three.js/manual/en/backgrounds.html

Title: Three.js Backgrounds and Skyboxes
Description: How to add a background in THREE.js
TOC: Add a Background or Skybox

Most of the articles here use a solid color for a background.

Adding as static background can be as simple as setting some CSS. Taking
an example from [the article on making THREE.js responsive](threejs-responsive.html)
we only need to change 2 things.

We need to add some CSS to our canvas to set its background to an image

```html
<style>
body {
    margin: 0;
}
#c {
    width: 100%;
    height: 100%;
    display: block;
+    background: url(resources/images/daikanyama.jpg) no-repeat center center;
+    background-size: cover;
}
</style>
```

and we need to tell the `WebGLRenderer` to use `alpha` so places we are not
drawing anything are transparent.

```js
function main() {
  const canvas = document.querySelector('#c');
-  const renderer = new THREE.WebGLRenderer({canvas});
+  const renderer = new THREE.WebGLRenderer({
+    canvas,
+    alpha: true,
+  });
```

And we get a background.

{{{example url="../threejs-background-css.html" }}}

If we want the background to be able to be affected by [post processing
effects](threejs-post-processing.html) then we need to draw the background using
THREE.js.

THREE.js makes this some what simple. We can just set the background of the scene to
a texture.

```js
const loader = new THREE.TextureLoader();
const bgTexture = loader.load('resources/images/daikanyama.jpg');
scene.background = bgTexture; 
```

which gives us

{{{example url="../threejs-background-scene-background.html" }}}

This gets us a background image but its stretched to fit the screen.

We can solve this issue by setting the `repeat` and `offset` properties of
the texture to show only a portion of image.

```js
function render(time) {

   ...

+  // Set the repeat and offset properties of the background texture
+  // to keep the image's aspect correct.
+  // Note the image may not have loaded yet.
+  const canvasAspect = canvas.clientWidth / canvas.clientHeight;
+  const imageAspect = bgTexture.image ? bgTexture.image.width / bgTexture.image.height : 1;
+  const aspect = imageAspect / canvasAspect;
+
+  bgTexture.offset.x = aspect > 1 ? (1 - 1 / aspect) / 2 : 0;
+  bgTexture.repeat.x = aspect > 1 ? 1 / aspect : 1;
+
+  bgTexture.offset.y = aspect > 1 ? 0 : (1 - aspect) / 2;
+  bgTexture.repeat.y = aspect > 1 ? 1 : aspect;

  ...

  renderer.render(scene, camera);

  requestAnimationFrame(render);
}
```

and now THREE.js drawing the background. There is no visible difference from
the CSS version at the top but now if we used a [post processing
effect](threejs-post-processing.html) the background would be affected too.

{{{example url="../threejs-background-scene-background-fixed-aspect.html" }}}

Of course a static background is not usually what we want in a 3D scene. Instead
we usually want some kind of *skybox*. A skybox is just that, box with the sky
draw on it. We put the camera inside the box and it looks like there is a sky in
the background.

The most common way to implement a skybox is to make a cube, apply a texture to
it, draw it from the inside. On each side of the cube put a texture (using
texture coordinates) that looks like some image of the horizon. It's also often
common to use a sky sphere or a sky dome with a texture drawn on it. You can
probably figure that one out on your own. Just make a cube or sphere, 
[apply a texture](threejs-textures.html), mark it as `THREE.BackSide` so we 
render the inside instead of the outside, and either put it in your scene directly 
or like above, or, make 2 scenes, a special one to draw the skybox/sphere/dome and the
normal one to draw everything else. You'd use your normal `PerspectiveCamera` to
draw. No need for the `OrthographicCamera`.

Another solution is to use a *Cubemap*. A Cubemap is a special kind of texture
that has 6 sides, the sides of a cube. Instead of using standard texture
coordinates it uses a direction from the center pointing outward to decide where
to get a color.

Here are the 6 images of a cubemap from the computer history museum in Mountain
View, California.

<div class="threejs_center">
  <img src="../resources/images/cubemaps/computer-history-museum/pos-x.jpg" style="width: 200px" class="border">
  <img src="../resources/images/cubemaps/computer-history-museum/neg-x.jpg" style="width: 200px" class="border">
  <img src="../resources/images/cubemaps/computer-history-museum/pos-y.jpg" style="width: 200px" class="border">
</div>
<div class="threejs_center">
  <img src="../resources/images/cubemaps/computer-history-museum/neg-y.jpg" style="width: 200px" class="border">
  <img src="../resources/images/cubemaps/computer-history-museum/pos-z.jpg" style="width: 200px" class="border">
  <img src="../resources/images/cubemaps/computer-history-museum/neg-z.jpg" style="width: 200px" class="border">
</div>

To use them we use `CubeTextureLoader` to load them and then use that as a the
scene's background.

```js
{
  const loader = new THREE.CubeTextureLoader();
  const texture = loader.load([
    'resources/images/cubemaps/computer-history-museum/pos-x.jpg',
    'resources/images/cubemaps/computer-history-museum/neg-x.jpg',
    'resources/images/cubemaps/computer-history-museum/pos-y.jpg',
    'resources/images/cubemaps/computer-history-museum/neg-y.jpg',
    'resources/images/cubemaps/computer-history-museum/pos-z.jpg',
    'resources/images/cubemaps/computer-history-museum/neg-z.jpg',
  ]);
  scene.background = texture;
}
```

At render time we don't need to adjust the texture like we did above

```js
function render(time) {

   ...

-  // Set the repeat and offset properties of the background texture
-  // to keep the image's aspect correct.
-  // Note the image may not have loaded yet.
-  const canvasAspect = canvas.clientWidth / canvas.clientHeight;
-  const imageAspect = bgTexture.image ? bgTexture.image.width / bgTexture.image.height : 1;
-  const aspect = imageAspect / canvasAspect;
-
-  bgTexture.offset.x = aspect > 1 ? (1 - 1 / aspect) / 2 : 0;
-  bgTexture.repeat.x = aspect > 1 ? 1 / aspect : 1;
-
-  bgTexture.offset.y = aspect > 1 ? 0 : (1 - aspect) / 2;
-  bgTexture.repeat.y = aspect > 1 ? 1 : aspect;

  ...

  renderer.render(scene, camera);

  requestAnimationFrame(render);
}
```

Let's add some controls in so we can rotate the camera.

```js
import {OrbitControls} from './resources/threejs/r132/examples/jsm/controls/OrbitControls.js';
```

```js
const fov = 75;
const aspect = 2;  // the canvas default
const near = 0.1;
-const far = 5;
+const far = 100;
const camera = new THREE.PerspectiveCamera(fov, aspect, near, far);
-camera.position.z = 2;
+camera.position.z = 3;

+const controls = new OrbitControls(camera, canvas);
+controls.target.set(0, 0, 0);
+controls.update();
```

and try it out. Drag on the example to rotate the camera and see the cubemap
surrounds us.

{{{example url="../threejs-background-cubemap.html" }}}

Another option is to use an Equirectangular map. This is the kind of picture a
[360 camera](https://google.com/search?q=360+camera) takes.

[Here's one](https://hdrihaven.com/hdri/?h=tears_of_steel_bridge) I found from
[this site](https://hdrihaven.com).

<div class="threejs_center"><img src="../resources/images/equirectangularmaps/tears_of_steel_bridge_2k.jpg" style="width: 600px"></div>

It's not much different. First we load the equirectangular image as a texture
and then, in the callback after it has loaded, we can call `WebGLCubeRenderTarget.fromEquirectangularTexture`
which will generate a cubemap from the equirectangular texture for us.
We pass in the size we want the cubemap to be to `WebGLCubeRenderTarget`.
Passing in the height of the equirectangular image seems like a good bet.

```js
{
-  const loader = new THREE.CubeTextureLoader();
-  const texture = loader.load([
-    'resources/images/cubemaps/computer-history-museum/pos-x.jpg',
-    'resources/images/cubemaps/computer-history-museum/neg-x.jpg',
-    'resources/images/cubemaps/computer-history-museum/pos-y.jpg',
-    'resources/images/cubemaps/computer-history-museum/neg-y.jpg',
-    'resources/images/cubemaps/computer-history-museum/pos-z.jpg',
-    'resources/images/cubemaps/computer-history-museum/neg-z.jpg',
-  ]);
-  scene.background = texture;
+  const loader = new THREE.TextureLoader();
+  const texture = loader.load(
+    'resources/images/equirectangularmaps/tears_of_steel_bridge_2k.jpg',
+    () => {
+      const rt = new THREE.WebGLCubeRenderTarget(texture.image.height);
+      rt.fromEquirectangularTexture(renderer, texture);
+      scene.background = rt.texture;
+    });
}
```

And that's all there is to it.

{{{example url="../threejs-background-equirectangularmap.html" }}}

Rather than do it at load time you can also convert an equirectangular image
to a cubemap beforehand. [Here's a site that will do it for you](https://matheowis.github.io/HDRI-to-CubeMap/).