Libgdx coordinate system

I just started with libgdx and I’m still figuring out how the coordinate system work.The code below would draw the image at [0,0] without the Orthographic camera

What I can’t understand is why the code doesn’t draw the image when I uncomment the camera and viewport. I thought the camera is at point [0,0] by default. But I’m not sure why I can’t see the image.

[Edit]
Reading the question I posted this morning, I guess what I want to ask is how camera and viewport affect the coorindates that is passed to the SpriteBatch.

Working code

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.GL20;
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.SpriteBatch;
import com.badlogic.gdx.utils.viewport.FitViewport;
import com.badlogic.gdx.utils.viewport.Viewport;

public class SpriteBatchSample extends GdxSample {
    private static final Color BACKGROUND_COLOR = new Color(0.39f, 0.58f, 0.92f, 1.0f);
    private static final float WORLD_TO_SCREEN = 1f;
    private static final float SCENE_WIDTH = 100f;
    private static final float SCENE_HEIGHT = 100f;

    private OrthographicCamera camera;
    private Viewport viewport;
    private SpriteBatch batch;
    private Texture cavemanTexture;
    private Texture sbTexture;
    private Color oldColor;

    @Override
    public void create() {      
        camera = new OrthographicCamera();
        viewport = new FitViewport(SCENE_WIDTH, SCENE_HEIGHT, camera);
        //Gdx.app.log("SpriteBatch",    camera.position.toString());
        batch = new SpriteBatch();
        oldColor = new Color();
        camera.position.set(100/2f, 100/2f,0);
        cavemanTexture = new Texture(Gdx.files.internal("data/sb.png"));
        cavemanTexture.setFilter(TextureFilter.Nearest, TextureFilter.Nearest);
    }

    @Override
    public void dispose() {
        batch.dispose();
        cavemanTexture.dispose();
        sbTexture.dispose();
    }

    @Override
    public void render() {      
        Gdx.gl.glClearColor(BACKGROUND_COLOR.r,
                BACKGROUND_COLOR.g,
                BACKGROUND_COLOR.b,
                BACKGROUND_COLOR.a);

        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        //batch.setProjectionMatrix(camera.combined);

        batch.begin();

        int width = cavemanTexture.getWidth();
        int height = cavemanTexture.getHeight();
        float originX = width * 0.5f;
        float originY = height * 0.5f;
        Gdx.app.log("SpriteBatchSample", "width " + width + "n"
            + "height: " + height + "n"
            + "originX" + originX + "n"
        );

        // Draw
        batch.draw(cavemanTexture,
                0, 0,                       // x, y
                originX, originY,                           // originX, originY
                cavemanTexture.getWidth(), cavemanTexture.getHeight(),// width, height
                WORLD_TO_SCREEN, WORLD_TO_SCREEN,           // scaleX, scaleY
                0.0f,                                       // rotation
                0, 0,                                       // srcX, srcY
                cavemanTexture.getWidth(),cavemanTexture.getHeight(),   // srcWidth, srcHeight
                false, false);                              // flipX, flipY



        batch.end();
    }

    @Override
    public void resize(int width, int height) {
        //viewport.update(width, height, false);
    }
}

Not Working

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.GL20;
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.SpriteBatch;
import com.badlogic.gdx.utils.viewport.FitViewport;
import com.badlogic.gdx.utils.viewport.Viewport;

public class SpriteBatchSample extends GdxSample {
    private static final Color BACKGROUND_COLOR = new Color(0.39f, 0.58f, 0.92f, 1.0f);
    private static final float WORLD_TO_SCREEN = 1f;
    private static final float SCENE_WIDTH = 100f;
    private static final float SCENE_HEIGHT = 100f;

    private OrthographicCamera camera;
    private Viewport viewport;
    private SpriteBatch batch;
    private Texture cavemanTexture;
    private Texture sbTexture;
    private Color oldColor;

    @Override
    public void create() {      
        camera = new OrthographicCamera();
        viewport = new FitViewport(SCENE_WIDTH, SCENE_HEIGHT, camera);
        //Gdx.app.log("SpriteBatch",    camera.position.toString());
        batch = new SpriteBatch();
        oldColor = new Color();
        camera.position.set(100/2f, 100/2f,0);
        cavemanTexture = new Texture(Gdx.files.internal("data/sb.png"));
        cavemanTexture.setFilter(TextureFilter.Nearest, TextureFilter.Nearest);
    }

    @Override
    public void dispose() {
        batch.dispose();
        cavemanTexture.dispose();
        sbTexture.dispose();
    }

    @Override
    public void render() {      
        Gdx.gl.glClearColor(BACKGROUND_COLOR.r,
                BACKGROUND_COLOR.g,
                BACKGROUND_COLOR.b,
                BACKGROUND_COLOR.a);

        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        batch.setProjectionMatrix(camera.combined);

        batch.begin();

        int width = cavemanTexture.getWidth();
        int height = cavemanTexture.getHeight();
        float originX = width * 0.5f;
        float originY = height * 0.5f;
        Gdx.app.log("SpriteBatchSample", "width " + width + "n"
            + "height: " + height + "n"
            + "originX" + originX + "n"
        );

        // Draw
        batch.draw(cavemanTexture,
                0, 0,                       // x, y
                originX, originY,                           // originX, originY
                cavemanTexture.getWidth(), cavemanTexture.getHeight(),// width, height
                WORLD_TO_SCREEN, WORLD_TO_SCREEN,           // scaleX, scaleY
                0.0f,                                       // rotation
                0, 0,                                       // srcX, srcY
                cavemanTexture.getWidth(),cavemanTexture.getHeight(),   // srcWidth, srcHeight
                false, false);                              // flipX, flipY



        batch.end();
    }

    @Override
    public void resize(int width, int height) {
        viewport.update(width, height, false);
    }
}