Hi!

I hope you can answer a question for me that has been bugging me for close to a week now. I cant seem to get the matrix operations correct. I am using android.graphics.Matrix.

I am trying to rotate the player char but it doesnt seem to work right. This is what i am doing at the moment.

matrix.preTranslate(lCenterX, lCenterY);
matrix.postRotate(entity.rotation);
matrix.postTranslate(entity.x-lCenterX,entity.y-lCenterY);

I am trying rotate the player vertices (a rectangle so it has 4 of them) by a differing amount of degrees (like 20d, 45d, 90d) but the player ends up displaced very far away. The higher amount of degrees the more the player is displaced. It gets really wierd.

public static Vector2D[] transform(Vector2D[] src, Vector2D[] dst, Matrix matrix){
for (int i = 0; i < src.length; i++){
transformBuffer[0] = src[i].x;
transformBuffer[1] = src[i].y;
matrix.mapPoints(transformBuffer);
dst[i].x = transformBuffer[0];
dst[i].y = transformBuffer[1];
}
return dst;
}

I am using matrix.mapPoints to move the vertices. Also worth knowing is that i have an array of vertices that represent the untransformed vertices (in model space) which i am transforming into a set of transformed vertices into world space.

Also as an ammendment I'd like to point out that i have also tried the following

matrix.preTranslate(-lCenterX, -lCenterY);
matrix.postRotate(entity.rotation);
matrix.postTranslate(entity.x+lCenterX,entity.y+lCenterY);

And i have also tried
matrix.postRotate(entity.rotation, lCenterX, lCenterY);
matrix.postTranslate(entity.x,entity.y);

And also (best results)
matrix.postRotate(entity.rotation, 0, 0);
matrix.postTranslate(entity.x,entity.y);

And the best results come from the above but it still ends up displacing the player by a big amount depending on how many degrees.

Well, it's annoying to me too. Even after decades i often stumble over such problems, and i have to trial and error each time. And it becomes worse with the years, not better. That just said for consolidation.

Btw, for 99% of work i dodge the problem by keeping rotation and translation separated. Using quaternions for rotation and a point for most things i never have such problems.
But ofc. if we want to transform a bunch of vertices, matrices are faster, so there is no way around it.

Gtadam said:
And also (best results)

matrix.postRotate(entity.rotation, 0, 0);

matrix.postTranslate(entity.x,entity.y);

Likely it's best because you rotate around the origin, eliminating one potential source of error. So that's a good start to begin the trial and error fun.

I'm not used to pre and post multiplication support, but here is an example of code i use to rotate around a given point:

Quat rotation = ...

vec3 centerOfRotation = ...

Mat4 xf;
xf.SetRotationFromQuat (rot); // sets the upper left 3x3 matrix 
xf.SetTranslation (centerOfRotation - xf.Rotate(centerOfRotation)); // need to rotate the desired cor by that

for (&object : objects) 
{
	onject.transform = xf * onject.transform; // transform multiple objets, would work for transforming just points too
}

I guess your mistake is not considering the rotation for setting the translation part. But i'm never sure. :D

Thanks JoeJ I'm chipping away at the problem and hopefully will find a solution.

I think i have managed to get the rotation matrix right this time. However, im still having the same problem as earlier with my char moving up and down as jittery movement. I'm not sure if im using the MTV (Minimum translation distance) from my SAT algorithm correctly. How is it usually done?

MinimumTranslationVector overlap = Shape.getOverlapSAT(this, that, false);

if(overlap != null){
this.x = x - overlap.min.x*overlap.overlap;
this.y = y - overlap.min.y*overlap.overlap;

if(mVelocity.y != 0f && overlap.min.y*overlap.overlap > 0f ){
mVelocity.y = 0;
mIsOnGround = true;
}
}