erence index refIdxLXN remain unchanged. 8.4.1.4 Derivation process for chroma motion vectors This process is only invoked when ChromaArrayType is not equal to 0. Inputs to this process are a luma motion vector mvLX and a reference index refIdxLX. Output of this process is a chroma motion vector mvCLX. A chroma motion vector is derived from the corresponding luma motion vector. The precision of the chroma motion vector components is 1 ( 4 * SubWidthC ) horizontally and 1 (4 SubHeightC ) vertically. NOTE - For example, when using the 4:2:0 chroma format, since the units of luma motion vectors are one-quarter luma sample units and chroma has half horizontal and vertical resolution compared to luma, the units of chroma motion vectors are one-eighth chroma sample units, i.e., a value of 1 for the chroma motion vector refers to a one-eighth chroma sample displacement. For example, when the luma vector applies to 8x16 luma samples, the corresponding chroma vector in 4:2:0 chroma format applies Rec. ITU-T H.264 (01/2012) to 4x8 chroma samples and when the luma vector applies to 4x4 luma samples, the corresponding chroma vector in 4:2:0 chroma format applies to 2x2 chroma samples. For the derivation of the motion vector mvCLX, the following applies: If ChromaArrayType is not equal to 1 or the current macroblock is a frame macroblock, the horizontal and vertical components of the chroma motion vector myCLX are derived as: mvCLX[ [0]=mvLX[0] (8-221) mvCLX[ [1]=mvLX[1] (8-222) Otherwise (ChromaArrayType is equal to 1 and the current macroblock is a field macroblock), only the horizontal component of the chroma motion vector mvCLX[ 0] is derived using Equation 8-221. The vertical component of the chroma motion vector mvCLX[ 1 is dependent on the parity of the current field or the current macroblock and the reference picture, which is referred by the reference index refIdxLX. mvCLX[ is derived from mvLX[ according to Table 8-10. Table 8-10 - Derivation of the vertical component of the chroma vector in field coding mode Parity