The Other Side of Compression

Post-processing does not seem to apply to the small frame-size, 30Kbps clips, which is a shame because it is needed most at such low bit rates. We also tried out the latest release, 4.0.2, which has a new de-ringing filter built-in, but it did not seem to apply to these test cases. According to Jerome "Gej" Rota, a founder of the company, the filters work adaptively based on an edge detection algorithm. In small frames, edges in turn are smaller and not detected as well. Also, according to Gej, the filters are really designed to improve image quality at higher frame sizes encoded at challenging bit rates.

Overall, the quality of the DivX clips is good, especially at broadband rates. Both the 604Kbps and 252Kbps tests achieved full-motion, 29.97 fps frame rates. At 30Kbps, there are more blocking artifacts and some color shifts introduced, especially in the Fash clip. With these kinds of results, it would be beneficial to take a closer look at DivX if full-screen, full-motion video is in your business plan.

Tips and Tricks for Encoding with DivX 4.0.1:

1. Use the slowest performance/quality setting and two-pass VBR mode whenever possible.
2. The post-processing is set in the decoder, so defaults are only mentioned for reference. For highly detailed, high-motion content like our Anim clip, applying the deblocking at level 2 will help at challenging bit rates, like our 252Kbps test.
3. When encoding streams at frame rates that are lower than the source frame rate in VirtualDub, you need to compensate for the application's interpretation of bits/frame. If you decimate by 2, double the target bit rate; decimate by 3 and you need to triple the target bit rate, etc. Note that this is an imprecise science that will only get you in the ballpark for your target.

Chapter 4: On2 Technologies’ VP3

VP3, On2 Technologies

PROS	CONS
High-quality video codec available as open source.	No two-pass VBR mode.
Full-motion, 30 fps achieved at broadband rates (250Kbps).	Requires frames sizes that are multiples of 16.
Built-in preprocessor with good choices for default parameters.	Dependent on third parties for improvements and applications.
Good support on On2’s open source Web site.	Access to all parameters requires coding and recompiling the source.

On2 Technologies has evolved from the gaming and CD-ROM market where its Truemotion codecs were marketed under the company’s previous name, The Duck Corporation. The company is now addressing two segments of the streaming media marketplace with its VP3 and VP4 codecs. Chapter 5 will discuss the applications for broadcast, set-top boxes and video on demand services available through On2’s special licensing arrangements of VP4. On the flip side is the company's offering of VP3.2.1.0 as an open source codec. Downloads have been steady since its initial posting over the summer, but we have yet to see any major improvements or product offerings using this codec. According to On2, curiosity has been high at multimedia production houses and some communications and telco firms.

Access to all of the VP3 source code is available at www.vp3.com, and fully operational codecs are available for QT on both the Mac and Windows platforms, as well as the Video for Windows/DirectShow platform in Windows. Playback platforms are equally well represented, and ports to Linux for the decoder are in the works. Bit streams generated on the different platforms are identical, although the manner in which DirectShow and QuickTime handle certain functions may vary slightly, and so results may not look identical.

We tested VP3.2 using QT for Windows, which produced exceptional quality images at broadband rates, and reasonable quality images at low bit rates.

For this round of tests, we were able to use the source frame rate of 29.97 fps for all 604Kbps clips, and all but the Anim clip encoded at 252Kbps. Most other competitors prefer 15 fps at this bit rate. When encoding the high-motion graphics clip, Anim, we favored the non-dropped frame approach so that the motion didn’t become jerky, and so lowered the frame rate. VP3’s parameter choices are basic, but certainly adequate for achieving good quality. Auto Keyframe Placement was checked and the default parameters were used throughout. This also meant the "Keyframe every xxx frames" checkbox was left off in the QT Pro interface.

This new generation of codecs is improving the motion rendition even at low bit rates, and VP3 is right up there with the best of them. We recommend 10 fps for the test clips here, but could have also used 15 fps in the low-motion talking head clip. More image detail can be achieved at the lower frame rate. In the very motion-intensive Anim clip, 10 fps overshot the target of 30Kbps, but if you can afford to go a little higher in your progressive download content, you can stick with the higher frame rate of 15 fps, even with high motion.

With VP3 easily capable of 30 fps even at our 252Kbps target, we wonder what it can do at full screen. In addition, this codec has lots of room to grow, with improvements already being discussed for two-pass VBR and building an interface to have more access to its built-in preprocessor and data rate controls. Clearly, the open source community now has a very competitive codec to work with for multi-platform support and delivery of broadband content.

Tips and Tricks for Encoding with VP3.2.1:

1. Keep the frame rate high and let the codec do some frame dropping. It will only drop the frame rate by half, meaning the streams won’t play back with uneven frame rates that bounce around from those chosen for the encode.
2. Let the codec do all keyframe control. Unselect the "Key Frame Every" box, or enter a very large number so the application doesn’t put in key frames where they are not needed.