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April 10, 2008

Mtg: H.264 Video Encoding Quality Optimizations

by @ 12:18 pm. Filed under ALL, Communications, Engineering Mgmt, Optics/Displays

TUESDAY April 22
SCV Consumer Electronics Chapter
Speaker: Guy Cote, Mobilygen
Time: Refreshments at 6:30 PM; Presentation at 7:00 PM
Cost: $5 for IEEE members, and $10 for non-members
Place: HP Oak Room, Bldg 48 (Hwy 280 at Wolfe Rd), Cupertino
RSVP: not required

State of the art video codecs such as MPEG-4 AVC / ITU-T H.264 have reached a very high level of sophistication, and when designing a video encoder, there are many encoder parameters to control. Some of the encoder parameters will be fixed, some will adapt during the encoding, and others may be left to the user to control externally. Encoding parameters could be configured intuitively or empirically based on limited experiments. Setting parameter values in this way, though simple, cannot guarantee optimal video encoding quality. In addition, the parameters are usually configured individually and sequentially, so even though the parameters are optimal individually, the overall setting of multiple parameters may not be optimal. A systematic framework of configuring encoder parameters is thus indispensable for optimal video encoding.

In this work, we introduce a systematic approach to optimize video encoding parameters for best video quality. The determination of optimal video encoding parameters is formulated as an optimization problem of maximizing the expected video encoding quality under a set of constraints that may include a video quality measure, a target bitrate, computation, memory bandwidth, etc. We use the Video Quality Metric (VQM), a measurement paradigm of video quality that is based on algorithms for objective measurement of video quality, to measure the expected video encoding performance. The optimization problem can be solved through an efficient multidimensional numerical search method, direct simplex search method, with encoding of various sequences with different encoding parameter settings. We illustrate the approach to determine parameters to enable optimal MB level quantization parameter adaptation, as well as the optimization of quantization scaling matrices, in an AVC / H.264 encoder.

Adapting quantization at the macroblock level and at the coefficient level is an important encoding tool for discrete cosine transform (DCT) based perceptual image / video encoding in that DCT coefficients can be quantized according to the sensitivity of the human visual system to the coefficientsâ?? corresponding spatial frequencies. A quadratic model is introduced to parameterize the quantization matrices. This model is then used to optimize quantization matrices for a specific bitrate or bitrate range by maximizing the expected encoding quality via a trial based multidimensional numerical search method. The model is simple yet it characterizes the slope and the convexity of the quantization matrices along the horizontal, the vertical and the diagonal directions. The advantage of the model for improving perceptual video encoding quality is demonstrated with simulations using H.264 / AVC video encoding.

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