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How can I clear the screen in openGL using Java

I don't understand how I can simply clear the screen in Java while using OpenGL. I have searched all over the internet, there is like no real good resource for OpenGL information. Basically I just want to clear the screen and re-draw a circle. Instead my code decides that it isn't going to clear the screen ever, and it most definitely isn't going to draw anything else.. I want it to clear the screen when I press "e", and then draw a new circle. I have two java files.. I will only post relevant code for the sake of any user's who can help me - but will post more code if needed.
In the beginning of my JOGLEventListener.java file I'm also declaring a global var
// Test
GLAutoDrawable test = null;
JOGLEventListener.java
#Override
public void display(GLAutoDrawable gLDrawable)
{
// Set a global variable to hold the gLDrawable
// May not need this?
test = gLDrawable;
GL2 gl = gLDrawable.getGL().getGL2();
gl.glClearColor(backrgb[0], 0, 1, 1);
gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);
backrgb[0]+=0.0005;
if (backrgb[0]> 1) backrgb[0] = 0;
// =============================================
// Draw my circle here
//
// =============================================
// =============================================
System.out.println("Drawing Circle..");
drawCircle(5.0f, 5.0f, 10.0f);
}
// Draw Circle
void drawCircle(float x, float y, float radius)
{
System.out.println("IN DRAWCIRCLE");
int i;
GL2 gl = test.getGL().getGL2();
int lineAmount = 100; //# of triangles used to draw circle
final
//GLfloat radius = 0.8f; //radius
float twicePi = (float) (2.0f * Math.PI);
gl.glBegin(gl.GL_LINE_LOOP);
for(i = 0; i <= lineAmount;i++) {
gl.glVertex2f(
x + (radius * (float)Math.cos(i * twicePi / lineAmount)),
y + (radius* (float)Math.sin(i * twicePi / lineAmount))
);
}
gl.glEnd();
}
#Override
public void keyTyped(KeyEvent e)
{
char key= e.getKeyChar();
System.out.printf("Key typed: %c\n", key);
GL2 gl = test.getGL().getGL2();
if(key == 'e')
{
// WHY ISNT THIS WORKING
// CLEAR THE SCREEN AND DRAW ME A NEW CIRCLE
gl.glClear( gl.GL_COLOR_BUFFER_BIT | gl.GL_DEPTH_BUFFER_BIT );
gl.glLoadIdentity();
//test
float x = 100.0f;
float y = 100.0f;
float twicePi = (float) (2.0f * Math.PI);
float radius = 100f;
System.out.println("Draw Another Circle...");
gl.glBegin(gl.GL_LINE_LOOP);
for(int i = 0; i <= 360;i++)
{
gl.glVertex2f(
x + (radius * (float)Math.cos(i * twicePi / 360)),
y + (radius* (float)Math.sin(i * twicePi / 360))
);
}
gl.glEnd();
}

1) That's deprecated OpenGL, don't use it
2) Don't save the gl object to one global value, always get it from the drawable or the GLContext
3) Use a shader program to render and a vertex buffer to hold the vertices position. But first, I'd suggest you to start a tutorial to learn the basic of OpenGL. Or if you want to get something working asap, clone this hello triangle of mine and start experiment on that

The problem is apparently that you don't swap the front and back buffers.
I'm not familiar with the OpenGL bindings for Java, but I guess that the library already does that for you after it calls the display() function. It doesn't do that after keyTyped().
The way you are supposed to do this is to always draw the scene from scratch inside the display() function based on some internal state. Then in keyTyped() you shall modify that internal state and invalidate the window, which will cause the display() to be called again and redraw the scene properly.
EDIT: Calling display() yourself won't be enough. I can't find how to invalidate the window in Java (in C this would be so much easier). As a dirty hack you can try calling temp.swapBuffers() manually in display, setting setAutoSwapBufferMode(false) and calling display from keyTyped().

LAG : not fluid movement on JAVA Box2D game

I'm a newbie game developer. I've been trying to develop a game using LibGDX and Box2D physics engine, but when I export my android application, it is not working as it should be.
This problem is happening in some devices; it does not matter if it's an old device or a new one.
For example the app is working well in Sony Xperia Z1, Motorola XT615, LG L1, LG L3, LG L5,etc. However, in Sony Xperia U, Motorola Moto E,Samsung Galaxy's is not working like I expect.
You can see a non-fluid behavior when the object is moving across the screen. That's the problem I have. It's like a stutter effect, not fluid.
I read on forums that It could be a FPS (frames per second) problem when I set the time step in the world. So, I am using a fixed time step method and with the remainder time that I get in "accumulator" I interpolate it. I read these articles:
http://gafferongames.com/game-physics/fix-your-timestep/
http://saltares.com/blog/games/fixing-your-timestep-in-libgdx-and-box2d/
to learn the fixed time step and interpolation method, but I think that there is a fail in my code that I can't see.
Here is my code:
public class MyGame extends ApplicationAdapter {
SpriteBatch batch;
Texture img;
Sprite sprite;
Body body;
Box2DDebugRenderer debugRenderer;
OrthographicCamera camera;
World world;
final float PixelsToMeters=100f;
float step = 1.0f / 60.0f ,alpha;
Vector2 currentPosition,lastPosition,position;
double currentTime,accumulator,lastAngle,currentAngle;
final float BODY_WIDTH=95f,BODY_HEIGHT=95f;
#Override
public void create () {
world= new World(new Vector2(0f,-9.8f),true);
batch = new SpriteBatch();
img = new Texture("circulo.png");
sprite=new Sprite(img);
//BODY DEFINITION
BodyDef bodydef=new BodyDef();
bodydef.type= BodyType.DynamicBody;
bodydef.position.set(-3.5f,-2.4f);
body=world.createBody(bodydef);
CircleShape shape=new CircleShape();
shape.setRadius((sprite.getWidth()/2)/PixelsToMeters);
FixtureDef fixturedef=new FixtureDef();
fixturedef.shape=shape;
fixturedef.density=0.1f;
fixturedef.restitution=1f;
fixturedef.friction=0.5f;
body.createFixture(fixturedef);
shape.dispose();
//SET SPRITE POSITION
sprite.setPosition(body.getPosition().x *PixelsToMeters - BODY_WIDTH/2,
body.getPosition().y * PixelsToMeters - BODY_HEIGHT/2);
//THROW CIRCLE
body.setLinearVelocity(3.5f,9.5f);
lastPosition=new Vector2();
position=new Vector2();
camera=new
OrthographicCamera(Gdx.graphics.getWidth(),Gdx.graphics.getHeight());
}
#Override
public void render () {
camera.update();
double newTime = TimeUtils.millis() / 1000.0;
double frameTime = Math.min(newTime - currentTime, 0.25);
float deltaTime = (float)frameTime;
currentTime = newTime;
accumulator+=deltaTime;
while (accumulator >= step) {
//SAVE LAST BODY POSITION AND ANGLE
lastPosition.x=body.getPosition().x;
lastPosition.y=body.getPosition().y;
lastAngle=body.getAngle();
world.step(step, 8, 3);
accumulator -= step;
}
//SAVE CURRENT BODY POSITION AND ANGLE
currentPosition= new Vector2(body.getPosition().x,body.getPosition().y);
currentAngle=body.getAngle();
alpha=(float) (accumulator/step);
position.x = lastPosition.x + (currentPosition.x - lastPosition.x) * alpha;
position.y = lastPosition.y + (currentPosition.y - lastPosition.y) * alpha;
sprite.setPosition(position.x * PixelsToMeters - BODY_WIDTH/2, position.y * PixelsToMeters
-BODY_HEIGHT/2);
sprite.setRotation((float)Math.toDegrees(lastAngle+(currentAngle-lastAngle)*alpha));
Gdx.gl.glClearColor(0, 0, 0, 0);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
batch.setProjectionMatrix(camera.combined);
batch.begin();
batch.draw(sprite, sprite.getX(), sprite.getY(),
sprite.getOriginX(), sprite.getOriginY(),
sprite.getWidth(), sprite.getHeight(), sprite.getScaleX(),
sprite.getScaleY(), sprite.getRotation());
batch.end();
}
#Override
public void dispose() {
world.dispose();
batch.dispose();
img.dispose();
super.dispose();
}
}

I had this exact problem before using box2d, small stuttering when physics steps took longer than 1/60 seconds.
I also tried interpolation and read lots of articles about it without any success.
In the end I am not using fixed timestep at all (just delta to the world step) and all goes smoothly with box2d.
But if you still want to use fixed step, i have some suggestions:
Use Gdx.graphics.getDeltaTime() as it gives you a smoothed delta with n previous frames.
Use deltaTime in the world step loop:world.step(deltaTime, 8, 3); and accumulator -= deltaTime.
Hope it helps!

java libgdx move perspective camera

I'm creating a 3D game but I don't know how to translate the camera according to my fingers. I'm creating a map(x = -30 to +30;y = -30 to 30;z = -1 to 1) where every coordinate is used for a model using .g3db files from my assets and put in place using model translation. This works so far, the map looks good and is viewed in a 67% angle. The map is so large it can't be viewed in the camera at once(no I don't want to zoom out). Whenever I'm touching the screen of my android phone it's just rotating around the center, but instead I want the gamemap to stay in place from my point of view and instead change the position of the perspective camera on the x and y axis. There's no game object that can be used for position tracking, everything should be depending on my finger actions. This way I want to move to visible screen to each x and y direction(think of it visualy like a 2D camera moving up, down and to the sides in front of a picture). Can you help me?
This is my code so far:
public class MyGdxGame extends ApplicationAdapter implements ApplicationListener{
//define variables
...
#Override
public void create() {
environment = new Environment();
environment.set(new ColorAttribute(ColorAttribute.AmbientLight, 0.4f, 0.4f, 0.4f, 1f));
environment.add(new DirectionalLight().set(0.8f, 0.8f, 0.8f, -1f, -0.8f, -0.2f));
modelBatch = new ModelBatch();
cam = new PerspectiveCamera(67, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
cam.position.set(10f, 10f, 10f);
cam.lookAt(0,0,0);
cam.near = 1f;
cam.far = 300f;
cam.update();
assets = new AssetManager();
//load assets
...
loading = true;
camController = new CameraInputController(cam);
camController.forwardTarget = true;
Gdx.input.setInputProcessor(camController);
}
private void doneLoading() {
//load models and add them to instances
loading = false
}
#Override
public void render() {
if (loading && assets.update())
doneLoading();
camController.update();
Gdx.gl.glViewport(0, 0, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT | GL20.GL_DEPTH_BUFFER_BIT);
modelBatch.begin(cam);
modelBatch.render(instances, environment);
modelBatch.end();
}
I'm using the spring tool suite 3.5.1 as IDE and libgdx 1.0.1

Dont use a CameraInputController, but instead implement InputProcessor and use that class in the call to Gdx.input.setInputProcessor.
For example:
public class MyGdxGame extends ApplicationAdapter implements InputProcessor {
And in the create method:
Gdx.input.setInputProcessor(this);
You'll have to implement all methods as shown in this link, except for the touchDown and touchDragged method which require additional code:
private int dragX, dragY;
#Override
public boolean touchDown (int x, int y, int pointer, int button) {
dragX = x;
dragY = y;
return true;
}
#Override
public boolean touchDragged (int x, int y, int pointer) {
float dX = (float)(x-dragX)/(float)Gdx.graphics.getWidth();
float dY = (float)(dragY-y)/(float)Gdx.graphics.getHeight();
dragX = x;
dragY = y;
cam.position.add(dX * 10f, dY * 10f, 0f);
cam.update();
return true;
}
Here the dX and dY values is the amount that user has dragged on the screen within the range between -1f and +1f. E.g. for dX a value of 0.5f means that the user dragged half the screen size to the right. Note that these are delta values since the last time the method was called. The method is likely to be called many times per drag operation. Therefor, the dX and dY will be small values.
The call to cam.position.add(...) will actually move the camera and the call to cam.update(); will recalculate the values needed for rendering. I've used a multiplier of 10f for both the X and Y translation of the camera. This means that if the user fully drags from the left edge to the right edge of the screen, that the camera will move a total amount of 10 units to the right. You can adjust the value as needed.
Note that this moves the camera on XY plane. If you need to move on e.g. the XZ plane, you need to adjust the call the cam.position.add(...) accordingly. Also, this doesn't change the direction of the camera. If you want to move the camera, while looking at the same location, you'll need to add cam.lookAt(0,0,0); just before cam.update();.

Sprite starts in lower left corner of screen - libgdx

I have a player setup with box2d and I am trying to draw a sprite over the player. The player spawns in the middle of the screen, while the sprite spawns in the lower left hand corner of the screen but does move along with the player entity, just starting at a different location.
GameScreen snippet:
#Override
public void render(float delta) {
super.render(delta);
player.update();
world.step(TIMESTEP, VELOCITY_ITERATIONS, POSITION_ITERATIONS);
}
#Override
public void show() {
player = new Player(world, 0, 0);
}
Player class snippet:
public Player(World world, float x, float y) {
texture = new Texture(Gdx.files.internal("sprites/Player.png"));
sprite = new Sprite(texture);
}
public void update() {
batch = new SpriteBatch();
batch.begin();
sprite.draw();
sprite.setPosition(body.getPosition().x, body.getPosition().y);
batch.end();
body.setLinearVelocity(impulse);
}
I tried setting the position of the sprite in the constructor based on the body's coordinates but it doesn't seem to be working. I have removed body & fixture code. Any push in the right direction is appreciated.

If you enable box2d debug render, you'll probably find that both the texture and the body are starting at the left corner of the screen. Actually, 0, 0 are supposed to be the coordinates for the left bottom corner. In order to set your body in the center of the screen, you should set something like
(SCREEN_WIDTH / 2) / PTM_RATIO, (SCREEN_HEIGHT / 2) / PTM_RATIO
as your body initial position.

LibGDX - properly using Polygon class

I have created Polygon object to wrap my airplane (size of airplane's TextureRegion is 256x74, but size of this one in a game is 70x20). So:
TextureRegion[] texRegsAirplane = TextureRegion.split(textureAirplane, 256, 74);
Rectangle bounds = new Rectangle(0, 0, 70, 20);
Polygon polygon = new Polygon(new float[]{0,0,bounds.width,0,bounds.width,bounds.height,0,bounds.height,0,0});
After that in my update function I update position of it:
public void update(float delta){
Vector2 v = getPosition();
v.add(velocity);
polygon.setPosition(v.x, v.y);
}
Then I render polygon to know where it is:
public void render(SpriteBatch spriteBatch, float pixelPerUnitX, float pixelPerUnitY){
spriteBatch.draw(testTexture,polygon.getX()*pixelPerUnitX, polygon.getY()*pixelPerUnitY,
polygon.getBoundingRectangle().width*pixelPerUnitX,polygon.getBoundingRectangle().height*pixelPerUnitY);
}
At the end I create 2 airplanes and make them fly to each other and every iteration I try to detect collision like below:
public void detectCollision(){
for(Airplane airplane1 : Airplanes){
for(Airplane airplane2 : Airplanes){
if(Intersector.overlapConvexPolygons(airplane1.getPolygon(), airplane2.getPolygon())){
//COLLISION DON'T HAPPEN!!!
}
}
}
I see that 2 rectangles move to each other and intersect, but overlapConvexPolygons function doesn't work! Why?

I've solved this problem. I incorrectly specified vertices. I needed to get rectangular polygon, so I had to use following:
polygon = new Polygon(new float[]{0,0,bounds.width,0,bounds.width,bounds.height,0,bounds.height});
and do not forget set origin if you are going to rotate polygon object:
polygon.setOrigin(bounds.width/2, bounds.height/2);
Now it works perfect!