ed with the corresponding luma samples and the nominal locations in a frame and in fields are as shown in Figures 6-3 and 6-4, respectively. Frame H.264(09)_F6-3 Location of luma sample Location of chroma sample Figure 6-3 - Nominal vertical and horizontal locations of 4:2:2 luma and chroma samples in a frame Rec. ITU-T H.264 (01/2012) Bottom field field Location of luma sample Location of chroma sample H.264(09)_F6-4 Figure 6-4 - Nominal vertical and horizontal sampling locations of 4:2:2 samples top and bottom fields When the value of chroma_format_idc is equal to 3, all array samples are co-sited for all cases of frames and fields and the nominal locations in a frame and in fields are as shown in Figures 6-5 and 6-6, respectively. Frame H.264(09)_F6-5 Location of luma sample Location of chroma sample Figure 6-5 - Nominal vertical and horizontal locations of 4:4:4 luma and chroma samples in a frame Rec. ITU-T H.264 (01/2012) Bottom field field Location of luma sample Location of chroma sample H.264(09)_F6-6 Figure 6-6 - Nominal vertical and horizontal sampling locations of 4:4:4 samples top and bottom fields The samples are processed in units of macroblocks. The luma array for each macroblock is 16 samples in both width and height. The variables MbWidthC and MbHeightC, which specify the width and height, respectively, of the chroma arrays for each macroblock, are derived as follows: If chroma_format_idc is equal to 0 (monochrome) or separate_colour_plane_flag is equal to 1, MbWidthC and MbHeightC are both equal to 0. Otherwise, MbWidthC and MbHeightC are derived as MbWidthC = 16 / SubWidthC (6-1) MbHeightC = 16 / SubHeightC (6-2) Spatial subdivision of pictures and slices This clause specifies how a picture is partitioned into slices and macroblocks. Pictures are divided into slices. A slice is a sequence of macroblocks, or, when macroblock-adaptive frame/field decoding is in use, a sequence of macroblock pairs. Each macroblock is comprised of one 16x16 luma array and, when the chroma sampling format is not equ