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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.

Installation

If you are installing this in an existing React Native app, make sure to install expo in your project.

Usage

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

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.

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 }}
        enableExperimentalWorkletSupport
        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.

Check Reanimated documentation to learn more.

Remote debugging and 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';

No API data file found, sorry!

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.