Calculating B-frame ratios

Dear Sirs, Is there any tool or formula to calculate b-vop settings in XviD to produce a file with a certain percentage of b-frames used? For example, encoding a movie (90min) to acheive 30% b-frames. The closest I have come is using b-vop settings of 1/1.50/1.00 with b-frame sensitivity of -23. Warm gratitude and best wishes.

Why exactly do you care what fraction of the frames are B? Meeting some arbitrary quota isn't going to improve quality or speed.

Don't touch bvop sensitivity unless you are certain about _you know_ what you're doing there. During 2nd pass there is no mode-decision anymore, the mode-decision (thus the I/P/B-frame structure) is fixed in the statsfile. If you're not satisfied with the amount of bframes, do a proper 2 pass encode again with changed values for max. bframes (and, only in case you know what you're doing, b-vop sensitivity). Doing a first pass based upon constant bitrate makes absolutely no sense in the terms of constant quality. That's the only valid argument and the reason why that's not going to be implemented into xvid. Regards Koepi

well said kopei!! :)

Hmm, would this mean the answer is no one knows for certain?

No, it means it is too complicated and partially unpredictable. How many b-frames and which sensitivity would be 'ideal' would vary from source to source. There is no indication that trying to reach a certain percentage of b-frames for your encode will help quality in any way BTW: More b-frames and higher sensitivity can quickly and quite unexpectedly lead to an increase in filesize while at the same time to a decrease in quality. The defaults are good, you probably won't gain anything by deviating from them. The only thing you're likely to get out of changing the defaults is a lot of work in comparing different settings before returning to the defaults. ;)

Dear sirs, I have more time today to respond. :D @akupenguin It is precisely because I care about the quality of the encode that prompts me to adjust b-frame controls. Speed is of no consequence of course, once quality is attained. @Blue_MiSfit I totally trust XviD to respond and encode the video at the settings I provide it. :) I'm positive XviD will hit whatever mark I ask of it, and well. @movmasty Thanks for the sharing of your knowledge, I can only confirm the same findings myself. @Koepi You are absolutely correct. Given what the laymen or XviD enthusiast is provided with by FAQs and other infomartion shared on this nice forum, that is how I set the encoding options for encoding, adjusted by personal experience and visual preferences. :p I've strictly followed the 2-pass procedure you documented as it seems only logical. @Teegedeck Sadly, I do wish there was an easier way or a tool invented just for this purpose. It isn't necessary of course, the old trial and error method does work, afterall. I simply submitted the query in hope of a suitable response. The defaults are very nice indeed, however, I believe none of us really use the default settings do we? ;) In closing, I sincerely thank all you gents for sharing your philosophy on XviD usage. It is gratifying to be able to share with developers and enthusiasts alike. Best wishes.

Dear unmei, Are we speaking of b-frame defaults, or XviD codec defaults? Visually I've always enjoyed none, or 1 b-frame much more than 2 b-frames. As for the rest of the default options, I've never used pack bitstream in my life. If you enjoy the actual defaults of XviD, I congradulate you. I was enjoying a mood of humour with Teegedeck about the XviD defaults, since he so highly values AQ, Q-pel, GMC and custom matrices, none of which are defaults for XviD. I suppose every one likes to interpret language as they wish. :rolleyes: Best wishes.

No i was strictly referring to the Bframe related settings only, because i thought this thread was about them. If you were talking about all the settings in general i'm sorry for the misunderstanding :D

I was referring to the b-frame defaults. Any yes, I use them.