updateBefore( frame ) {

		const { renderer } = frame;
		const { scene, camera, stereo, renderTarget } = this;

		_rendererState = RendererUtils.resetRendererState( renderer, _rendererState );

		//

		this._pixelRatio = renderer.getPixelRatio();

		stereo.cameraL.coordinateSystem = renderer.coordinateSystem;
		stereo.cameraR.coordinateSystem = renderer.coordinateSystem;
		stereo.update( camera );

		const size = renderer.getSize( _size );
		this.setSize( size.width, size.height );

		renderer.autoClear = false;

		this._cameraNear.value = camera.near;
		this._cameraFar.value = camera.far;

		for ( const name in this._previousTextures ) {

			this.toggleTexture( name );

		}

		renderer.setRenderTarget( renderTarget );
		renderer.setMRT( this._mrt );
		renderer.clear();

		renderTarget.scissorTest = true;

		renderTarget.scissor.set( 0, 0, renderTarget.width / 2, renderTarget.height );
		renderTarget.viewport.set( 0, 0, renderTarget.width / 2, renderTarget.height );
		renderer.render( scene, stereo.cameraL );

		renderTarget.scissor.set( renderTarget.width / 2, 0, renderTarget.width / 2, renderTarget.height );
		renderTarget.viewport.set( renderTarget.width / 2, 0, renderTarget.width / 2, renderTarget.height );
		renderer.render( scene, stereo.cameraR );

		renderTarget.scissorTest = false;

		// restore

		RendererUtils.restoreRendererState( renderer, _rendererState );

	}
This method is used to render the stereo effect once per frame.

@param {NodeFrame} frame - The current node frame.