function WebGLMultiview( requested, gl, canvas, extensions, capabilities ) {

  this.isAvailable = function () {

    return capabilities.multiview;

  }

  this.getMaxViews = function () {

    return capabilities.maxMultiviewViews;

  }

  this.isEnabled = function () {

    return requested && this.isAvailable();

  }


	if ( requested && ! this.isAvailable() ) {

		console.warn( 'WebGLRenderer: Multiview requested but not supported by the browser' );

	} else if ( requested !== false && this.isAvailable() ) {

		console.info( 'WebGLRenderer: Multiview enabled' );

  }

	var numViews = 2;
  var framebuffer; // multiview framebuffer.
	var viewFramebuffer; // single views inside the multiview framebuffer.
	var framebufferWidth = 0;
	var framebufferHeight = 0;

	var texture = {
		color: null,
		depthStencil: null
	};


	this.createMultiviewRenderTargetTexture = function () {

    var halfWidth = Math.floor( canvas.width * 0.5 );

		framebuffer = gl.createFramebuffer();
		gl.bindFramebuffer( gl.FRAMEBUFFER, framebuffer );

    var ext = extensions.get( 'OVR_multiview2' );

		texture.color = gl.createTexture();
		gl.bindTexture( gl.TEXTURE_2D_ARRAY, texture.color );
		gl.texParameteri( gl.TEXTURE_2D_ARRAY, gl.TEXTURE_MAG_FILTER, gl.NEAREST );
		gl.texParameteri( gl.TEXTURE_2D_ARRAY, gl.TEXTURE_MIN_FILTER, gl.NEAREST );
		gl.texImage3D( gl.TEXTURE_2D_ARRAY, 0, gl.RGBA8, halfWidth, canvas.height, numViews, 0, gl.RGBA, gl.UNSIGNED_BYTE, null );
		ext.framebufferTextureMultiviewOVR( gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, texture.color, 0, 0, numViews );

		texture.depthStencil = gl.createTexture();
		gl.bindTexture( gl.TEXTURE_2D_ARRAY, texture.depthStencil );
		gl.texParameteri( gl.TEXTURE_2D_ARRAY, gl.TEXTURE_MAG_FILTER, gl.NEAREST );
		gl.texParameteri( gl.TEXTURE_2D_ARRAY, gl.TEXTURE_MIN_FILTER, gl.NEAREST );
		gl.texImage3D( gl.TEXTURE_2D_ARRAY, 0, gl.DEPTH24_STENCIL8, halfWidth, canvas.height, numViews, 0, gl.DEPTH_STENCIL, gl.UNSIGNED_INT_24_8, null );
		ext.framebufferTextureMultiviewOVR( gl.FRAMEBUFFER, gl.DEPTH_STENCIL_ATTACHMENT, texture.depthStencil, 0, 0, numViews );

		viewFramebuffer = new Array( numViews );
		for ( var viewIndex = 0; viewIndex < numViews; ++ viewIndex ) {

			viewFramebuffer[ viewIndex ] = gl.createFramebuffer();
			gl.bindFramebuffer( gl.FRAMEBUFFER, viewFramebuffer[ viewIndex ] );
			gl.framebufferTextureLayer( gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, texture.color, 0, viewIndex );

  }
		framebufferWidth = halfWidth;
		framebufferHeight = canvas.height;
};

	this.bindMultiviewFrameBuffer = function ( camera ) {

    var halfWidth = Math.floor( canvas.width * 0.5 );
    if (camera.isArrayCamera) {

    } else {
      halfWidth *= 2;
    }

		if ( framebufferWidth < halfWidth || framebufferHeight < canvas.height ) {
			console.log( 'WebGLMultiview: Updating multiview FBO with dimensions: ', halfWidth, canvas.height );
			gl.bindTexture( gl.TEXTURE_2D_ARRAY, texture.color );
      gl.texImage3D(gl.TEXTURE_2D_ARRAY, 0, gl.RGBA8, halfWidth, canvas.height, numViews, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
			gl.bindTexture( gl.TEXTURE_2D_ARRAY, texture.depthStencil );
			gl.texImage3D(gl.TEXTURE_2D_ARRAY, 0, gl.DEPTH24_STENCIL8, halfWidth, canvas.height, numViews, 0, gl.DEPTH_STENCIL, gl.UNSIGNED_INT_24_8, null);
			framebufferWidth = halfWidth;
			framebufferHeight = canvas.height;

    }

		gl.bindFramebuffer( gl.DRAW_FRAMEBUFFER, framebuffer );

	};

	this.unbindMultiviewFrameBuffer = function ( camera ) {

    gl.bindFramebuffer( gl.DRAW_FRAMEBUFFER, null );

    if ( camera.isArrayCamera ) {


      for ( var i = 0; i < camera.cameras.length; i ++ ) {

        var bounds = camera.cameras[ i ].bounds;

        var x = bounds.x * canvas.width;
        var y = bounds.y * canvas.height;
        var width = bounds.z * canvas.width;
        var height = bounds.w * canvas.height;

        gl.bindFramebuffer( gl.READ_FRAMEBUFFER, viewFramebuffer[ i ] );
        gl.blitFramebuffer( 0, 0, width, height, x, y, x + width, y + height, gl.COLOR_BUFFER_BIT, gl.NEAREST );

      }

    } else {

      gl.bindFramebuffer( gl.READ_FRAMEBUFFER, viewFramebuffer[ 0 ] );
      gl.blitFramebuffer( 0, 0, canvas.width, canvas.height, 0, 0, canvas.width, canvas.height, gl.COLOR_BUFFER_BIT, gl.NEAREST );

    }

	};


  if ( this.isEnabled() ) {

    this.createMultiviewRenderTargetTexture();

  }

}



export { WebGLMultiview };