Tutorial: Shooting Top-Quality Streaming Video—How to Minimize Motion
The Motion/Data Rate Connection
The first point is illustrated in Figure 3. In my studies, the RealVideo codec did the best job of all handling motion. Still, if you throw enough motion at the codec at a low enough data rate, the video gets ugly.
Figure 3 (below): RealVideo codec compressing the same sequence at 56Kbps, 100Kbps, and 300Kbps (from left to right).
Conversely, throw enough data rate at the problem and motion-related degradation minimizes and then largely disappears. At higher data rates, this means that you really don’t have to do anything special to produce high quality video.
How high do you have to go to avoid visible artifacts? Well, for their 1080i Miami Vice trailer, Universal Studios encoded at 9.89Mbps, which is higher than a Hollywood DVD (see Figure 4). At that data rate, though there is some graininess during dark sequence, and banding around lights, most motion is very well preserved and artifacts are limited.
Figure 4 (below): Universal Studios encoded their Miami Vice QuickTime trailer at 9.89Mbps, which is higher than a Hollywood DVD.
At 56Kps, even at a resolution of 160x120, you need to carefully manage the motion in the video, irrespective of the video resolution. Trying to produce at higher resolutions would produce very ugly video.
At 100Kps, video quality at 160x120 should be very good, even with lots of motion, though the video will be very small, even if doubled during playback. If you limit the motion in your video, and plan your background carefully, you can render at 320x240 resolution and produce very good quality video, as shown in Figure 5 on the right. Too much motion results in the blurriness and artifacts seen on the left.
Figure 5 (below): Too much background motion degrades the quality on the left, while limited motion and plain background looks good on the right. Both videos at 320x240@100Kbps.
At 1Mbps, assuming a 640x480 video resolution and 30fps, you should be able to produce very good quality irrespective of the amount of motion in the video. Ditto for 320x240 video at 30fps and 500Kbps.
At 300Kbps, even relatively high motion video should look pretty good at 320x240, while lower resolutions show the caution orange because you’re being unnecessarily conservative. At 500Kbps, 320x240 video looks absolutely pristine, but you’re still pushing the envelop a bit for 640x480 video, which really starts to look quite good at 750Kbps and above.
The Codec/Quality Connection
Of course, to a degree, this depends upon the codec used to encode the video, which is illustrated in Figure 6, a very high motion sequence of two combined shots of a marimba player, encoded to 320x240@100Kbps to Windows Media, Real, Flash, and Apple’s H.264 codec.Figure 6 (below): Codecs handle motion differently, which you have to factor into your planning.
After studying these codecs, patterns emerge. Windows Media gets very blocky, while H.264 tends to "blow up" during challenging sequences at these data rates (though it looks wonderful at much higher rates). The VP6 preserves a bit more detail than Real, but consequently has more visible artifacts.