教程:编码的屏幕截图
Editor's Note: This article appears in the October/November issue of 流媒体 杂志. 点击 在这里 免费订阅.
在八月/九月号 流媒体, I described how to script, record, and edit a screencam presentation for fun or profit. 在本文中, I’ll detail which codec does the best job compressing screencams for internet delivery, which encoding parameters work best and why, and which encoding tools do the best job producing the compressed screencam.
顺便说一下背景, the starting point for all encodings was a file I exported from my video editor in Apple Animation format. I detailed how I produced the file and why I used the Apple Animation codec in the previous article.
哪种编解码器是最好的?
Many times when producing screencams, the target codec is 既成事实. 这是, you have to encode into Silverlight or Flash format for website design or other similar reasons. 当然，今天，用H.264-capable Flash player penetration exceeding 80%, you can consider both VP6 and H.264时，为Flash制作. 除了, some casual producers won’t be locked into a codec decision beforehand and just want to use the best technology for the job. 我们来讨论一下哪个是最好的.
When I’m producing screencams, I define best using four parameters. 第一个, the technology must easily enable random access to various points in the video file to accommodate viewers who want to skip or repeat a section. All codecs meet the first test, and you can drag, stop, and restart any of the formats at will. Obviously, to a degree, this will only be possible if the player you create offers these controls. 但是在Windows Media Player中, FLV球员, 和QuickTime播放器, I could move to any point within the file and start playback with no lag or distortion.
Second, I consider the convenience of the player itself. 例如, I find the QuickTime Player most usable since you can open multiple windows and use the arrow keys to drive frame-by-frame progress in either direction, a function that is critical for file analysis, 如果没有教程浏览. I won’t discuss the FLV player in detail since most Flash-based tutorials will use a custom Flash player, but it does support multiple open instances.
不幸的是, though previous versions of the Windows Media Player did let you open multiple instances at once, you can’t do so with current versions, meaning that your viewers can’t open more than one tutorial at a time. Media Player also doesn’t enable frame-by-frame access via the arrow keys. 再一次。, this is more of a problem for file analysis than for tutorial viewing, 但以防雷德蒙德有人,洗., is reading, arrow key support would be awesome.
How many folks have the player is always an issue, and if you’re targeting Mac and Windows customers, Flash在这方面有很大的优势. I personally think that Silverlight is a better option than Media Player for cross-platform, browser-based applications because at least it’s Microsoft’s own playback module. Though the Flip4Mac Windows Media plug-in has worked well for me on my Macs, no one knows its overall penetration.
最后, when considering a codec for a screencam video, you also need to consider comparative quality at the selected data rate. 检验质量, I encoded the same 1024x768 @ 15 fps test file to the same ridiculously low data rate: 200Kbps with 32Kbps audio. I’ll delve more into the specific encoding parameters that I used with each encoder later. I produced each format with a range of encoders, 稍后再讨论, and compared the best-quality file available for each codec.
我产生了H.264和FLV文件, but I borrowed heavily from Ben Waggoner’s excellent tutorial, "Encoding Screen Recordings for Silverlight in VC-1 With Expression Encoder 2,在生成VC-1文件时. You can read the tutorial at http://on10.net/blogs/benwagg/21750; I’ll describe how I diverged from his procedures in the VC-1 section of this article.
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两个小时.264 and VP6 were much clearer than VC-1. 有趣的是, all implementations of VC-1 began well but seemed to lose their way somew在这里 after the halfway point, starting with small pockets of distortion, 如图所示, that often worsened into highly noticeable color artifacts. VP6和H.264偶尔有不好的伸展, they typically lasted only a moment or two and, if they were in a portion of the screen away from the cursor motion, 常常不引人注意.
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