Re: [Openexr-devel] Re: [Openexr] KeyKode offsets

[Daniel Fort]

The years I spent working in Hollywood seem to have caused my math  
brain cells to atrophy. In addition, I sent this directly to Ken  
instead of the list so I would like to share this along with some new  
comments:

> Ken McGaugh said:
> ....
> > Your description is excellent and I think I'm starting to understand
> > this.  But I think there is conflicting terminology going on and I'm
> > trying to get it straight.
> ....
>
> It looks like I got it all wrong. Apparently the perf offset is always
> counting from the perf identified as the zero perf in Kodak's key  
> code. On
> 35mm this would count from 0 to 63 on 16mm it would count from 0 to 19  
> and
> on 65mm it counts from 0 to 119.

It looks like I'm still wrong. All I did was to elaborate on Kodak's  
key code documentation, not perf offset.

> I have never worked with a system that counted in these perf  
> offests--in
> editorial we count in frames, not perfs. The only time we refer to  
> perfs
> is when talking about which perf number the "zero perf" falls on a  
> frame.
> In the case of 35mm 3-perf the entire foot that follows that zero perf
> frame has the same perf offset number.

Thanks to Ken for clearing this up and posting a formula to convert  
between perfOffset and frame numbers:
------------------------------------------------------------------------ 
--------------------------------
As for the perfOffset, we will interpret it as the number of  
perforations
between the reference mark the *last* perf in the frame.  Using integer
math, we can compute the frame and alignment as follows:

    frameOffset = perfOffset / perfsPerFrame
    frameAlignment = perfsPerFrame - (perfOffset % perfsPerFrame)

where frameOffset is the "+ 00" bit in the human readable keycode,
and frameAlignment is the "p1" at the end.
------------------------------------------------------------------------ 
--------------------------------

It seems that where I got it all wrong was that perf numbers we use in  
editorial and telecine count from the first perf of the frame and here  
we need to reference the last perf in the frame.

Hopefully this clears things up to others that may be in the same boat.  
Does anyone have a paddle?

--Dan