Hello,

I have a class which is supposed to except a touchEvent in its update method, and then rotate a bitmap to face the location of that touchEvent.

For some reason the way I have it written now the image just continues to rotate in the direction of the touchEvent. I've tried just about everything, wondering if someone could help.

package com.turretgame.mainpackage;

import android.graphics.Color;
import android.graphics.Matrix;
import android.util.Log;

import com.example.gamedevchapter3.Input.TouchEvent;
import com.example.gamedevchapter3.Pixmap;
import com.example.gamedevchapter3.Graphics;

public class Turret {

	Pixmap turretImage;
	Matrix matrix;

	float direction, tempDirection = 0;
	int xpos, ypos = 100;
	float eventSlope = 0;
	int screenWidth, screenHeight;

	public Turret(Pixmap image, int x, int y, int worldWidth, int worldHeight) {
		turretImage = image;

		matrix = new Matrix();

		xpos = 360;
		ypos = 1040;
		matrix.postTranslate(360, 1040);
	}

	public void update(TouchEvent event, float deltaTime) {

		event.x -= 360;
		eventSlope = (((float) event.x) / ((float) event.y));

		if (eventSlope != 0) {
			tempDirection = ((float) (Math.atan2((double)1 / (double)eventSlope, 0)));
		} else {
			eventSlope = 0;
		}
		
		Log.d("direction", Float.toString(direction));
        
		direction = direction + tempDirection;
		
		matrix.postRotate(direction, xpos + 8, ypos + 20);
	
		direction = tempDirection;
	}

	public void draw(Graphics g) {

		g.drawPixmap(turretImage, matrix);

	}
}

An asside, if event.y=0 you have a divide by zero.

When you say "continues to rotate", I assume that is because you don't have anything that turns the rotation off. You simply call postRotate every time, so the thing gets rotated every time. Is that intended?

Also, you don't give much context but I think you may want a turret that slowly rotates toward a touch. You might be better off adjusting the angle by first taking the arrcos of the dot product between the normalized current direction and the normalized touch's direction, clamping that value to a maximum (so it moves slowly), and then using it for the rotation.

An asside, if event.y=0 you have a divide by zero.

When you say "continues to rotate", I assume that is because you don't have anything that turns the rotation off. You simply call postRotate every time, so the thing gets rotated every time. Is that intended?

Also, you don't give much context but I think you may want a turret that slowly rotates toward a touch. You might be better off adjusting the angle by first taking the arrcos of the dot product between the normalized current direction and the normalized touch's direction, clamping that value to a maximum (so it moves slowly), and then using it for the rotation.

I did have a check to make sure I didn't end up dividing by 0, but I most of accidentally deleted when I moved some code around. My goal is to have the turret snap to the angle for it face the point. Right now it does rotate slowly. What I mean by continues to rotate is that it begins rotating in the correct direction, but then continues passed the desired angle. Sorry, I should have explained my intention better.

Thanks for the help Marc. Right now my graphics framework is written using on board graphics instead of opengl. Will this make much of a difference?

Ok, I have this code working. I translated some things to the standard graphics library, but it still works well. This is what I have now.

package com.turretgame.mainpackage;

import android.graphics.Color;
import android.graphics.Matrix;
import android.util.Log;

import com.example.gamedevchapter3.Input.TouchEvent;
import com.example.gamedevchapter3.Pixmap;
import com.example.gamedevchapter3.Graphics;

public class Turret {

	Pixmap turretImage;
	Matrix matrix;
	
	final int SHIP_X_POS = 360;
	final int SHIP_Y_POS = 1040;
	
	final int SHIP_X_ROTATEPOS = 360 + 8;
	final int SHIP_Y_ROTATEPOS = 1040 + 20;

	final int TOUCH_X_TRANSLATION = 360;
	
	float direction, tempDirection = 0;
	int xpos, ypos = 100;
	float eventSlope = 0;
	int screenWidth, screenHeight;
	
	float shipPosition[] = {5.0f, 7.0f};
	float shipRotation[] = {0.0f, 0.0f, 0.0f, 1.0f};
	
	float touchPosition[] = {0.0f, -5.0f, 0.0f};
	
	float blendedRotation  = 0.0f;
	float startRotation    = 0.0f;
	float targetRotation   = 180.0f;
	float sliderTime       = 0.0f;
	float incrementTime    = 0.0f;
	 
	float distanceBetweenModels[] = {0.0f, 0.0f, 0.0f};

	public Turret(Pixmap image, int x, int y, int worldWidth, int worldHeight) {
		turretImage = image;

		matrix = new Matrix();

		xpos = 360; 
		ypos = 1040;
		matrix.postTranslate(360, 1040);
	}

	public void update(TouchEvent event, float deltaTime) {
		
		
		matrix.reset();	
		matrix.postTranslate(SHIP_X_POS, SHIP_Y_POS);
		
		touchPosition[1] = event.y;
		touchPosition[0] = event.x - TOUCH_X_TRANSLATION;
		
		
		//============================= - The next seven lines of code seems to have round 1 taken care of.
	       incrementTime += 0.001;     
	       //--------------------------------    
	       
	       distanceBetweenModels[1] = touchPosition[1] - shipPosition[1];                                      
	       distanceBetweenModels[0] = touchPosition[0] - shipPosition[0];                                                                  
	 
	       //------------------------------------------------------------------------------------------------------
	 
	       targetRotation = (float) ((float)Math.atan2(distanceBetweenModels[0], distanceBetweenModels[1])* 180 / 3.14159);                    
	 
	       
	       blendedRotation = (float) ((startRotation * (1.0 - sliderTime)) + (targetRotation * sliderTime));                                   
	 
	       matrix.postRotate(blendedRotation, SHIP_X_ROTATEPOS, SHIP_Y_ROTATEPOS);
	       
	       if(sliderTime <= 1.0) // This will prevent the rotation from over-shooting
	       {
	             sliderTime += incrementTime;                                                                                                   
	       }
	}

	public void draw(Graphics g) {

		g.drawPixmap(turretImage, matrix);
	}
}
 

I added some constant variables for the sake of readability. The only thing I have noticed is that if a touchEvent is too close or too far away from the turret it loses a great deal of accuracy. If the touchEvent is about half of the distance from the turret to the top of the screen its dead on.

The only thing I have noticed is that if a touchEvent is too close or too far away from the turret it loses a great deal of accuracy. If the touchEvent is about half of the distance from the turret to the top of the screen its dead on.

Edit: Ignore all the following confused musing, I've been awake for a day now, I'm hung-over and I did not read your code first. I guess I must think I'm Richard Feynman or something, like I can just imagine the solution without looking at anything first. Dunh!.. Half of what I was wondering about is written right there at the beginning of your code post. I had just assumed that it was exactly as I had written. Silly! I'm just going to leave the mess below for now so I can laugh at myself later.

Haha, don't worry about it. I know the feeling, and appreciate the help. I'm going to put some time into it and see if I can come up with something.