TABLE OF CONTENTS The slice_start_code has values in the range from (0x00000101 through 0x000001AF). The least significant byte, the slice_vertical_position, gives the vertical position of the first macroblock in the slice. In general. more than one slice can start in the same row and every slice must contain at least one macroblock. However, in restricted environments slices must cover the image area and are restricted to be a single macroblock row. Main profile is a restricted environment.
Macroblocks are not skipped in I-pictures, and in any type of picture the first and last macroblock of each slice may not be skipped. In B-pictures, macroblocks may not be skipped immediately after I-blocks.
The product of 16 lines per block row x 175 block rows is 2800 lines. If the image vertical_size is greater than 2800 lines, a three-bit slice_vertical_position_extension is used to extend the range of the slice vertical position. However the least significant byte of the slice code is then restricted to 1 through 128. Note that since the main profile at high level (MP@HL) has a maximum of 1920 lines, the extension is not needed for main profile. The mb_row parameter starts at zero, and thus is the slice_ vertical_position decremented by one. Figure 10.19 shows how to calculate the macroblock row.
mb_row = slice_vertical_position;/* last byte of slice start code */ mb_row = mb_row -...
