MPEG
Rawesak Tanawongsuwan
ccrtw@mahidol.ac.th
Video Compression
• Spatial (intra-frame) compression
– Compress each frame in isolation, treating it
as a bitmapped image
• Temporal (inter-frame) compression
– Compress sequences of frames by only
storing differences between them
Spatial Compression
• Image compression applied to each frame
• Can be either lossless or lossy
• Motion JPEG
– Purely spatial compression
– Apply JPEG to each frame
– Used by most analogue capture cards
– No standard, but MJPEG-A format widely
supported
Temporal Compression
• A video consists of a time-ordered
sequence of frames
• Contents do not change rapidly between
frames
• A solution to video compression would be
predictive coding based on previous
frames
Temporal Compression
• Compression proceeds by subtracting images
– subtract in time order and code the residual error
Temporal Compression
• We can do better by searching for parts of the
image to subtract from the previous frame.
Video Compression with Motion
Compensation
• Exploit temporal redundancy
• Steps of Video compression based on
Motion Compensation (MC)
– Motion Estimation (motion vector search)
– MC-based Prediction
– Derivation of the prediction error, i.e., the
difference.
Motion Compensation
• Each image is divided into macroblocks of
size NxN
Motion compensation
• Target frame: the current image frame
• Reference frame: either previous or future frame
• Goal:
– predict the target macroblock from the reference
macroblocks
– For each macroblock in the target frame, find most
similar macroblock in the reference frame
• Motion vector (MV): displacement of the
reference macroblock to the target macroblock
• Prediction error: difference of the two
corresponding macroblocks
Motion Compensation
Search for Motion Vectors
• The difference between two macroblocks
can be measured by several criteria, e.g.
– Mean Absolute Difference (MAD)
• Goal is to find a closest match within the
searching window
Search methods
• Sequential (simple)
• Logarithmic
• Hierarchical
H.261
• An earlier digital video compression standard
• Its principle of MC-based compression is
retained in all later
• Video compression standards designed for
videophone, video conferencing, other
audiovisual services over ISDN
H.261 Frame Sequence
• Intra-frames (I-frames) vs. Inter-frames (P-frames)
• I-frames: independent images, spatial redundancy
coding, coding method similar to JPEG, usually sent a
couple of times in each second of video
• P-frames: not independent, coded by a forward
predictive coding method, temporal redundancy coding
• Motion vectors in H.261 are always measured in units of
full pixel and they have a limited range of about 15 pixels
I-frame coding
• Macroblocks are of size 16x16 pixels for the Y frame,
and 8x8 for Cb and Cr frames, since 4:2:0 chroma
subsampling is employed. A macroblock consists of four
Y, one Cb, and one Cr 8x8 blocks.
P-frame (predictive) coding
MPEG
• MPEG: Moving Pictures Experts Group,
established in 1988 for the development of
digital video
• MPEG-1,-2,-4,-7,-21,A,B,C,D,E,…
MPEG-1
• MPEG-1 adopts the CCIR601 digital TV format
also known as SIF (Source Input Format)
• MPEG-1 supports only non-interlaced video.
• Normal picture resolution is
– 352x240 for NTSC video at 30 fps
– 352x288 for PAL video at 25 fps
• 4:2:0 chroma subsampling
Motion compensation
• Forward prediction
Problem finding the good match
Motion compensation
• Future frame may help find a good match
• MPEG introduces B-frame
• Bi-directional motion compensation
Bi-directional motion compensation
average
MPEG Frame Sequence
Sizes of MPEG-1 Frames
• The typical size of compressed P-frames is
significantly smaller than that of I-frames
– temporal redundancy is exploited in inter-frame
compression
• B-frames are even smaller than P-frames
– the advantage of bi-directional prediction
MPEG-2
• Higher quality video at a bit-rate of more than 4 Mbps
• Support interlaced video because of the options for
digital broadcast TV and HDTV
• Support various picture resolutions as defined by DVD,
ATV and HDTV
MPEG-4
• MPEG-4 departs from its predecessors in adopting an
object-based coding
• Higher compression ratio, also beneficial for digital video
composition, manipulation, indexing, and retrieval
• The bit-rate for MPEG-4 video now covers a large range
between 5 kbps to 10 Mbps.
Block-based vs Object-based coding
MPEG-4 Video Principle
• Retained from predecessor
– Block based natural video coding
– Motion estimation and compensation
– Variable length coding
• MPEG-4 Specific
– Object based coding
– Arbitrary shape coding
– 2D & 3D mesh and texture coding
