digital video processing

Digital Video Processing

Vasant ManoharComputer Science and Engineering

University of South Florida

Digital Image Processing – Fall 2008 Prof. Dmitry Goldgof

http://www.csee.usf.edu/~vmanohar

[email protected]

Outline

• Basics of Video• Digital Video• MPEG• Summary

Basics of Video

Static scene capture ImageBring in motion Video

• Image sequence: A 3-D signal– 2 spatial dimensions & 1 time dimension

– Continuous I (x, y, t) discrete I (m, n, tk)

Video Camera

• Frame-by-frame capturing• CCD sensors (Charge-Coupled Devices)

– 2-D array of solid-state sensors– Each sensor corresponds to a pixel– Stored in a buffer and sequentially read out– Widely used

Video Display

• CRT (Cathode Ray Tube)– Large dynamic range– Bulky for large display

• CRT physical depth has to be proportional to screen width

• LCD Flat-panel display– Use electrical field to change the optical properties,

thereby the brightness/color of liquid crystal– Generating the electrical field

• By an array of transistors: active-matrix thin-film transistors

“Active-matrix TFT display has a transistor located at each pixel, allowing display to be switched more frequently and with less current to control pixel luminance. Passive matrix LCD has a grid of conductors with pixels located at the grid intersections”

Composite vs. Component Video

• Component video– Three separate signals for tri-stimulus color representation or

luminance-chrominance representation– Pro: higher quality– Con: need high bandwidth and synchronization

• Composite video– Multiplex into a single signal– Historical reason for transmitting color TV through monochrome

channel– Pro: save bandwidth– Con: cross talk

• S-video– Luminance signal + single multiplexed chrominance signal

Progressive vs. Interlaced Videos

• Progressive– Every pixel on the screen is refreshed in order (monitors) or

simultaneously (films)

• Interlaced – Refreshed twice every frame; the little gun at the back of your

CRT shoots all the correct phosphors on the even numbered rows of pixels first and then odd numbered rows

– NTSC frame-rate of 29.97 means the screen is redrawn 59.94 times a second

– In other words, 59.94 half-frames per second or 59.94 fields per second


• How interlaced video could cause problems– Suppose you resize a 720 x 480 interlaced video to 576

x 384 (20% reduction)– How does resizing work?

• takes a sample of the pixels from the original source and blends them together to create the new pixels

– In case of interlaced video, you might end of blending scan lines of two completely different images!


Image in full 720 x 480 resolution Observe distinct scan lines


Image after being resized to 576x384 Some scan lines blended together!

Aspect Ratio

• When you view pure NTSC video on your monitor, people look a little fatter than normal ?

• TV video stored in 3:2 aspect ratio, while monitors store picture data in 4:3 aspect ratio

• A lot of capture cards crop off the 16 pixels in the horizontal edges and capture in 704 x 480 or 352 x 480

• Aspect ratios in movies– 5:3 mostly used in animation movies– 16:9 academy ratio– 21:9 cinescope

Aspect Ratio

• Converting widescreen pictures to 4:3 TV format– letterbox format (black bars above and below the picture)– losing parts of the picture

• If we convert a 21:9 picture, we might lose a large part of the picture (blue – 16:9, red – 4:3)

DIGITAL VIDEO

Why Digital?

• “Exactness”– Exact reproduction without degradation– Accurate duplication of processing result

• Convenient & powerful computer-aided processing– Can perform rather sophisticated processing through

hardware or software

• Easy storage and transmission– 1 DVD can store a three-hour movie !!!– Transmission of high quality video through network in

reasonable time

Digital Video Coding

• The basic idea is to remove redundancy in video and encode it

• Perceptual redundancy– The Human Visual System is less sensitive to color and

high frequencies

• Spatial redundancy– Pixels in a neighborhood have close luminance levels

• Low frequency

• How about temporal redundancy?– Differences between subsequent frames are very less.

Shouldn’t we exploit this?

Hybrid Video Coding

• “Hybrid” ~ combination of Spatial, Perceptual, & Temporal redundancy removal

• Issues to be handled– Not all regions are easily inferable from previous frame

• Occlusion ~ solved by backward prediction using future frames as reference

• The decision of whether to use prediction or not is made adaptively

– Drifting and error propagation• Solved by encoding reference regions or frames at constant intervals

of time

– Random access• Solved by encoding frame without prediction at constant intervals of

time

– Bit allocation• according to statistics• constant and variable bit-rate requirement

MPEG combines all of these features !!!

MPEG

• MPEG – Moving Pictures Experts Group– Coding of moving pictures and associated audio

• Picture part– Can achieve compression ratio of about 50:1 through storing only

the difference between successive frames– Even higher compression ratios possible

• Audio part– Compression of audio data at ratios ranging from 5:1 to 10:1– MP3 ~ “MPEG-1 audio Layer-3”

MPEG Generations

Bit Rate

• Defined in two ways– bits per second (all inter-frame compression algorithms)– bits per frame (most intra-frame compression algorithms

except DV and MJPEG)

• What does this mean?– If you encode something in MPEG, specify it to be 1.5

Mbps; it doesn’t matter what the frame-rate is, it takes the same amount of space lower frame-rate will look sharper but less smooth

– If you do the same with a codec like Huffyuv or Intel Indeo, you will get the same image quality through all of them, but the smoothness and file sizes will change as frame-rate changes

Data Hierarchy

• Sequence: entire video sequence• Group of Pictures: basic unit allowing for random access• Picture: primary coding unit with three color components and different picture formats

progressive or interlaced scanning modes• Slice or Group of Blocks: basic unit for resynchronization refresh and error recovery

(skipped if erroneous)• Macro-block: motion compensation unit• Block: transform and compression unit

MPEG-1 Compression Aspects

• Lossless and Lossy compression are both used for a high compression rate

• Down-sampled chrominance – Perceptual redundancy

• Intra-frame compression – Spatial redundancy– Correlation/compression within a frame– Based on “baseline” JPEG compression standard

• Inter-frame compression– Temporal redundancy– Correlation/compression between like frames

• Audio compression– Three different layers (MP3)

Perceptual Redundancy

• Here is an image represented with 8-bits per pixel

• The same image at 7-bits per pixel


• At 6-bits per pixel


• It is clear that we don’t all these bits!– Our previous example illustrated the eye’s sensitivity

to luminance

• We can build a perceptual model– Give more importance to what is perceivable to the

Human Visual System• Usually this is a function of the spatial frequency


Video Coloring Scheme

• Translate the RGB system into a YUV system• Human perception is less sensitive to chrominance

than to brightness– Translate brightness into chrominance and then the

resolution does not have to be as good lower necessary bit-rate

Translation formulas:Y = WrR + WbB + WgGCr = Wr’ (R - Y)Cb = Wb’ (B - Y)Cg =Wg’ (G - Y)

Normal

Red

Green

Blue

JPEG Coloring Blocks

Luminance

Cr

Cb

Cg

Coloring Scheme:

Video Coloring Scheme

• Chrominance means “the difference between one color and a reference color of the same brightness and chromaticity.”

• Block: composed of six blocks (4:2:0 or 4:1:1 format)– Four blocks of yellow (luminance)– One block of Cb (blue chrominance)– One block of Cr (red chrominance)

• Down-sampled chrominance– Y Cb Cr coordinate and four sub-sampling formats

Ref: Y. Wang, J. Osterman, Y-Q Zhang: Digital Video Processing & Communications, Prentice-Hall, 2001

Intra-frame Compression

• Intra-frame Coding:– Reduces spatial redundancy to reduce necessary

transmission rate– Encoding I-blocks are practically identical to JPEG

standard– Makes use of the DCT transform along with zigzag

ordering• Lossy data compression

Fundamentals of JPEG

DCT Quantizer Entropy coder

IDCT Dequantizer Entropy

decoder

Compressed

image data

Encoder

Decoder

Fundamentals of JPEG

• JPEG works on 8×8 blocks• Extract 8×8 block of pixels• Convert to DCT domain• Quantize each coefficient

– Different stepsize for each coefficient• Based on sensitivity of human visual system

• Order coefficients in zig-zag order– Similar frequencies are grouped together

• Run-length encode the quantized values and then use Huffman coding on what is left

Random Access and Inter-frame Compression

Temporal Redundancy – Only perform repeated encoding of the parts of a picture

frame that are rapidly changing– Do not repeatedly encode background elements and still

elements

Random access capability– Prediction that does not depend upon the user accessing the first

frame (skipping through movie scenes, arbitrary point pick-up)

3-D Motion -> 2-D Motion

2-D MV

3-D MV

Sample (2D) Motion Field

Anchor FrameTarget Frame

Motion Field

2-D Motion Corresponding to Camera Motion

Camera zoom Camera rotation around Z-axis (roll)

General Considerationsfor Motion Estimation

• Two categories of approaches:– Feature based (more often used in object tracking, 3D

reconstruction from 2D)– Intensity based (based on constant intensity

assumption) (more often used for motion compensated prediction, required in video coding, frame interpolation)

• Three important questions– How to represent the motion field?– What criteria to use to estimate motion parameters?– How to search motion parameters?

Motion Representation

Global:Entire motion field is represented by a few global parameters

Pixel-based:One MV at each pixel, with some smoothness constraint between adjacent MVs.

Region-based:Entire frame is divided into regions, each region corresponding to an object or sub-object with consistent motion, represented by a few parameters.

Block-based:Entire frame is divided into blocks, and motion in each block is characterized by a few parameters.

Also mesh-based (flow of corners, approximated inside)

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Half-pel Exhaustive Block Matching Algorithm (EBMA)

Examples

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Three-level Hierarchical Block Matching Algorithm

Examples

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EBMA vs. Mesh-based Motion Estimation

Examples

Motion Compensated Prediction

• Divide current frame, i, into disjoint 16×16 macroblocks

• Search a window in previous frame, i-1, for closest match

• Calculate the prediction error• For each of the four 8×8 blocks in the

macroblock, perform DCT-based coding• Transmit motion vector + entropy coded

prediction error (lossy coding)

Decoding with non-random access

• Decoding and playing sub-frames located in section G, all frames before section G must be decoded as well• Synchronization algorithm issues • If section G is far along in the movie, this could take a considerable amount of time

Decoding with random access

• When decoding any frame after an I frame (frame G in this example)– we only have to decode past frames until we reach an I-frame – saves time when skipping from frame to frame

• I-frames are not predictively encoded – reduction in compression ratio

• Depending on the concentration of I frames, there is a tradeoff:– More I frames faster random access time – Less I frames better compression ratio

Introduce “I” frames, frames that are NOT predictively encoded by design

Frames that are still encoded using a prediction algorithm are called “P” frames

MPEG-1 Video Coding

• Most MPEG1 implementations use a large number of I frames to ensure fast access– Somewhat low compression ratio by itself

• For predictive coding, P frames depend on only a small number of past frames– Using less past frames reduces the propagation error

• To further enhance compression in an MPEG-1 file, introduce a third frame called the “B” frame bi-directional frame– B frames are encoded using predictive coding of only two other

frames: a past frame and a future frame

• By looking at both the past and the future, helps reduce prediction error due to rapid changes from frame to frame (i.e. a fight scene or fast-action scene)

Predictive coding hierarchy:I, P and B frames

• I frames (black) do not depend on any other frame and are encoded separately– Called “Anchor frame”

• P frames (red) depend on the last P frame or I frame (whichever is closer)– Also called “Anchor frame”

• B frames (blue) depend on two frames: the closest past P or I frame, and the closest future P or I frame– B frames are NOT used to predict other B frames, only P frames

and I frames are used for predicting other frames

MPEG-1 Temporal Order of Compression

• I frames are generated and compressed first– Have no frame dependence

• P frames are generated and compressed second– Only depend upon the past I frame values

• B frames are generated and compressed last– Depend on surrounding frames– Forward prediction needed

Adaptive Predictive Coding inMPEG-1

• Coding each block in P-frame– Predictive block using previous I/P frame as reference– Intra-block ~ encode without prediction

• use this if prediction costs more bits than non-prediction• good for occluded area• can also avoid error propagation

• Coding each block in B-frame– Intra-block ~ encode without prediction– Predictive block

• use previous I/P frame as reference (forward prediction)• or use future I/P frame as reference (backward prediction)• or use both for prediction

Codec Adjustments

• For smoothing out bit rate– A few applications prefer approx. constant bit rate video stream

(CBR)• e.g., prescribe number of bits per second• very-short-term bit-rate variations can be smoothed by a buffer• variations cannot be too large on longer term, else buffer overflow

• For reducing bit rate by exploiting Human Vision System (HVS) temporal properties– Noise/distortion in a video frame would not be very much visible

when there is a sharp temporal transition (scene change)• can compress a few frames right after scene change with less bits

• Changing the frame types– I I I I I I … lowest compression ratio (like MJPEG)– I P P … P I P P … moderate compression ratio– I B B P B B P B B I … highest compression ratio

MPEG Library

• The MPEG Library is a C library for decoding MPEG-1 video streams and dithering them to a variety of color schemes.

• Most of the code in the library comes directly from an old version of the Berkeley MPEG player (mpeg_play)

• The Library can be downloaded fromhttp://starship.python.net/~gward/mpeglib/mpeg_lib-1.3.1.tar.gz

• It works good on all modern Unix and Unix-like platforms with an ANSI C compiler. I have tested it on “grad”.

NOTE - This is not the best library available. But it works good for MPEG-1 and it is fairly easy to use. If you are inquisitive, you should check MPEG Software Simulation Group at http://www.mpeg.org/MPEG/MSSG/ where you can find a free MPEG-2 video coder/decoder.

MPEG Library

• Using the Library is very similar to the way in which files are normally handled– Open an MPEG stream and initialize internal data

structures– Read frames until the stream is done in– If need be, you can rewind the stream and start over– When done, you close the stream and clean up

NOTE: You cannot randomly access the stream. This is a limitation of the Library because of the nature of the decoding engine on which the Library is built.

NOTE: Berkeley decoding engine profusely depends on global variables and hence cannot decode more than one MPEG at a time.

MPEGe Library

• The MPEGe(ncoding) Library is designed to allow you to create MPEG movies from your application

• The library can be downloaded from the files section ofhttp://groups.yahoo.com/group/mpegelib/

• The encoder library uses the Berkeley MPEG encoder engine, which handles all the complexities of MPEG streams

• As was the case with the decoder, this library can write only one MPEG movie at a time

• The library works good with most of the common image formats– To keep things simple, we will stick to PPM

MPEGe Library Functions

• The library consists of 3 simple functions – MPEGe_open for initializing the encoder. – MPEGe_image called each time you want to add a frame

to the sequence. The format of the image pointed to by image is that used by the SDSC Image library

• SDSC is a powerful library which will allow you to read/write 32 different image types and also contains functions to manipulate them. The source code as well as pre-compiled binaries can be downloaded at ftp://ftp.sdsc.edu/pub/sdsc/graphics/

– MPEGe_close called to end the MPEG sequence. This function will reset the library to a sane state and create the MPEG end sequences and close the output file

Note: All functions return non NULL (i.e. TRUE) on success and Zero (or FALSE) on failure.

Usage Details

• You are not required to write code using the libraries to decode and encode MPEG streams

• Copy the binary executables from– http://www.csee.usf.edu/~vmanohar/DIP/readframes– http://www.csee.usf.edu/~vmanohar/DIP/encodeframes

• Usage– To read frames from an MPEG movie (say test.mpg) and store them in a directory

extractframes (relative to your current working directory) with the prefix testframe (to the filename)

• readframes test.mpg extractframes/testframe This will decode all the frames of test.mpg into the directory extractframes with the

filenames testframe0.ppm, testframe1.ppm …– To encode,

• encodeframes 0 60 extractframes/testframe testresult.mpg This will encode images testframe0.ppm to testframe60.ppm from the directory

extractframes into testresult.mpg• In order to convert between PPM and PGM formats, copy the script from

– http://www.csee.usf.edu/~vmanohar/DIP/batchconvert• Usage

– To convert all the PPM files in the directory extractframes to PGM• batchconvert extractframes ppm pgm

– To convert all the PGM files in the directory extractframes to PPM• batchconvert extractframes pgm ppm

MPEG-1 Summary

• 3-type frame structures– I-frame ~ intra-coded (without prediction)

• Like JPEG– P-frame ~ predictive-coded (with previous frame as reference)– B-frame ~ bidirectional predictive coded

• Group-of-Pictures (GOP)– Frame order

• I1BBP1BBP2BBI2

– Coding order• I1P1BBP2BBI2BB

• User selectable parameters– Distance between I-frames in a GOP– Distance between P-frames in a GOP– Bit-rate (Constant/Variable, Bandwidth)

digital video processing

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