Compressibility Check and IVTC: Which Framerate?

[gio]

I'm dealing with a source where I have to perform IVTC.

When I open the DVD2AVI project in GKnot, it says of course 29.970 frames. But as we all know, the movie will have 23.976 frames at the end.

When interpreting the results from the Compressibility Test, I don't know which framerate gives the correct result...
It makes quite a difference (e.g. 78.7% vs. 62.8%)!

As I really didn't find the answer anywhere, I thought I'd ask :) Thanks for your help!

[jggimi]

Whenver I manually IVTC, I always adjust the framerate in GKnot to 23.976 after opening the .d2v.

I must admit, I've never even thought about your question. I always make sure that the compression check uses the same script (except for the SelectEvery command used for the test) that the encoding will use. To date, I've not had any filesize issues with those results.

This may not be the definitive answer, but it's the methodology I use.

[gio]

Originally posted by jggimi
To date, I've not had any filesize issues with those results.
Thank you very much for your answer :)

But actually the set resolution shouldn't change the filesize. It changes the bitrate used per pixel.
Thing is, I try to encode a movie, but get rather disappointing results, so I began asking myself the question if I'm using the right values...

[theReal]

I think when you don't change the frame rate to 23.976 then Gknot will complain that the number of frames from the compressibility test does not match with the given number of frames.

IMO you get the correct results when you change the frame rate to 23.976 manually before the compress test.

[gio]

Originally posted by theReal
I think when you don't change the frame rate to 23.976 then Gknot will complain that the number of frames from the compressibility test does not match with the given number of frames.
No, it doesn't complain. It opens the DVD2AVI project and displays the correct number of 29.970 frames.
How should GKnot 'know', that you're going to perform IVTC later?
Actually it doesn't care at all about the chosen framerate.

IMO you get the correct results when you change the frame rate to 23.976 manually before the compress test.
The value returned from the check is the same for every framerate. Only the calculated percentage depends on it. So it's the same to do the CC with 29.970 frames and then switch to 23.976 and to do it with 23.976 frames from the beginning.

You could be right, that 23.976 is the framerate to choose, but I don't know.

[manono]

Hi-

Of course you want the percentage for the final 23.976fps movie, but it doesn't much matter if you select the 23.976 before or after the compress test. Because 23.976fps is 20% fewer frames than 29.97fps, the percentage figure for 23.976fps will be 25% greater than that for 29.97fps.

So, if you got 78.7% then you should be very pleased with the final result. If not, then perhaps something else went wrong.

[gio]

Originally posted by manono

Of course you want the percentage for the final 23.976fps movie, but it doesn't much matter if you select the 23.976 before or after the compress test. Because 23.976fps is 20% fewer frames than 29.97fps, the percentage figure for 23.976fps will be 25% greater than that for 29.97fps.

Ok, thanks. I'll use the 23.976 frames-value.
As said before, it doesn't matter at all what framerate is chosen while CC is running.

[theReal]

How should GKnot 'know', that you're going to perform IVTC later?Because, when ivtc is used in the avs script, the compress test is only performed on 5% of the 23.976 fps total amount of frames (which is less than what Gknot thinks the total amount of frames should be). Maybe it doesn't complain because Gknot doesn't know how many percent of the movie's frames where used for the test - it will certainly complain after the first pass when the total amount of frames found in the first-pass log is much smaller than expected.

23.976 fps should be used because the avs script that you're using for the compress test and for encoding is a 23.976 fps movie. The selectrangeevery() of the compress test is performed after the ivtc, so it selects 14 out of every X frames from the ivtc'ed 23.976 fps output.

btw. not that it would actually do any harm if you set the frame rate to 29.97 - it doesn't do anything except that it will show wrong values for the compress test. The movie will encode fine and all, but you also want correct numbers for the test, right? Doesn't matter if you change to 23.976 fps before or after the test, just do it at some point to get the right numbers.

[OvERaCiD23]

if you're using IVTC in Gordian Knot, after making the AVS files, select "IVTC in frameserver ~>correct frame count". this will correct the # of frames so Gordian Knot doesn't give errors after the 1st pass. or you could adjust the framerate after opening the D2V file.

[gio]

Originally posted by OvERaCiD23
if you're using IVTC in Gordian Knot, after making the AVS files, select "IVTC in frameserver ~>correct frame count". this will correct the # of frames so Gordian Knot doesn't give errors after the 1st pass. or you could adjust the framerate after opening the D2V file.
Yes, but that was not the question.

I just wanted to know, which framerate to use for the CC and thereal gave me the answer.
Actually this was what I thought myself, but I got bad results although the percentage value (23.976) was pretty high and so I started doubting it (Because you have of course to choose a lower resloution / higher bitrate to get the same value for 29.97 frames).