Reducing computational complexity when video encoding uses bi-predictively encoded frames

A digital processing system comprising:

• at least one processor;
• and
• a non-transitory computer readable storage medium storing a program for execution by the at least one processor, the program including instructions to:
• receive a first macro-block of an image frame;
• compute a cost function of the first macro-block, comprising instructions to:
• determine a potential best-match macro-block in a region of a reference frame by comparing a pixel value of the potential best-match macro block to a pixel value of the first macro-block;
• and
• compute the cost function of the first macro-block based on a sum-of-absolute differences (SAD) values of pixel value differences between pixels of the first macro-block and the potential best-match macro-block and a predicted motion vector (PMV) of the first macro-block, in response to determining that the first macro-block is a B-frame;
• perform fractional pixel refinement for motion estimation on the reference frame based on the computed cost function of the first macro-block;
• compute a motion vector for the first macro-block based on the cost function of the first macro-block, wherein the instructions to compute the motion vector include instructions to perform sub-pixel interpolation;
• and
• encode the image frame based on the motion vector for the first macro-block.