A projection-based frame is generated according to an omnidirectional video frame and a triangle-based projection layout. The projection-based frame has a 360-degree image content represented by triangular projection faces assembled in the triangle-based projection layout. A 360-degree image content of a viewing sphere is mapped onto the triangular projection faces via a triangle-based projection of the viewing sphere. One side of a first triangular projection face has contact with one side of a second triangular projection face, one side of a third triangular projection face has contact with another side of the second triangular projection face. One image content continuity boundary exists between one side of the first triangular projection face and one side of the second triangular projection face, and another image content continuity boundary exists between one side of the third triangular projection face and another side of the second triangular projection face.