diff --git a/DOCS/xml/en/encoding-guide.xml b/DOCS/xml/en/encoding-guide.xml index 1aa21d8182..9a01a59730 100644 --- a/DOCS/xml/en/encoding-guide.xml +++ b/DOCS/xml/en/encoding-guide.xml @@ -292,7 +292,8 @@ Note: Most codecs which support ABR encode only support two pass encode - while some others such as x264 + while some others such as x264, + XviD and libavcodec support multipass, which slightly improves quality at each pass, yet this improvement is no longer measurable nor noticeable after the @@ -303,28 +304,31 @@ - In each of these modes, libavcodec + In each of these modes, the video codec (such as + libavcodec) breaks the video frame into 16x16 pixel macroblocks and then applies a quantizer to each macroblock. The lower the quantizer, the better the - quality and higher the bitrate. The method - libavcodec uses to determine + quality and higher the bitrate. + The method the movie encoder uses to determine which quantizer to use for a given macroblock varies and is highly tunable. (This is an extreme over-simplification of the actual process, but the basic concept is useful to understand.) - When you specify a constant bitrate, libavcodec will encode the video, discarding + When you specify a constant bitrate, the video codec will encode the video, + discarding detail as much as necessary and as little as possible in order to remain lower than the given bitrate. If you truly do not care about file size, you could as well use CBR and specify a bitrate of infinity. (In practice, this means a value high enough so that it poses no limit, like 10000Kbit.) With no real restriction on bitrate, the result is that - libavcodec will use the lowest + the codec will use the lowest possible quantizer for each macroblock (as specified by - , which is 2 by default). As soon as you specify a - low enough bitrate that libavcodec + for + libavcodec, which is 2 by default). + As soon as you specify a + low enough bitrate that the codec is forced to use a higher quantizer, then you are almost certainly ruining the quality of your video. In order to avoid that, you should probably downscale your video, according @@ -333,12 +337,13 @@ - With constant quantizer, libavcodec uses the same quantizer, as - specified by the option, on every macroblock. If - you want the highest quality rip possible, again ignoring bitrate, you can - use . This will yield the same bitrate and PSNR - (peak signal-to-noise ratio) as CBR with + With constant quantizer, the codec uses the same quantizer, as + specified by the option (for + libavcodec), on every macroblock. + If you want the highest quality rip possible, again ignoring bitrate, + you can use . + This will yield the same bitrate and PSNR (peak signal-to-noise ratio) + as CBR with =infinity and the default of 2. @@ -960,7 +965,7 @@ If you will not be encoding in constant quantizer mode, you need to select a bitrate. The concept of bitrate is quite simple. - It's the (average) number of bits that will be consumed to store your + It is the (average) number of bits that will be consumed to store your movie, per second. Normally bitrate is measured in kilobits (1000 bits) per second. The size of your movie on disk is the bitrate times the length of the @@ -980,10 +985,10 @@ Perceptual: You notice MPEG - artifacts more if they're scaled up bigger! + artifacts more if they are scaled up bigger! Artifacts appear on the scale of blocks (8x8). Your eye will not see errors in 4800 small blocks as easily as it - sees errors in 1200 large blocks (assuming you'll be scaling both + sees errors in 1200 large blocks (assuming you will be scaling both to fullscreen). @@ -1009,7 +1014,7 @@ However this has not been verified with theoretical or empirical rigor. Further, given that movies vary greatly with regard to noise, detail, - degree of motion, etc., it's futile to make general recommendations + degree of motion, etc., it is futile to make general recommendations for bits per length-of-diagonal (the analog of bits per pixel, using the square root).