直接使用 WebGLRenderer 时，背景会自动清除为白色。但是，当使用 EffectComposer 时，背景会被清除为黑色。
这是一个示例：http: //jsfiddle.net/wereHamster/Lt2DX/。场景显示白色背景上的红色立方体。如果您注释掉第 41 行并取消注释第 42 行（启用 EffectComposer），画布将在黑色背景上显示一个红色立方体。
这是代码：
# A simple scene with a red cube in the center.
scene = new THREE.Scene
camera = new THREE.PerspectiveCamera 75, 200/200, 0.01, 1000
camera.position.z = 5
scene.add camera
geometry = new THREE.CubeGeometry 1, 1, 1
material = new THREE.MeshBasicMaterial { color: 0xFF0000 }
scene.add (mesh = new THREE.Mesh geometry, material)
# The barebone WebGL renderer.
renderer = new THREE.WebGLRenderer
renderer.setSize 200, 200
$('body').append renderer.domElement
# Composer with two passes, RenderPass and ShaderPass.
composer = new THREE.EffectComposer renderer
renderPass = new THREE.RenderPass scene, camera
composer.addPass renderPass
shaderPass = new THREE.ShaderPass THREE.CopyShader
shaderPass.renderToScreen = true
composer.addPass shaderPass
# The main animation loop.
animate = ->
requestAnimationFrame animate
mesh.rotation.x += 0.01
mesh.rotation.y += 0.01
# If we use the composer to render the scene then the
# background is black instead of white.
renderer.render scene, camera
#composer.render 0.1
animate()