1. Multimedia Processing Lab MSEE, University of Texas @ Arlington
H.264 to VC 1 Transcoding
Vidhya Vijayakumar
Multimedia Processing Lab
MSEE, University of Arlington
Guided by Dr. KR Rao

2. What is… H.264 VC 1 The new industry standard
Massive quality, Minimal files
Scalable from 3G to HD and Beyond
VC 1
Informal name of the SMPTE 421M video codec
Standard initially developed by Microsoft
Supported standard for HD DVDs, Blu-ray Discs, and Windows Media Video

3. What is…
Transcoding
Converting a previously compressed video signal into another one with different format
Change in bit rate, frame rate, frame size, or even compression standard
2 Ways
Decode fully and encode in target standard
Change the bit stream format from one standard to another without its undergoing the complete decoding and encoding process.
Limitations
Compression artifacts are cumulative

4. Why Trancode H.264 to VC-1?
The two high definition DVD formats HD-DVD and Blue ray have mandated MPEG-2, H.264 and VC-1 as video compression formats
As H.264 based and VC-1 based content and products become available, transcoding in both directions will become widely used capabilities.
From an end user point of view, any VC-1 decoder can now become twice as powerful as it was earlier.

5. Why VC1?
Requires less computational power and can be decoded at full 1080i/p resolution on today’s off-the-shelf PC
Advanced Profile delivers compression efficiencies far superior to MPEG-2
Delivers HD content at bit rates as low as 6-8 Mbps
Better visual quality against H.264 and MPEG-2 demonstrated in independent tests

6. More of VC1… DCT-based video codec design
Coding tools for interlaced video sequences as well as progressive encoding
8-bit, 4:2:0 format
Uses block based transform and motion compensation with quantization and entropy coding.

7. Decoder – Simple & Main profile

8. Decoder – Advanced Profile

9. Block Transforms (Integer DCT)
8x8 blocks can be encoded using
1_8x8 2_8x4
2_4x8 4_4x4
Frame / Macroblock/Block signaling
Block level for coarse and fine level specification
Frame level for overhead reduction
Only 8x8 used for I frames

10. 8x8 Integer DCT Matrices
WMV 9
H.264 HP

11. Key features of the Transforms
The norms of vectors of the ratio 288:289:299
The variation in the norm accounted for in the encoder itself
At the decoder, inverse transform (rows) -> rounding-> inverse transform (columns) ->rounding (to operate in the 16 bit range)

12. Quantization Same rule applied to all block sizes
Both types with (bit savings at low bit rates) and without dead zone available
Type used signaled at the frame level to the decoder
At the encoder side automatic switch from uniform quantization to dead zone quantization as Q – parameter increases
Other factors like noise and rate control can be used to control this switch

13. Loop Filtering
Done to remove blocky artifacts and thus quality of current frame for future prediction
Operates on pixels on the border of blocks
The process involves a discontinuity measurement
Checks are computationally expensive so done only for one set of pixel per boundary

14. Motion Estimation and Compensation
Max resolution of ¼ pixel (i.e. ¼, ½, ¾) allowed
16x 16 motion vectors used by default but 8x8 allowed
Bicubic filter with 4 taps/ Bilinear filters with 2 taps to generate subpixel precision.
4 combined modes
1.Mixed block size (16x16 and 8x8), ¼ p ,bicubic
2.16x16, ¼ p, bicubic
3.16x16, ½ p, bicubic
4.16x16, ½ p, bilinear
Bilinear filters for chroma components

15. Advanced entropy coding
Simple VLC codes
Multiple code tables for encoding each particular alphabet out
A possible set of code tables is chosen (based on frame level quantization parameter) and signaled in the bitstream
Additional information e.g. motion vectors resolution coded using bitplane coding

16. Interlaced coding
Supports field and frame coding

17. Advanced B frame coding
B frames:- employ bi-directional prediction
Fractional position definition with respect to the reference frames for better scaling of motion vectors
Intra coded B frames for scene changes
Allow inter field reference

18. Overlap smoothing
The deblocking filter smoothens out the block as well as true edges and it may be disabled in less complex profiles
A lapped transform (input spans to pixels from other blocks as well) is used at the edges
Used in spatial domain as pre and post processing
Used only at low bit rates where blocking artifacts are higher
Signaled at macroblock level so can be turned off in smooth regions

19. Low rate tools (<100 Kbps)
Code frames at multiple resolutions (both in X and Y direction)
A frame can be downscaled at the encoder and then upscaled at the decoder for LBR transmission
The downscaling factor needs to remain same from the start of 1 I frame to the start of next I frame.
The frame must be upscaled prior to display (upscaling out of scope of the standard).

20. Fading compensation
Large amount of bits required for scenes having effects like fade-to-black ,fade-from-black
Not possible to predict motion using normally used techniques.
Effective fading detection (original reference image- current video image > threshold = fading)
If detected then encoder computes fading parameters which specify a pixel-wise first order linear transform for the reference image.
Also signaled to the decoder

21. Profiles Simple Main Advanced Baseline intra frame compression Yes
Variable-sized transform
16-bit transform
Overlapped transform
4 motion vector per macroblock
¼ pixel luminance motion compensation
¼ pixel chrominance motion compensation No
Start codes
Extended motion vectors
Loop filter
Dynamic resolution change
Adaptive macroblock quantization
B frames
Intensity compensation
Range adjustment
Field and frame coding modes
GOP Layer
Display metadata

22. Comparison of H.264 and VC-1 Overview VC 1 H.264 Goals
Designed to offer very
high image quality with
excellent compression efficiency
Designed to meet a variety of industry needs with many profiles and
levels, allowing for varying compression, quality and CPU usage levels,
where the lowest level is for portable devices, designed with low CPU
usage in mind, while the high levels are designed with very high quality
and compression efficiency in mind
Example industry use
Supports 4:2:0
compression / color space
Supports studio archiving requirements with 4:4:4 color space;
separate black and white (BW) video mode
Licensing costs
Similar
Documentation
Not free. Reference
decoder, which is not
free by itself, comes with
external documentation.
FFMPeg project gives a
free decoder.
Free. Reference encoder and decoder free as well. Plus JVT, M4IF mail-
lists where one may receive answers on the AVC related questions.

23. Comparison of H.264 and VC-1 Features VC-1 H.264 Bitstream formats
single bit stream
NAL and byte stream
Bitstream format
In advanced profile each Bit stream Data Unit has its
own header. Simple and Main profile do not provide
neither sequence nor entry point headers.
SPS (sequence parameter set),
PPS (picture parameters set),
slice header, macroblock
Deblocking filter
In-loop and out-of-loop algorithms, overlap transform
In-loop only
CABAC No
Only supported in Main and higher profiles
Variable transform
Size
Yes
Only in High profile and above
VLC
Slice
Contiguous (integer number of macro block rows only)
Contiguous/non contiguous
B frame used for
predicting other pictures
Sub pixel
Interpolation
methods
bicubic, bilinear
6-tap filter for half pixel, averaging for quarter pixels

24. Comparison of H.264 and VC-1 Feature VC-1 H.264 Partition sizes
16x16, 16x8, 8x16, and 8x8
16x16, 16x8, 8x16, 8x8, 8x4, 4x8, and 4x4
Integer transform
8x8, 4x8, 8x4, and 4x4
4x4; 8x8 available in High Profile only
Frame
Used for interlace content. Consists of bottom and top field
Used for progressive or interlaced content
Macroblock sizes
16x16 only
Motion vector
Two dimensional vector offset from current position to reference frame
Picture
A field or frame
Skipped Mb
No data is encoded for macroblock

25. 16-bit integer transforms
Graphically…
8x8, 4x8, 8x4, 4x4 adaptive block size
Frequency-independent dequantization scaling
VLC-based entropy coding
4 tap bicubic filters for MC
Relatively-simple loop filter
Overlap intra filtering
Range reduction/expansion
Resolution red./exp.
8x8 and 4x4 adaptive block size
Frequency-dependent dequantization matrix
CABAC or VLC
Long filters for MC
Complex loop filter
Spatial intra prediction
Multi-picture arbitrary-order referencing
Intra PCM
VC-1
H.264
Block motion
16-bit integer transforms
Bit-exact spec
Fading prediction
Loop filter

26. Transcoding point of view
Adaptive In High profile

27. Stepping forward…
Algorithm to deduce the picture type in VC-1 from H.264 picture types
Algorithm to effectively handle transform size mismatch between H.264 and VC-1
Algorithm to choose the best reference picture of H.264 to be used for MC in VC-1

28. References
An efficient algorithm for VC-1 to H.264 video transcoding in progressive compression - Jae-Beom Lee and Hari Kalva
Windows Media Video 9: overview and applications
Sridhar Srinivasan, Pohsiang (John) Hsu, TomHolcom b, Kunal Mukerjee, Shankar L. Regunathan, Bruce Lin, Jie Liang, Ming-Chieh Lee, Jordi Ribas-Corbera
Windows Digital Media Division, Microsoft Corporation, Redmond, WA 98052, USA, available online at

29. Vidhya Vijayakumar vidhya.vijayakumar@mavs.uta.edu
Thank You
Vidhya Vijayakumar