January 8, 2014 Sam Siewert

Digital Media and Interactive Systems

Deeper Dive into MPEG Digital Video Encoding

MPEG Encode/Decode

Tools

Sam Siewert

2

FFMPEG FAQ

Read It!! http://ffmpeg.org/faq.html You should know how to Decode Video (recorded from your camera or pre-recorded by someone else) You should know how to Encode Video (to turn in with your labs) On Ubuntu – do “apt-get install ffmpeg” to get it!

Sam Siewert 3

Ffmpeg (avconv) Notes sudo apt-get install ffmpeg ffmpeg -i movie.mpg –ss 30 –t 30 movie%d.ppm –- 30 seconds @ 30 sec

ssiewert@ssiewert-VirtualBox:~/a485/media$ ffmpeg -i big_buck_bunny_480p_surround-fix.avi -ss 30 -t 30 bbb%d.ppm ffmpeg version 0.8.6-4:0.8.6-0ubuntu0.12.04.1, Copyright (c) 2000-2013 the Libav developers built on Apr 2 2013 17:02:36 with gcc 4.6.3 Input #0, avi, from 'big_buck_bunny_480p_surround-fix.avi': Duration: 00:09:56.45, start: 0.000000, bitrate: 2957 kb/s Stream #0.0: Video: mpeg4 (Simple Profile), yuv420p, 854x480 [PAR 1:1 DAR 427:240], 24 tbr, 24 tbn, 24 tbc Stream #0.1: Audio: ac3, 48000 Hz, 5.1, s16, 448 kb/s Incompatible pixel format 'yuv420p' for codec 'ppm', auto-selecting format 'rgb24' [buffer @ 0x907700] w:854 h:480 pixfmt:yuv420p [avsink @ 0x9054c0] auto-inserting filter 'auto-inserted scaler 0' between the filter 'src' and the filter 'out' [scale @ 0x905b60] w:854 h:480 fmt:yuv420p -> w:854 h:480 fmt:rgb24 flags:0x4 Output #0, image2, to 'bbb%d.ppm': Metadata: encoder : Lavf53.21.1 Stream #0.0: Video: ppm, rgb24, 854x480 [PAR 1:1 DAR 427:240], q=2-31, 200 kb/s, 90k tbn, 24 tbc Stream mapping: Stream #0.0 -> #0.0 Press ctrl-c to stop encoding ... Last message repeated 719 times -0kB time=29.00 bitrate= -0.0kbits/s frame= 720 fps= 38 q=0.0 Lsize= -0kB time=30.00 bitrate= -0.0kbits/s video:864686kB audio:0kB global headers:0kB muxing overhead -100.000002% ssiewert@ssiewert-VirtualBox:~/a485/media$

Sam Siewert 4

Now with PPM Frames PPM is Simple, but No Compression – Good for CV – http://en.wikipedia.org/wiki/Netpbm_format - Read this! – JPEG, PNG are Compressed – TIFF is an Alternative, but More Complex

Sam Siewert 5

Simple Re-encode When Quality is not a Concern, Keep it Simple

ffmpeg -f image2 -i bbb%d.ppm bbbtrans.mpg vlc bbbtrans.mpg

Sam Siewert 6

Quality Encoding is Tricky Use MPEG4 HQ Settings, Encode 480p, AR=4:3 ffmpeg -f image2 -i bbb%d.ppm -maxrate 20000k -bufsize 32M -s 640x480 -vcodec mpeg4 -qscale 1 bbbtranshq.mp4

Sam Siewert 7

MPEG Encode/Decode

Theory and Algorithms

Sam Siewert

8

Overview MPEG-2 Standards – 13818-1: Transport Streams for Video & Audio

Container for Program Streams (188 Byte Packets) Multiplexed Video and Audio Elementary Streams PSI – Program Specific Information System Clock (PCT, PTS/DTS)

– 13818-2: Elementary Video Stream Encode/Decode

Color Format Macro-blocks, Video DCT GoP (I-Frame, B-Frame, P-Frame) Motion Compensation and Vector Quantization

Differences Between MPEG-2 and MPEG-4

Sam Siewert 9

MPEG-2: Order Of Operators

Sam Siewert 10

#1: POINT (Pixel) Encoding #2 A-C: Macro-Block Lossy Intra-Frame Compression #3: Motion-Based Compression in Group of Pictures

#1

#2A

#2B #2C #3

Sam Siewert 11

Step #1 – RGB to YCrCb 4:4:4 24-bit (Lossless)

For every Y sample in a scan-line, there is also one CrCb sample – Each Y (Y7:Y0), Cr (Cr7:Cr0), & Cb (Cb7:Cb0) Sample is 8 bits – No compression between RGB and YCrCb 4:4:4 (both 24 bits/pixel)

Typically a Post Production, CEDIA or DCI format

… 0 319

… 76,480 76,799

…

= Y, Cr, and Cb sample = Y sample only

48 bit to 32 bit

Sam Siewert 12

Step #1 – RGB to YCrCb 4:2:2 (Lossy) For every 2 Y samples in a scan-line, one CrCb sample – Each Y (Y7:Y0), Cr (Cr7:Cr0), & Cb (Cb7:Cb0) Sample is 8 bits – Two RGB Pixels = 48 bits, Whereas Two YCrCb is 32 bits, or 16

bits per pixel vs. 24 bits per pixel (33% smaller frame size)

… 0 319

… 76,480 76,799

…

= Y, Cr, and Cb sample = Y sample only

Sam Siewert 13

Step #1 – RGB to YCrCb 4:2:0 (Lossy) For every 4 Y samples in a scan-line, one CrCb sample – Each Y (Y7:Y0), Cr (Cr7:Cr0), & Cb (Cb7:Cb0) Sample is 8 bits – Two RGB Pixes = 48 bits, Whereas Four YCrCb is 48 bits, or 12

bits per pixel on average vs. 24 bits per pixel (50% smaller)

… 0 319

… 76,480 76,799

…

= Cr, Cb sample = Y sample only

Step #2 – Convert to 8x8 Macroblocks and Transform

Aspect Ratios Designed to Fit 8x8 Macroblock E.g. 640 x 480 => 80 x 60 Macroblocks Discrete Cosine Transform Applied to Each 8x8 – Spatial Intensity to Frequency Transform – Applied on X Axis (Row) – Applied on Y Axis (Column)

Set up for Intra-frame (I-frame) Compression

Sam Siewert 14

Convolution Concepts Math operation on 2 functions, that produces a 3rd Point Spread Function “Sharpen” meets this Definition So do Many Mask Operations applied to Pixel Neighborhoods

Sam Siewert 15

2 impulses, f(t), g(X – t)

Area inside intersection

f convolved with g over t

DCT – Discrete Cosine Transform Convolution of Image with Discrete Cosine See http://www.cse.uaa.alaska.edu/~ssiewert/a490dmis_code/example-dct1/ De-convolved to restore image from Convolved Image

Sam Siewert 16

DCT

Inverse DCT

DCT Concepts F(x) is a sum of sinusoids (with frequency, amplitude) DCT operates of a discrete number of samples Can derive DC sum at any x, even where F(x) not known N x N Macro-block has Zero Frequency DC at 0,0 Increasing Horizontal Frequency Increasing Vertical Frequency Can De-convolve (inverse DCT, or iDCT) Can Eliminate High Frequency Horizontal and Vertical Terms – Minimal Losses from Truncation (otherwise lossless) – Loss of High Frequency Image Features (What are These?)

Sam Siewert 17

Basic Concept of Waveforms Complex Waveform is Sum of Simple Fundamentals Simple Fundamentals Can Be Derived from Complex

Sam Siewert 18

Scanline DCT Example Small Losses Due to DCT, iDCT Numerical Truncation Larger Losses Due to H.O.T. Quantization and Truncation http://www.cse.uaa.alaska.edu/~ssiewert/a490dmis_doc/1D-DCT-N-Fundamentals.xlsx

Sam Siewert 19

What Is Lost with DCT Quantization? Noise More Than Anything Else Complex XY Variable Patterns (Real Science Data?)

Sam Siewert 20

Complex Tiling Higher Frequency X Higher Frequency Y Terms Can Still be Ignored

Complex Wood Texture Most Detail in X Far Less in Y

Randomized Texture Image High X Detail High Y Detail Most Loss of Detail, But Noisy

Step #2A: Macro-block Discrete Cosine Transform

8x8 Pixel Block – Macro-block – SD NTSC 720x480 (90x60 Macro-blocks), 3:2 Aspect Ratio – HD 720 1280x720 (160x90 Macro-blocks), 16:9 AR – HD 1080 1920x1080 (240x135 Macro-blocks), 16:9 AR

Sam Siewert 21

Step #2B: Macro-block Quantization (Lossy)

Apply Weighting and Scaling 8x8 to DCT Produces Lots of Repeated Values (and Zeros) Compared to Original

Sam Siewert 22

Decode Process for #2A-B

Sam Siewert 23

How Lossy is the Decode Macro-Block?

Sam Siewert 24

OpenCV Macroblock DCT Example Same Cactus 320x240 with 80x80 DCT Macroblocks

Sam Siewert 25

DCT iDCT

Same Cactus 320x240 Again with 8x8 DCT Macroblocks

DCT iDCT

Mathematics for 2D DCT Frequency Variation on X and Y axes from top left to bottom right Straight-forward Algorithm Based on 2D Equation is O(n2) per dimension Like Cooley-Tukey for DFT, a DCT Algorithm that is O(n*log2(n)) has been formulated (Arai, Y.; Agui, T.; Nakajima, M. - Numerical Recipes: The Art of Scientific Computing (3rd ed.)) http://www.cse.uaa.alaska.edu/~ssiewert/a490dmis_code/dct2/dct2.c

Sam Siewert 26

http://en.wikipedia.org/wiki/File:Dctjpeg.png

Step #2C: Macro-block Run-Length and Huffman Encoding

Zig-Zag Run-Length Encoding to Exploit Repeated Data and Zeros found in H.O.T. of Quantized DCT

– 86, 1, 7, -5, -1, 0, 1, 0, 0, 2, -1, 1, 0, -1, 0 , 0, 0, 0, -1, 0, 0, …

Becomes:

Sam Siewert 27

Huffman Applied to RLE Data Huffman Tables for MPEG-2 Macro-Blocks Defined in 13818-2 (Lossless) Compression Based on Probability of Occurance Shannon’s Source Coding Theory: log2(P), P=probability of occurrence, Binary encoding of Symbols

Sam Siewert 28

Step #3: Group of Pictures Concept – Transmit Change-Only Data I-Frame Compressed Only Intra-Frame By Methods #2A-2C to Macro-Blocks I-Frame Can Be Decoded Alone P-Frame is Differences Only Over the GoP B-Frame is Differences Only Between Both I-Frame and Closest P-Frame Difference Data Can be Further Encoded with Lossless Methods Without Steps 2A-C, Specifically Quantization, and With High Motion Video, Could Blow-Up

Sam Siewert 29

Group of Pictures: High Level View

Sam Siewert 30

Overall MPEG YCrCb Compression Performance

Standard Definition 720x480x2 (675KB/frame) @ 30fps – Requires 20MB/sec (200 Mbps) Uncompressed – Typical MPEG-2 @ 3.75 Mbps, > 50x Compression – Typical MPEG-4 @ 1.5 Mbps, > 100x Compression – 10 to 20 Programs on QAM 256 (48Mbps, 6MhZ/Ch) – ≈10 MPEG-4 Programs on ATSC 8VSB (19.39 Mbps, 6MhZ/Ch)

HD 720p (1280x720x2,1800KB/frame) @ 30fps – Requires 53MB/sec (530Mbps) Uncompressed – Typical MPEG-2 @ 20 Mbps, > 25x Compression – Typical MPEG-4 @ 10 Mbps, > 50x Compression

HD 1080p (1920x1080x2, 4050KB/frame) @ 30fps – Requires 120MB/sec (1200Mbps) Uncompressed – Typical MPEG-2, VC-1 @ 45 Mbps, > 30x Compression – Typical MPEG-4 @ 20 Mbps, > 60x Compression

Sam Siewert 31

13818-2 Defines Elementary/Program Streams

13818-2: Elementary Video Stream Encode/Decode – Defines Color Sub-Sampling Formats – 8x8 Macro-Block Encoding – Video DCT – Post DCT Macro-Block Quantization Weighting and Scaling

Coefficients – RLE Zig-Zag Macro-Block Sampling – Huffman Encoding Table – Group of Pictures:I-Frame, B-Frame, P-Frame – Presentation and Decode Time Stamps (PTS/DTS) – Order of Encode and Decode Operations

Not Suitable for Transport over Networks, but Sufficient for Local Playback (DVD, PC HDD, Flash-Memory Media)

Sam Siewert 32

13818-1 13818-1: Transport Streams for Video & Audio – Container for Program Streams (188 Byte Packets) – Multiplexed Video and Audio Elementary Streams – PSI – Program Specific Information

PID – Program ID Guide Data Emergency Broadcast

– System Clock (PCR, PTS/DTS) – Sequence Headers – Resolution and Format Information, Bit-Rate

GoP Header, Frame Header Slices of Macro-Blocks for Resolution Decoder Information (Color, Quantization Tables)

– Can Be Multiple Programs or Combined Audio and Video as a Program

MPEG-2 Video Elementary Stream AC-3 Audio Elementary Stream Secondary Audio Stream (Different Language) Up to 10 or More Audio+Video in One Transport Stream for Virtual Channels

Sam Siewert 33

Parsing an Elementary Video Stream

Sam Siewert 34

Many 188-Byte Packet Types and Header Allows for Multi-plexing of many Video and Audio Streams on a Carrier

MPEG-4 vs. MPEG-2 MPEG-2 – Defined by ISO 13818-1, 13818-2 – Leverages MPEG-1 (Motion Picture Experts Group – 1988) – Widely Used for Digital Video – Digital Cable TV, DVD – Transport Stream designed for Broadcast (Lossy, No Beginning or End of

Stream) ATSC – Advanced Television Systems Committee (HDTV Broadcast)

– 8VSB Modulation – 8 level Vestigal Sideband Modulation, 6MhZ channel, 19.39 Mbps, Reed-Solomon Error Correction

– Up to 1080p (1920x1080) Video Resolution – AC-3 (Dolby) Audio

DVB – Digital Video Broadcast (Europe, Satellite) – Program Stream designed for Playback Media (DVD, Flash, HDD, etc.)

MPEG-4 – Defined by ISO 14496 (1998) – Leverages MPEG-2 Standards for Program/Transport, Encode/Decode – Better Compression Rates (improved motion prediction for P,B frames),

MPEG-4 Part-10 (H.264), e.g. Blu-Ray – Extensions for Digital Rights Management – Advanced Audio Encoding – Becoming More Widely Deployed for HD and Because of Lower Bit-Rate

Transport Streams

Sam Siewert 35