[argyllcms] Re: Calculating film LUTs for a film recorder

  • From: Philip Pemberton <philpem@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 21 Aug 2012 01:39:50 +0100

On 14/08/12 02:37, Graeme Gill wrote:
> Philip Pemberton wrote:
>> A while ago I managed to pick up a Polaroid HR-6000 film recorder.
>> Basically, it's a white CRT, an RGB filter wheel and a 35mm (or Polaroid
>> packfilm) camera and a controller board with a SCSI port.
> 
> Ah yes - the DICENet RIP had a driver for something very like that
> (except it used GPIB as the interface - rather slow).

Sounds like that was probably one of the AGFA / Matrix film recorders.
Lasergraphics, Mirus, MGI and Polaroid all used SCSI or Centronics (or
both); Agfa used GPIB.

> One issue I see is that you can't directly set any 11 bit value for
> the test chart. There are certainly ways of doing this, but
> you would need to create a suitable 8:11 LUT and then encode the
> test chart values accordingly. Of course you may just
> skip this by loading the LUT with a linear ramp and then
> being content with <= 256 levels evenly spread.

Unfortunately that's just one of many problems at the moment...

I've reverse engineered enough of the command set to load a custom film
table and print something (e.g. a grey card) but unfortunately the film
table file format isn't documented anywhere.

I've figured out Polaroid's obfuscation scheme, but now I need to figure
out what the "film flags" mean (I know 0x10 is "black and white film")
and how the two data tables relate to each other. Figuring that out will
probably involve reverse-engineering the firmware. That's probably not
an easy task, even with the service manual sitting on my desk :(

> I suspect that you can get a fair way with Argyll, if you make use of
> the print calibration tool <http://www.argyllcms.com/doc/printcal.html>,
> except that it's not really set up for density. A spectroscan T would
> be the ideal instrument. I guess that density could be converted
> to a pseudo-L*a*b* and fed into printcal (ie. convert to linear density
> then apply a fake gamma = 1 RGB profile).
> See <http://www.argyllcms.com/doc/Scenarios.html#PC1>.

I've actually got a Spyder2Print calibrator, though as I understand it
that isn't supported by Argyll :(

> It's always hard to know what a calibration is intended to do. Is it
> technical (ie. linear by some objective measure), or is there some
> level of art in it, intended to make certain things "look nice".

It's a linearisation LUT. From what I can tell, it's intended to counter
the effects of the film's characteristic curve (H-D, or
"Hurter-Driffield" curve). Although as you say, matching to sRGB may be
easier and more productive.

Thanks,
-- 
Phil.
philpem@xxxxxxxxxxxxx
http://www.philpem.me.uk/

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