Runlength encoding

• Since low frequencies are preserved by DCT/quantisation process and high frequencies dropped, non-zero components occur in top left of matrix
• For efficient RLE require long unbroken runs of 0
• Zigzag sequence used

• DC (mean) coefficient does not typically vary much from block to block, so difference from last block is stored
• DC stored as (SIZE, AMPLITUDE) pair
• Assuming DC component of previous block was +12, difference is +3, hence DC encoded as (2,3), ie. 2 bits, amplitude 3
• AC components stored as (NUMBER OF PRECEEDING ZEROES, SIZE, AMPLITUDE)
• For above therefore:
  (2,3) (1,2)(-2) (0,1)(-1) (0,1)(-1) (0,1)(0,1) (2,1)(-1) (0,0)
• (0,0) is EOB (end of block)