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A fast multiple reference assignment scheme is proposed to reduce the motion search complexity in the H.264 video standard in this work. The relationship between the temporal search range and the block size is exploited to develop a new algorithm, called the block-size adaptive referencing (BAR) scheme, that assigns a different number of references in a block-size adaptive manner. The expected rate-distortion (RD) coding loss due to the use of fewer references in the BAR scheme is first analyzed. Furthermore, it is found that the RD coding loss of BAR is related to the RD gradient through a model. Thus, a proper BAR scheme can be chosen to minimize the reference frame numbers while keeping the expected RD loss under a target level. Finally, fast motion search is performed within selected references in a block-size adaptive manner. Experimental results show that the proposed BAR scheme can save approximately 25-70% of the encoding complexity in H.264 (JM10.1) motion estimation with negligible degradation in the RD performance.