How to properly use setposition in libgdx? - libgdx

Question

This is the java file created by gdx-setup-ui.jar of v0.9.7
sprite.setPosition(-sprite.getWidth()/2, -sprite.getHeight()/2);
Why we need to set position to negative value in order to center the picture?
Where is the reference base point of the libgdx picture? (left-bottom corner?)
I was told that the origin of libgdx is left-bottom corner. Given the above values, part of the picture should have been outside the screen....It turns out not! I am very confused.
Thanks in advance
Complete listing:
package com.packtpub.libgdx.basic;
import com.badlogic.gdx.ApplicationListener;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.GL10;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.Texture.TextureFilter;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.graphics.g2d.TextureRegion;
public class Basic implements ApplicationListener {
private OrthographicCamera camera;
private SpriteBatch batch;
private Texture texture;
private Sprite sprite;
#Override
public void create() {
float w = Gdx.graphics.getWidth();
float h = Gdx.graphics.getHeight();
camera = new OrthographicCamera(1, h/w);
batch = new SpriteBatch();
texture = new Texture(Gdx.files.internal("data/libgdx.png"));
texture.setFilter(TextureFilter.Linear, TextureFilter.Linear);
TextureRegion region = new TextureRegion(texture, 0, 0, 512, 275);
sprite = new Sprite(region);
sprite.setSize(0.9f, 0.9f * sprite.getHeight() / sprite.getWidth());
sprite.setOrigin(sprite.getWidth()/2, sprite.getHeight()/2);
sprite.setPosition(-sprite.getWidth()/2, -sprite.getHeight()/2);
}
#Override
public void dispose() {
batch.dispose();
texture.dispose();
}
#Override
public void render() {
Gdx.gl.glClearColor(1, 1, 1, 1);
Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
batch.setProjectionMatrix(camera.combined);
batch.begin();
sprite.draw(batch);
batch.end();
}
#Override
public void resize(int width, int height) {
}
#Override
public void pause() {
}
#Override
public void resume() {
}
}

Answer 1

It's because the default center point for the orthographic camera is (0,0,0), so if you just draw your image at (0,0) its bottom left cornet will be in the center of the screen.
You can change this center point by using cam.position.set(w / 2, h / 2, 0) method
See here for more detailed example of OrthographiCamera use: https://github.com/libgdx/libgdx/wiki/Orthographic-camera