three.js/src/renderers/shaders/ShaderLib/cube_frag.glsl

uniform samplerCube tCube;
uniform float tFlip;
uniform float opacity;

varying vec3 vWorldDirection;

void main() {

	gl_FragColor = textureCube( tCube, vec3( tFlip * vWorldDirection.x, vWorldDirection.yz ) );
	gl_FragColor.a *= opacity;

}