Camera在Libgdx中以及与视口一起工作的方式

Question

If you work with LibGdx it goes not long until you come to Camera and viewport. If you work the first time with camera and Viewport you get some questions about how it works and how to use it. So:

• How can I use a Camera in LibGdx? What's viewport width and height?
• What is a Viewport, how can I use it and how it works together with the Camera?

Answer 1

How can I use a Camera in LibGdx? What's viewport width and height?

Firstly it's important that you know the Camera works with World units not with Pixels. World units are not a regular Unit. You self can define how much one World Unit is. Later more.

First, we create an OrthographicCamera a SpriteBatch and a Texture :

private OrthographicCamera camera;
private SpriteBatch batch;
private Texture img;

@Override
public void create () {
    //We create a OrthographicCamera through which we see 50x50 World Units
    camera = new OrthographicCamera(50,50);
    batch = new SpriteBatch();
    img = new Texture("badlogic.jpg");
}

We create a OrthographicCamera and in the Constructor we define how many World Units we see if we look through this camera into our world. In our example 50 x 50 World Units these are the viewport width and height. So we have created a Camera with a viewport width and height of 50.

In the render() method we render our image:

@Override
public void render () {
    //Clear the screen (1)
    Gdx.gl.glClearColor(1, 1, 1, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

    //Set ProjectionMatrix of SpriteBatch (2)
    batch.setProjectionMatrix(camera.combined);

    batch.begin();
    //Draw image on position 0, 0 with width 25 and height 25 (3)
    batch.draw(img, 0, 0, 25, 25); 
    batch.end();
}

(1) Clear the Screen, if we don't do that every Texture will draw over the other and if we draw a Animation we will see the old Frames.

(2) The batch is our Drawer he draws our Images, Animations etc. Default he draws a World which has so many World Units like the Screen has Pixels so in this case 1 World Unit = 1 Pixel. But now we will see 50 x 50 World Units doesn't matter how big the screen is. To say the Batch that he should draw what we see through our camera we must call: batch.setProjectionMatrix(camera.combined);

(3) Now we draw our img on Position 0,0 But 0, 0 doesn't mean on Pixel position 0,0 it means the Image will be drawn on World position 0,0 also width and height are not in Pixels they are in World units so the img will be drawn on Position 0,0 25x25 World Units big. So on a 50x50 viewport, the image fills one-quarter of the whole screen.

The Image fill one-quarter of the whole screen exactly as expected. But why it is on the right top and not on the bottom left?

The Problem is that the center of the Camera point on the position 0,0

So our Image is drawn on position 0,0 he fills the top right corner. We must set the position of the camera so 0,0 is in the bottom left corner:

camera = new OrthographicCamera(50,50);
camera.position.set(camera.viewportWidth / 2, camera.viewportHeight / 2, 0);

In the render() method we must add camera.update() because every time we change the position or the scale or what else of the camera we must update the camera.

Now the Image is in the bottom left.

Where are the Pixels?

We always speak about World units but where are the Pixels? The pixels are still there. If we have a Screen size of 200 x 200 pixels the batch will always draw 200 x 200 pixels. With the method batch.setProjectionMatrix(camera.combined); we only say the batch how much World Units are one Pixel.

If we have a Screen with 200 x 200 pixels and we create a Camera with a viewport of 50 x 50 world units the SpriteBatch know 1 WorldUnit = 4 Pixels. Now we draw a Image which is 25 x 25 World Units big the SpriteBatch knows he must draw the image 25 * 4 = 100 pixel big.

So the pixels still there but it's easier to think in World Units. If it's not clear enough here is a little bit more detailed description: Libgdx's World Units

Box2d

It's also very important to think in World Units if you use Box2d because Box2d works with Meters. So if you create a Body with a Force off 5 on the x axis, the Body is 5 m/s fast.

And now it's very cool to work with World Units because you can say 1 World Unit = 1 Meter so you can create a object with a width of 10 and you know after one second the Body will be in the Center of the Object. If you work with Pixels you will have a Problem if you have a different Screensize.

What is a Viewport, how can I use it and how it works together with the Camera?

Now we have the big Problem about different Screen sizes. Suddenly we have a Screen size of 350 x 200 pixels, now the Image will be stretched and don't look so nice as before.

For this Problem we use Viewports a few Viewports are StretchViewport , FitViewport and ExtendViewport . All viewports you can find here: https://github.com/libgdx/libgdx/wiki/Viewports .

Firstly what is a Viewport.

Imagine the camera is a speaker who speaks English. Different Screen Sizes are other People who speak German, French, Chinese etc. and the Viewport is the translator. The Translator doesn't change the sense of that what the English Speaker says but he adapts it so the others can understand it. Same are camera and Viewport. Viewport doesn't say or change what you can see on your screen if you run the program. He only handles that you always see the same on different Screen sizes. A Camera can life without Viewport. A Viewport not without Camera.

Add a viewport Object:

private Viewport viewport;

and the resize() method:

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

StretchViewport

Create a StretchViewport:

camera = new OrthographicCamera(50, 50);
camera.position.set(camera.viewportWidth / 2, camera.viewportHeight / 2, 0);
viewport = new StretchViewport(camera.viewportWidth, camera.viewportHeight, camera);

In the StretchViewport Constructor, we define the viewport width and height and the Camera.

Now we get the same result as before if we have different Screensizes the Images will be stretched.

FitViewport

Maybe we won't stretch our Images we will matter about the ratio of x and y.

The ratio of x and y means: is an Object 2 width and 1 height he will always twice as wide as high for example 200x100, 30x15 but not 20x15.

Create a FitViewport:

camera = new OrthographicCamera(50, 50);
camera.position.set(camera.viewportWidth / 2, camera.viewportHeight / 2, 0);
viewport = new FitViewport(camera.viewportWidth, camera.viewportHeight, camera);

Now the Image will always be a square. To see the Bars on the Side lets draw the Image as big as our viewport:

batch.draw(img, 0, 0, 50, 50);

The Image has a Ratio of 1 because of 50(width)/50(height) = 1 so the Image will always have the same width and height. The Bars on the side are outside of our Viewport and will be drawn in the color you define here: Gdx.gl.glClearColor(1, 1, 1, 1);

ExtendViewport

Maybe we won't Bars on the Side then we can take a ExtendViewport. The ExtendViewport keeps the world aspect ratio without bars by extending the world in one direction. Means on a screen where the aspect ratio between width and height are bigger you will see more of the world.

On a screen 400x200 aspect ration = (400/200 = 2) you will see more than on a screen of 300x200 (300/200 = 1.5);

To show this create a ExtendViewport and draw the Image bigger than the viewport and a second small Image:

camera = new OrthographicCamera(50, 50);
camera.position.set(camera.viewportWidth / 2, camera.viewportHeight / 2, 0);
viewport = new ExtendViewport(camera.viewportWidth, camera.viewportHeight, camera);

// in render() method
batch.begin();
batch.draw(img, 0, 0, 100, 50);
batch.draw(img, -20, 0, 20, 20);
batch.end();

If we now start our Program with a Screen size of 200x200 we see:

And if we resize the Screen on x axis To make the screen wider:

Now we can see more off the first Image and additinal the Second image but the ratio will always be the same. The Image is only stretched because we draw it 100x50 not because of resizing.

I hope this will clear some Questions about Camera and Viewport if you will learn more, read and look some tutorials and read the LibGdx wiki: https://github.com/libgdx/libgdx/wiki