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renfield sonia

Use of magenta filters

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Hi, I'm preparing a feature film that's going to be shot with two vipers and recorded onto a OB1 (444 RGB 10 bit log) to be transfered into a 422 rgb signal (captured via downconverter). Would like to know if anyone knows the basic workflow (including transfer codecs, if to apply a correction LUT with the transfer station or wait till it's done in postproduction)

Would like to know if anyone has an opinion about the use of magenta filters to correct the green cast of the log uncompressed signal, I've heard that it not only color corrects the image, but it also increases the dynamic range and reduces noise.

Would like to know if anyone has used this filters and what correction factor to use in different light situations (day, night, interiors)

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Various people have various opinions about magenta filters on vipers. The technical discussion revolves around the idea that if you leave the thing unfiltered, you'll get somewhat better per-channel dynamic range, because of the greater sensitivity (therefore lower noise at a given exposure) green channel. However, having better DR in green than in red or (particularly) blue may not be very useful if you're not shooting predominantly green objects. Since most real scenes are actually quite low in saturation this is quite unlikely to be the case, and I believe it's this analysis which prompted the decision not to correct for the green cast in Vipers.


Workflow depends to an extent on exactly what data-security requirements are required by your insurance and completion guarantee contracts. The usual Viper workflow is to:


- record the unadulterated camera output onto some sort of recorder

- make redundant backups of that recording, which probably comes as part of the recorder rental in your case

- make offline versions of the recording, almost certainly applying a LUT to approximate your intended final grade. This can get complex depending on what you're doing the offline on and whether there are additional requirements for DVDs and so forth.

- edit

- conform the picture, then final grade.

It is possible to do all of this on anything from a desktop computer (with a US$500 RAID in it) right up to a full bore post facility ending up on a Baselight colour grading system. Where and when you do these stages, whose responsibility they are, and what technical approaches are used for each, are currently fairly poorly standardised. Storage and HD edit gear is now sufficiently cheap, if you're canny, that you might even choose to cut the uncompressed material, but it depends on what quantity of rushes you anticipate and that's manly done for shorts. It does obviate a big lump of complexity at the offline stage, though.



The chap who shot Benjamin Button (possibly the most natural-looking digital movie ever) has been known to post here; he'll have more practical advice.


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Thanks a lot for the answer, I've been making some research about magenta filtration and found out that the main reason to use those filters would be to lower down the green channel exposure so to make it closer to blue and red one, to avoid green channel clipping in the high lights (with the consecuent magenta halo of the very bright parts of the image) and this way having a little more of range in the highlights.. and another thing would be to avoid possible noise in postproduction due to the rising of the red and blue channels signal (to match green channel one).

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Yeah, that's the usual thought people have.


What it doesn't take into account is the fact that the different channels have markedly different sensitivity curves, noise floors, and other variances. People have done tests on this and arrived at various conclusions; I suggest you do the same.



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