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Question about frame rate embedded in DPX files


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#1 Dirk DeJonghe

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Posted 07 January 2017 - 05:21 AM

We work on a project where some sequences were filmed at 18 fps, others at 12 fps and others at 6 fps. The customer wants this shooting framerate embedded in the metadata of the files. On our Baselight, the software embeds the project framerate, this could be 18 or anything else, but the same for the whole project; I think it is the projection framerate that should be embedded (if any) and not the shooting framerate. This is for old archive material, the purpose of the job is to have long-term storage digital masters.

How would a projectionist (who is not the author of the film) know where to switch to 6 fps if he was even able to do this, Wasn't the intention of the author to have a faster pace when he shoots at 6fps?

 

 


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#2 Phil Rhodes

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Posted 07 January 2017 - 05:27 AM

Well, it might be the intention of the author to have more staccato movement by shooting at 6fps, too.

 

I can't imagine variable frame rate will be very well supported by anything, but all I can do is imagine. You'll have to try it.


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#3 Dirk DeJonghe

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Posted 07 January 2017 - 06:52 AM

Phil,

 

Is there any software that allows easy editing of this metadata parameter in DPX? We could create a project for 6fps, another one for 12 fps etc. Then reconcile the frames after renumbering etc. There must be an easier way.


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#4 Phil Rhodes

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Posted 07 January 2017 - 08:21 AM

I don't know anything in particular, but it's trivial to do. Almost a ten minute scripting project.
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#5 Dirk DeJonghe

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Posted 07 January 2017 - 08:35 AM

Since you volunteered, send me some information, we can talk.


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#6 Phil Rhodes

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Posted 07 January 2017 - 11:25 AM

There are three fixed headers in all DPX files, one of which is called "file header" and is 768 bytes long, the second of which is called "image header" and is 640 bytes long, and the third of which is called "orientation header" and is 256 bytes long.

 

The frame rate is stored in the fourth header, which is called "film information". Assuming your files use the "film information" header as opposed to the following "television information" header to store the frame rate, the frame rate information is held 60 bytes into the header, and thus 1724 bytes into the file itself, in a four-byte single precision floating point field. All that's required is to write a single-precision float 1724 bytes into the file. I may be a few bytes out. You'd want to test it in a hex editor.

 

Similar things apply to the "television information" header.

 

Interestingly, the immediately preceding value is called "HeldCount" and describes how many frames the frame in the file is to be held for, which could theoretically be used to create variable frame rate effects, but I suspect it is very poorly supported in most software.

 

As to actually how to write said script, it depends what sort of system you're on. On Linux, many people would do it in Python or something like that.

 

P


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#7 Bruce Greene

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Posted 07 January 2017 - 02:52 PM

How about inserting slate frames before each shot that describe the frame rate?  I don't think the .dpx system is designed for variable frame rates in a single folder anyway.  I wouldn't count on some future generation checking the metadata...


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#8 Phil Rhodes

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Posted 07 January 2017 - 05:48 PM

I have a feeling that certain aspects of the DPX system, such as it is, were not really designed at all!

 

The spec tells us to store frame rates in each frame, leaving open the possibility that they, and almost any other aspect of the file, down to resolution, colorimetry, etc. may change mid-sequence, but I'm not sure it talks about how these mid-sequence changes should be handled. Is a DPX sequence defined as a group of files in which nothing changes but the picture data and timecode? If so, what is the purpose of the hold duration count?

 

It is a puzzlement.


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