A video processing method includes a step of receiving a bitstream, and a step of decoding a part of the bitstream to generate a decoded frame, including parsing a plurality of syntax elements from the bitstream. The decoded frame is a projection-based frame that includes a plurality of projection faces packed at a plurality of face positions with different position indexes in a hemisphere cubemap projection layout. A portion of a 360-degree content of a sphere is mapped to the plurality of projection faces via hemisphere cubemap projection. Values of the plurality of syntax elements are indicative of face indexes of the plurality of projection faces packed at the plurality of face positions, respectively, and are constrained to meet a requirement of bitstream conformance.