webgl刷底色clearColor

var canvas = document.getElementById("mycanvas")
			canvas.width = window.innerWidth
			canvas.height = window.innerHeight
			const gl = canvas.getContext('webgl')
			gl.clearColor(0,0,0,0.5)//0 - 1范围
			gl.clear(gl.COLOR_BUFFER_BIT)

个人认为实际上刷的是它所属的区域，如果事先有背景色的时候还是会在底部