"compressibility of 60%" = "q=3" ?

[spyce]

hello,

as i have heard, it's recommended to encode with a compressibilty value of about 50-80%. if 100% relates to files, which are encoded with a quantizer of "2" and the output of a file encoded with quantizer "3" is about 60% of this size, can i say: i have to adjust/reduce the resolution and use the soften/denoise filters so far that i get my desired file size when i encode with quantizer "3"?

am i right?

[jggimi]

Hello, and welcome to the forum.

You are confusing two different "percentage" scales. That's understandable, because at 100%, they mean the same thing.

For GK the percentage is based on a compressibility test, and is comparing bits/pixels*frames values of the maximum possible quality (Q=1 or Q=2, can't recall which), with the value at your currently chosen resolution and bitrate. It is this value for which we have best practice ranges.

For codecs like DivX and XviD, these "percentages" convert directly to Quantizer values for single pass fixed quant encodings, and have nothing to do with multipass best practice.

[spyce]

i'll ask this way, does the output of a 2pass encode have a constant quantizer for the whole film or would it be exactly the same if had encoded it with a single pass at a certain quantizer? i thought the first pass just looks which quantizer to chose (for example "4.563") to reach a specified size. if it is the same i don't understand why these are different scales.

edit:

For codecs like DivX and XviD, these "percentages" convert directly to Quantizer values for single pass fixed quant encodings
so in this case i was right?

[jggimi]

Multipass encoding uses variable quantizers. The first pass is an analysis, the second (and any subsequent) passes are encoding based upon that analysis.

Think of it this way: single pass encodings use either fixed Quantizer (constant quality) or fixed bitrate (constant bitrate). Multipass encodings vary both. The bitrate selected by the operator is an average bitrate request.

I recommend reading through DivX Network's "DivX Users Guide" -- its helpful for all MPEG encoding, even if you don't use DivX at all.

It will help you understand how single pass and multipass encodings work, as well as how MPEG compression works, and how and why MPEG-4 ASP configuration settings effect the outcomes. Plus, its available in 4 languages. The link is fairly well hidden if you start at their home page, so click here to reach the approprate download link: http://support.divx.com/cgi-bin/divx.cfg/php/enduser/std_adp.php?p_faqid=444&p_created=1081444519&p_sid=4tSeQKVh&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9ODkmcF9wcm9kcz0wJnBfY2F0cz0mcF9wdj0mcF9jdj0mcF9zZWFyY2hfdHlwZT1hbnN3ZXJzLnNlYXJjaF9ubCZwX3BhZ2U9MSZwX3NlYXJjaF90ZXh0PXVzZXJzIGd1aWRl&p_li=&p_topview=1

[spyce]

ok, thank you.

maybe i'll come back to this topic later. ;)

[jggimi]

Well, this is about as "spoon-fed" as it gets around here. When someone is confused about a technical issue, they'll get an opinion, followed by a pointer to a document with background information. If they are unlucky, they might get many conflicting opinions, and no background information.

I hope you eventually decide to click on the link. Its an easy to read, easy to understand document, with great graphics and even better explanations, .... available in English, French, German, and Japanese.

[spyce]

now i read through it.

you said: "Think of it this way: single pass encodings use either fixed Quantizer (constant quality) or fixed bitrate (constant bitrate). Multipass encodings vary both."

why should multipass change the quantizer within the video? the only case it can happen, i guess, is if i change the "bitrate modulation".