Coding spherical video data

A method is described of forming a bitstream by an encoder apparatus comprising: a processor of the encoder apparatus receiving a projected video frame including one or more pixel regions, the one or more pixel regions representing pixels of spherical video data projected onto one or more 2D projection surfaces of a projection model, the projection model being adapted to project spherical video data onto a rectangular 2D surface of a video frame; the processor generating a plurality of different candidate video frames on the basis of the received projected video frame, each candidate video frame being formed by applying at least one operation to one or more groups of pixels in one or more pixel regions of the projected video frame, wherein said at least operation includes a shift, mirror and/or rotation operation; and, wherein said operation is different for each of said plurality of different candidate video frames; the processor applying a rate distortion analysis to each of the candidate video frames to determine a rate distortion cost for each of the candidate video frames; and, the processor selecting a candidate video frame from said plurality of candidate video frames on the basis of the rate distortion cost; and, generating an output bitstream on the basis of the selected candidate video frame.