotate around it actual x and y - not it's 'rendering x and y
• and so on

So as you can see, it's being rotated, not around the coordinates that it's been drawn at but at it's actual coordinates. I can't rotate it around it's render coordinates, because the rotation is done within the logic update, not during the render call, therefore, it would be rotating it around it 'old' render position, not the next one and I won't know the new render coordinates until the next render call which means.........

That the only way I can do this, is to actually translate / rotate the sprite within the render routine and not the logic routine.

I've tried this and it works, but not sure I'm happy about piling more 'logic' into my render routine.

Is this an 'acceptable' model? How else can I achieve rotation while keeping the object nicely interpolated?!

Answer 1

from looking at the docs I have found your issue

public static void rotateM (float[] m, int mOffset, float a, float x, float y, float z) Added in API level 1

Rotates matrix m in place by angle a (in degrees) around the axis (x, y, z).

Parameters:

• m source matrix
• mOffset index into m where the matrix starts a angle to rotate in degrees
• x X axis component
• y Y axis component
• z Z axis component

Matrix.rotateM(mRotationMatrix, 0, angle, 0, 0, 0.1f);

looks like this function is meant to rotate around an axis and your passing the axis of ( 0,0,0.1f ) when you probably mean to be rotating this object around its local origin

local origin rotation: (this is what you want to be doing)

rotation around world point: (this is what you are currently doing)

this would very probably change position, thus ruining your interpolation

you can change your rotateM call to look like this

Matrix.rotateM(mRotationMatrix, 0, angle, ORIGIN_X, ORIGIN_Y, ORIGIN_Z);