Expo Docs

GLView

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.

Other than the regular View props for layout and touch handling, the following props are available:
  • onContextCreate
    A function that will be called when the OpenGL ES context is created. The function is passed a single argument gl that has a WebGLRenderingContext interface.
  • msaaSamples
    GLView can enable iOS's built-in multisampling. This prop specifies the number of samples to use. By default this is 4. Setting this to 0 turns off multisampling. On Android this is ignored.

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

  • options (object) -- A map of options:
    • framebuffer (WebGLFramebuffer) -- Specify the framebuffer that we will be reading from. Defaults to underlying framebuffer that is presented in the view.
    • rect ({ x: number, y: number, width: number, height: number }) -- Rect to crop the snapshot. It's passed directly to glReadPixels.
    • flip (boolean) -- Whether to flip the snapshot vertically. Defaults to false.

Returns { uri, localUri, width, height } where uri is a URI to the snapshot. localUri is a synonym for uri that makes this object compatible with texImage2D. width, height specify the dimensions of the snapshot.

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.

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. However, 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 be presented. 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 Expo.Asset object could be 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 could cause a native crash. We plan 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 and, even otherwise, checks performed by the underlying OpenGL ES implementation are often sufficient.