HORZ_SAMPLING_FACTOR
HORZ_SAMPLING_FACTOR | (VERT_SAMPLING_FACTOR_422));
HORZ_SAMPLING_FACTOR | (VERT_SAMPLING_FACTOR_420));
header_write(ctx, buffer, offset, 1, (HORZ_SAMPLING_FACTOR >> 1) | 1);
header_write(ctx, buffer, offset, 1, (HORZ_SAMPLING_FACTOR >> 1) | 1);