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
Terminal
import{View}from'react-native';import{GLView}from'expo-gl';exportdefaultfunctionApp(){return(<Viewstyle={{flex:1,justifyContent:'center',alignItems:'center'}}><GLViewstyle={{width:300,height:300}}onContextCreate={onContextCreate}/></View>functiononContextCreate(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 programconst 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:
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.
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.
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.
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.
GLViewProps
enableExperimentalWorkletSupport
Android
iOS
Web
Optional • Type: boolean • Default: false
Enables support for interacting with a gl object from code running on the Reanimated worklet thread.
msaaSamples
iOS
Optional • 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.
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.
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.
Note that the context created using createContextAsync has to be destroyed using destroyContextAsync.
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.
A 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)
boolean
Whether 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.
Note: When using WebP format, the iOS version will print a warning, and generate a 'png' file instead.
It is recommended to use platform-specific code in this case.
Specify 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
Android
iOS
Web
Property
Type
Description
nativeEvent
{
exglCtxId: number
}
-
WebGLObject
Android
iOS
Web
Property
Type
Description
id
number
-
Enums
GLLoggingOption
Android
iOS
Web
DISABLED
GLLoggingOption.DISABLED ＝ 0
Disables logging entirely.
METHOD_CALLS
GLLoggingOption.METHOD_CALLS ＝ 1
Logs method calls, their parameters and results.
GET_ERRORS
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.
RESOLVE_CONSTANTS
GLLoggingOption.RESOLVE_CONSTANTS ＝ 4
Resolves parameters of type number to their constant names.
TRUNCATE_STRINGS
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.
ALL
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:
Expo
GLView