Guides
Plan-enterprise-icon
Expo Application Services
API Reference

GLView

expo-gl provides a View that acts as an OpenGL ES render target, useful for rendering 2D and 3D graphics. On mounting, an OpenGL ES context is created. Its drawing buffer is presented as the contents of the View every frame.

Platform Compatibility

Android DeviceAndroid EmulatoriOS DeviceiOS SimulatorWeb
Status-success-iconStatus-success-iconStatus-success-iconStatus-success-iconStatus-success-icon

Installation

Terminal
→ npx expo install expo-gl

If you're installing this in a bare React Native app, you should also follow these additional installation instructions.

Usage

Basic GL usage
import React from 'react';
import { View } from 'react-native';
import { GLView } from 'expo-gl';

export default function App() {
  return (
    <View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
      <GLView style={{ width: 300, height: 300 }} onContextCreate={onContextCreate} />
    </View>
  );
}

function onContextCreate(gl) {
  gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
  gl.clearColor(0, 1, 1, 1);

  // Create vertex shader (shape & position)
  const vert = gl.createShader(gl.VERTEX_SHADER);
  gl.shaderSource(
    vert,
    `
    void main(void) {
      gl_Position = vec4(0.0, 0.0, 0.0, 1.0);
      gl_PointSize = 150.0;
    }
  `
  );
  gl.compileShader(vert);

  // Create fragment shader (color)
  const frag = gl.createShader(gl.FRAGMENT_SHADER);
  gl.shaderSource(
    frag,
    `
    void main(void) {
      gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0);
    }
  `
  );
  gl.compileShader(frag);

  // Link together into a program
  const program = gl.createProgram();
  gl.attachShader(program, vert);
  gl.attachShader(program, frag);
  gl.linkProgram(program);
  gl.useProgram(program);

  gl.clear(gl.COLOR_BUFFER_BIT);
  gl.drawArrays(gl.POINTS, 0, 1);

  gl.flush();
  gl.endFrameEXP();
}

High-level APIs

Since the WebGL API is quite low-level, it can be helpful to use higher-level graphics APIs rendering through a GLView underneath. The following libraries integrate popular graphics APIs:
  • expo-three for three.js
  • expo-processing for processing.js
Any WebGL-supporting library that expects a WebGLRenderingContext could be used. Some times such libraries assume a web JavaScript context (such as assuming document). Usually this is for resource loading or event handling, with the main rendering logic still only using pure WebGL. So these libraries can usually still be used with a couple workarounds. The Expo-specific integrations above include workarounds for some popular libraries.

Integration with Reanimated worklets

To use this API inside Reanimated worklet, you need to pass the GL context ID to the worklet and recreate the GL object like in the example below.
GL usage in reanimated worklet
import React from 'react';
import { View } from 'react-native';
import { runOnUI } from 'react-native-reanimated';
import { GLView } from 'expo-gl';

function render(gl) {
  'worklet';
  // add your WebGL code here
}

function onContextCreate(gl) {
  runOnUI((contextId: number) => {
    'worklet';
    const gl = GLView.getWorkletContext(contextId);
    render(gl);
  })(gl.contextId);
}

export default function App() {
  return (
    <View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
      <GLView style={{ width: 300, height: 300 }} onContextCreate={onContextCreate} />
    </View>
  );
}
For more in-depth example on how to use expo-gl with Reanimated and Gesture Handler you can check this example.

Limitations

Worklet runtime is imposing some limitations on the code that runs inside it, so if you have existing WebGL code, it'll likely require some modifications to run inside a worklet thread.
  • Third-party libraries like Pixi.js or Three.js won't work inside the worklet, you can only use functions that have 'worklet' added at the start.
  • If you need to load some assets to pass to the WebGL code, it needs to be done on the main thread and passed via some reference to the worklet. If you are using expo-assets you can just pass asset object returned by Asset.fromModule or from hook useAssets to the runOnUI function.
  • To implement a rendering loop you need to use requestAnimationFrame, APIs like setTimeout are not supported.
  • It's supported only on Android and iOS, it doesn't work on Web.
Check Reanimated documentation to learn more.

Remote Debugging & GLView

This API does not function as intended with remote debugging enabled. The React Native debugger runs JavaScript on your computer, not the mobile device. GLView requires synchronous native calls that are not supported in Chrome.

API

import { GLView } from 'expo-gl';

Component

GLView

Type: Component<GLViewProps>

A View that acts as an OpenGL ES render target. On mounting, an OpenGL ES context is created. Its drawing buffer is presented as the contents of the View every frame.

Props

Only for:
Apple-iconiOS

msaaSamples

Type: number • Default: 4

GLView can enable iOS's built-in multisampling. This prop specifies the number of samples to use. Setting this to 0 turns off multisampling.

onContextCreate

Type: (gl: ExpoWebGLRenderingContext) => void

A function that will be called when the OpenGL ES context is created. The function is passed a single argument gl that extends a WebGLRenderingContext interface.

Inherited Props

Static Methods

createContextAsync()

Imperative API that creates headless context which is devoid of underlying view. It's useful for headless rendering or in case you want to keep just one context per application and share it between multiple components. It is slightly faster than usual context as it doesn't swap framebuffers and doesn't present them on the canvas, however it may require you to take a snapshot in order to present its results. Also, keep in mind that you need to set up a viewport and create your own framebuffer and texture that you will be drawing to, before you take a snapshot.

Returns

  • Undo-iconPromise<ExpoWebGLRenderingContext>

A promise that resolves to WebGL context object. See WebGL API for more details.

destroyContextAsync(exgl)

NameTypeDescription
exgl
(optional)
number | ExpoWebGLRenderingContext

WebGL context to destroy.

Destroys given context.

A promise that resolves to boolean value that is true if given context existed and has been destroyed successfully.

takeSnapshotAsync(exgl, options)

NameTypeDescription
exgl
(optional)
number | ExpoWebGLRenderingContext

WebGL context to take a snapshot from.

options
(optional)
SnapshotOptions-

Takes a snapshot of the framebuffer and saves it as a file to app's cache directory.

A promise that resolves to GLSnapshot object.

Component Methods

destroyObjectAsync(glObject)

NameTypeDescription
glObjectWebGLObject-

takeSnapshotAsync(options)

NameTypeDescription
options
(optional)
SnapshotOptions-

Same as static takeSnapshotAsync(), but uses WebGL context that is associated with the view on which the method is called.

Interfaces

ExpoWebGLRenderingContext

Extends: WebGL2RenderingContext

flushEXP()

  • Undo-iconvoid
NameTypeDescription
contextIdnumber-

Types

ComponentOrHandle

Acceptable values are: null, number, Component<any, any>, ComponentClass<any>.

GLSnapshot

NameTypeDescription
heightnumberHeight of the snapshot.
localUristringSynonym for uri. Makes snapshot object compatible with texImage2D.
uristring | Blob | nullURI to the snapshot.
widthnumberWidth of the snapshot.

SnapshotOptions

NameTypeDescription
compress
(optional)
numberA value in range 0 to 1.0 specifying compression level of the result image. 1.0 means no compression and 0 the highest compression.
Default: 1.0
flip
(optional)
booleanWhether to flip the snapshot vertically.
Default: false
format
(optional)
'jpeg' | 'png' | 'webp'Specifies what type of compression should be used and what is the result file extension. PNG compression is lossless but slower, JPEG is faster but the image has visible artifacts.
Info-icon
Note: When using WebP format, the iOS version will print a warning, and generate a 'png' file instead. It is recommendable to use platform dependant code in this case. You can refer to the documentation on platform specific code.
Default: 'jpeg'
framebuffer
(optional)
WebGLFramebufferSpecify the framebuffer that we will be reading from. Defaults to underlying framebuffer that is presented in the view or the current framebuffer if context is headless.
rect
(optional)
{ height: number, width: number, x: number, y: number }Rect to crop the snapshot. It's passed directly to glReadPixels.

SurfaceCreateEvent

NameTypeDescription
nativeEvent{ exglCtxId: number }-

WebGLObject

NameTypeDescription
idnumber-

Enums

GLLoggingOption

GLLoggingOption.DISABLED = 0

Disables logging entirely.

GLLoggingOption.METHOD_CALLS = 1

Logs method calls, their parameters and results.

GLLoggingOption.GET_ERRORS = 2

Calls gl.getError() after each other method call and prints an error if any is returned. This option has a significant impact on the performance as this method is blocking.

GLLoggingOption.RESOLVE_CONSTANTS = 4

Resolves parameters of type number to their constant names.

GLLoggingOption.TRUNCATE_STRINGS = 8

When this option is enabled, long strings will be truncated. It's useful if your shaders are really big and logging them significantly reduces performance.

GLLoggingOption.ALL = 15

Enables all other options. It implies GET_ERRORS so be aware of the slowdown.

WebGL API

Once the component is mounted and the OpenGL ES context has been created, the gl object received through the onContextCreate prop becomes the interface to the OpenGL ES context, providing a WebGL API. It resembles a WebGL2RenderingContext in the WebGL 2 spec.
Some older Android devices may not support WebGL2 features. To check whether the device supports WebGL2 it's recommended to use gl instanceof WebGL2RenderingContext.
An additional method gl.endFrameEXP() is present, which notifies the context that the current frame is ready to present. This is similar to a 'swap buffers' API call in other OpenGL platforms.
The following WebGL2RenderingContext methods are currently unimplemented:
  • getFramebufferAttachmentParameter()
  • getRenderbufferParameter()
  • compressedTexImage2D()
  • compressedTexSubImage2D()
  • getTexParameter()
  • getUniform()
  • getVertexAttrib()
  • getVertexAttribOffset()
  • getBufferSubData()
  • getInternalformatParameter()
  • renderbufferStorageMultisample()
  • compressedTexImage3D()
  • compressedTexSubImage3D()
  • fenceSync()
  • isSync()
  • deleteSync()
  • clientWaitSync()
  • waitSync()
  • getSyncParameter()
  • getActiveUniformBlockParameter()
The pixels argument of texImage2D() must be null, an ArrayBuffer with pixel data, or an object of the form { localUri } where localUri is the file:// URI of an image in the device's file system. Thus, an Asset object is used once .downloadAsync() has been called on it (and completed) to fetch the resource.
For efficiency reasons, the current implementations of the methods don't perform type or bounds checking on their arguments. So, passing invalid arguments may cause a native crash. There are plans to update the API to perform argument checking in upcoming SDK versions.
Currently, the priority for error checking is low since engines generally don't rely on the OpenGL API to perform argument checking; otherwise, checks performed by the underlying OpenGL ES implementation are often sufficient.
  • Message-iconAsk a question on the forums about GLView
  • Github-iconView open bug reports for GLView
  • Code-iconView source code for GLView
  • Build-iconView package in npm Registry
  • Edit-iconEdit this page

Was this doc helpful?