GLSL sampling from the scene background video?

If background is set like this

const video = document.getElementById( 'videoBg' );;
const texture = new THREE.VideoTexture( video );
scene.background = texture;

How do I sample from it in glsl?

vec3 normal = normalize(vNormal); //renormalize vertex normals
vec3 vectorBounceToEnvironment = normalize(reflect(-cameraViewDirection,normal)); //bounce vector
vec3 colorSampleOnBackgroundVideo = texture2D(bgVideo,?????????????).xyz; //color of video

is ? == vUv ? or gl_FragCoord.xy if you want to sample in screenspace…

Note you would need the screen size if you go the gl_FragCoord route.