Streaming Media - August/September 2008 - (Page 27)

Traditional deinterlace methods distorted the text on the cards too much, as illustrated in Figure 2. However, the softness increase from that filter wound up causing a slight visual discontinuity when it kicked in midshot. So, I broke out the two sequences with interlacing into layers, and then used a five-frame cross-dissolve from the original progressive frames to the first interlaced frame, hiding the slight loss of focus (Figure 3). Both interlaced sequences ended on a hard cut, so I was able to switch back to the original video without a transition. I then rendered the new version out from After Effects into the Lagarith codec in YV12 mode, which uses the native 8-bit 4:2:0 color space of VC-1 and other codecs. This means that Expression Encoder doesn’t need to do any color space conversion, making compression slightly faster. Markers and Manual Keyframes As most of you know, efficient codecs make use of both intra- and interframe compression. You can think of intraframe compression as being like a .jpg—it is a self-contained frame of video. These self-contained frames are I-frames, which are also known as keyframes. Interframe compression predicts the contents of a frame based on previous and subsequent I-frames. These are called intermediate frames. Together, this collection of frames is called a group of pictures (GOP). Keyframes are required for seeking or random access. If you jump to a point in a video that is a keyframe, the seek will be instant. If you jump to any other kind of frame (P for progressive, or B for bidirectional, described below), the codec will need to decode all the P-frames before the current frame, back to the previous keyframe. As a result, fewer keyframes means worse seek performance, but fewer keyframes can also reduce keyframe popping and can increase compression efficiency. Like everything else Figure 4. Adding keyframe markers provided a further boost in quality. in compression, we face a classic tradeoff. Note that modern codecs will insert “natural keyframes” at hard cuts and moments of significant motion. The other notable issue with the original clip was keyframe popping—when an obvious “jump” in the video happens at the keyframe rate of the video. Watch the original FLV in the online version of this article (www.streamingmedia.com/article.asp?id=10409), and you’ll see it during any of the longer static shots. Since the whole section with the cards is a single shot lasting more than 3 minutes without any hard cuts, there wasn’t a place for natural keyframes (automatically inserted at a hard cut) to go. Thus, keyframe transitions would happen while the cards were otherwise static, making even a slight change visible. And the VP6 codec is known for big jumps in image quality at keyframes. I also wanted to show off the Expression Encoder templates a bit by doing thumbnail navigation, which provides a graphical interface to set markers on particular frames and flag them to be keyframes or thumbnails. A thumbnail becomes an image file that, with the supported templates, automatically gets included in the menus for navigation (think of a chapter menu on a DVD). Normally you also want to make the chapter points keyframes, since keyframes support immediate random access to that frame, hence hitting a chapter button yields a nearly immediate start. This was an opportunity to kill two birds with one stone: If I set the markers on the first static frame of every card, it would be a nice, high-quality image that the later frames of that GOP will be based on, propagating quality forward. If I set my keyframe spacing long enough, there wouldn’t be any other keyframes in that interval to cause keyframe popping, and so the static card would be very consistent and efficiently encoded. So, I set a marker for each person and flagged each to be a thumbnail and a keyframe. The audio didn’t always sync up exactly, so that the person’s name began after his or her card was down or sometimes the first name was cut off. This was easy to fix by just delaying the audio a second. You can also use nonthumbnail keyframe markers; these become keyframes without showing up in navigation. I stuck a few of those in the intro and outro sections, and on the first full frames after the logo gets built. Since the sponsor pays the bills (RipCode in this case), I always want to make sure that the logos remain nice and crisp. The ability to set keyframes has been around in compression projects for ages now; I did a lot of this in the Cinepak era, as that codec was prone to keyframe popping issues. Modern codecs such as VC-1 do a much better job of finding good natural keyframes and WWW.STREAMINGMEDIA.COM 27 http://www.streamingmedia.com/article.asp?id=10409 http://WWW.STREAMINGMEDIA.COM

Table of Contents Feed for the Digital Edition of Streaming Media - August/September 2008

Streaming Media - August/September 2008 Table of Contents No Doubt - Editor's Note Mo’ Videos, Mo’ Money: Solving the Problem of Network Cost Why Good Design Matters More Than Ever: Just. Press. Play. Technology and Business Trends by the Numbers High-Touch Encoding With Microsoft Expression Encoder 2 The State of the Content Delivery Market, 2008 Get a Life: New Tools Put Lifecasting Within Anyone’s Reach So Many Codecs, So Little Time: Comparing H.264 Files Using Codecs From Apple, Dicas, and MainConcept Writing an RFP for Maximum ROI: Things to Consider and Questions to Ask When Writing a Request for Proposal for a Corporate Webcasting System The Patent Wars Producing Screencams in Camtasia Studio Analyze That Using Video to Communicate Change for Business Success Intellectuals’ Property Rights

Streaming Media - August/September 2008

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