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About this item. Condition Brand New. Quantity 3 available. Type Multimedia Tools. Language English. Platform Windows. Format CD. Brand Provided by Sharper Jacks. See all. A couple of the "personal preference" options may even cause changes that look better to some people, but look worse to others. Before continuing, you need to understand that this guide uses only one quality metric: global PSNR. Any time you read a claim about PSNR, one of the assumptions behind the claim is that equal bitrates are used. Nearly all of this guide's comments assume you are using two pass.
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When comparing options, there are two major reasons for using two pass encoding. Secondly, testing options by doing direct quality comparisons with one pass encodes introduces a major confounding factor: bitrate often varies significantly with each encode. It is not always easy to tell whether quality changes are due mainly to changed options, or if they mostly reflect essentially random differences in the achieved bitrate.
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If you are interested in tweaking either speed or quality, these are the first options you should consider. On the speed dimension, the frameref and subq options interact with each other fairly strongly. This is usually enough to be visible. Note that it is still recommended to always set bframes to something other than zero see below. Since the tradeoff encoding time vs. Merely raising frameref to 2 gains around 0. Unfortunately, diminishing returns set in rapidly.
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At such high frameref values, the only really good thing that can be said is that increasing it even further will almost certainly never harm PSNR, but the additional quality benefits are barely even measurable, let alone perceptible. Raising frameref to unnecessarily high values can and usually does hurt coding efficiency if you turn CABAC off. With CABAC on the default behavior , the possibility of setting frameref "too high" currently seems too remote to even worry about, and in the future, optimizations may remove the possibility altogether.
If you care about speed, a reasonable compromise is to use low subq and frameref values on the first pass, and then raise them on the second pass. Typically, this has a negligible negative effect on the final quality: You will probably lose well under 0. However, different values of frameref can occasionally affect frametype decision.
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Most likely, these are rare outlying cases, but if you want to be pretty sure, consider whether your video has either fullscreen repetitive flashing patterns or very large temporary occlusions which might force an I-frame. Adjust the first-pass frameref so it is large enough to contain the duration of the flashing cycle or occlusion.
For example, if the scene flashes back and forth between two images over a duration of three frames, set the first pass frameref to 3 or higher. This issue is probably extremely rare in live action video material, but it does sometimes come up in video game captures. Altering this option provides a straightforward quality-vs-speed tradeoff. At high values of frameref e. This option is rather useless in source containing only low motion, however in some high-motion source, particularly source with lots of small moving objects, gains of about 0.
It is interesting to note that using B-frames usually speeds up the second pass somewhat, and may also speed up a single pass encode if adaptive B-frame decision is turned off. With adaptive B-frame decision on the default behavior , it is safe to use higher values; the encoder will reduce the use of B-frames in scenes where they would hurt compression. The encoder rarely chooses to use more than 3 or 4 B-frames; setting this option any higher will have little effect.
With this option enabled, the encoder will use a reasonably fast decision process to reduce the number of B-frames used in scenes that might not benefit from them as much. The speed penalty of adaptive B-frames is currently rather modest, but so is the potential quality gain. It usually does not hurt, however.
Note that this only affects speed and frametype decision on the first pass. Note that these videos cannot be read by libavcodec-based decoders older than about March 5, However, in crossfades or fade-to-black scenes, weighted prediction gives rather large bitrate savings. In MPEG-4 ASP, a fade-to-black is usually best coded as a series of expensive I-frames; using weighted prediction in B-frames makes it possible to turn at least some of these into much smaller B-frames.
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Encoding time cost is minimal, as no extra decisions need to be made. Also, contrary to what some people seem to guess, the decoder CPU requirements are not much affected by weighted prediction, all else being equal. Unfortunately, the current adaptive B-frame decision algorithm has a strong tendency to avoid B-frames during fades. Two pass encoding : Above, it was suggested to always use two pass encoding, but there are still reasons for not using it. For instance, if you are capturing live TV and encoding in realtime, you are forced to use single-pass. Also, one pass is obviously faster than two passes; if you use the exact same set of options on both passes, two pass encoding is almost twice as slow.
Still, there are very good reasons for using two pass encoding. For one thing, single pass ratecontrol is not psychic, and it often makes unreasonable choices because it cannot see the big picture. For example, suppose you have a two minute long video consisting of two distinct halves. The first half is a very high-motion scene lasting 60 seconds which, in isolation, requires about kbps in order to look decent.
Immediately following it is a much less demanding second scene that looks good at kbps. Suppose you ask for kbps on the theory that this is enough to accomodate both scenes. Single pass ratecontrol will make a couple of "mistakes" in such a case. First of all, it will target kbps in both segments. The first segment may end up heavily overquantized, causing it to look unacceptably and unreasonably blocky.
The second segment will be heavily underquantized; it may look perfect, but the bitrate cost of that perfection will be completely unreasonable. What is even harder to avoid is the problem at the transition between the two scenes. The first seconds of the low motion half will be hugely over-quantized, because the ratecontrol is still expecting the kind of bitrate requirements it met in the first half of the video.
This "error period" of heavily over-quantized low motion will look jarringly bad, and will actually use less than the kbps it would have taken to make it look decent. There are ways to mitigate the pitfalls of single-pass encoding, but they may tend to increase bitrate misprediction. Multipass ratecontrol can offer huge advantages over a single pass. Using the statistics gathered from the first pass encode, the encoder can estimate, with reasonable accuracy, the "cost" in bits of encoding any given frame, at any given quantizer.
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This allows for a much more rational, better planned allocation of bits between the expensive high-motion and cheap low-motion scenes. See qcomp below for some ideas on how to tweak this allocation to your liking.