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Old December 17th, 2004, 04:17 PM   #1
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Ground Glass Grain

Greetings everyone,

Something just recently hit me up side the head. Something that I should have thought of a LONG time ago.

In still photography people have been known to take images with the lens cap on. These images (various processes for various forms of noise) are then subtracted from the original accurately removing thermal and dark current noise. Could something similar work with a ground glass? That would be a _very_ helpful means as we could use just about and GG and since the ground glass is static (I'm trying for a static version. Can't stand having to power one of these things) the grain would be consistent in its placement.

EDIT:

Does anyone have a GG handy that they can experiment with? Maybe you could attach the adapter to your camera and then point it at a light source so the only thing visible is the glass and its imperfections. Then pull out a single frame (would work best with a progressive scan camera), invert it and subtract it from the main footage?
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Old December 17th, 2004, 04:45 PM   #2
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I think this COULD work with a non compressed format, the MiniDV has a sight compression that IMHO make useless to subtract the plain noise fo the GG.
I think using a digital camera with cap on whould lead to a complete dark video, without the grain image. Keep in mind that photography has a way higher resolution that a DV video...

That's of course IMHO...maybe someone else can add something :D
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Old December 17th, 2004, 04:49 PM   #3
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Why do you think the compression would cause that much of an effect (not saying you're wrong just curious)?

Quite right about the lens cap idea. I was just suggesting that we try an approach similar to that used to remove dark current noise (which is achieved by shooting a frame with the lens cap on). In our case you would have to point the video camera at a light source or relfected light source so you only pick up the ground glass and its imperfections.
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Old December 19th, 2004, 12:41 PM   #4
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This is a great idea -- something Rai and Markus talk about in their HD setup which, on a very cursory, theoretical level I understand, but implementing the reality of it in practice is over my head. It's like sampling the noise of a room and using an audio tool to extract those sounds from dialogue -- see, I'm great with a metaphor but terrible with the code/filters for doing so.

I'm right now climbing Sysiphus' mountain of Flash and ActionScript, so I can't volunteer much time to work on the job, but someone with a working knowledge of VDub or AviSynth should be able to cook something like this up.

Please keep us posted -- if someone's willing to take the job, shows results, and lives near NYC (or visits occasionally), I'll buy you a beer(s) or your beverage of choice :)

- jim
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Old December 19th, 2004, 02:49 PM   #5
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question about oil

this i took from http://medfmt.8k.com/mf/vision.html

Quote:
When using ground glass focusing screens, as on a 4x5" view camera, the ground glass is hard to focus critically due to the coarse grain size of the glass. One old timer's press camera trick is to use a drop of oil to smooth out the surface of the ground glass. The result is an oil spot on the glass that is smoother, rather brighter than the untreated glass, and useful for critical focusing efforts. I presume the brighter oiled screen is due to less light being scattered from the coarse ground glass surface, thanks to the oil.
The author suggests Linseed Oil. Could this be used to elminate the problem of grain, or will the oil show up too much or make too soft of an image?
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Old December 19th, 2004, 04:15 PM   #6
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Matt: I believe someone mentioned this idea somewhere else on this board as well. It would certainly work. I'm not sure how practical it would be for a mini35 type system though as oil won't stay on the GG for very long.

Jim: I was unaware that Rai and Marcus had discussed this in respect to their HD cam. I assume they were doing this to remove dark current noise, not grain as with a GG?

I'm going to give it a shot sometime this week. I'll post any results.
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Old December 20th, 2004, 12:13 AM   #7
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I'm not sure if it was from GG or what, but it was a "fixed pattern noise" that they worked around. From the Drake camera thread:

Quote:
The software is capable of eliminating issues like fixed pattern noise and gain noise. It does that already almost perfect, what is left in terms of noise is changed to random noise, that looks like film grain, so not too bad.
Quote:
I knew that everybody would jump on the fixed pattern noise thing. Oh, well. We know that it's there and we found a way to eliminate it. thing is that for every gamma curve you apply during shooting you have to define a huge set of paramters to get enough data so the software is able to filter the noise out.
- jim
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Old December 20th, 2004, 01:13 AM   #8
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I wonder is electronic noise and gain a pattern or is it random? Anyone have factual info on this?

Shaw-
Try a few lighting conditions and mix and match to see if this will work in the real world. As mentioned in the other thread the image of the GG will be unique under each lighting condition even at each lens stop. Never the less I'm interested in hearing what you find.
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Old December 20th, 2004, 06:44 AM   #9
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<<<-- Originally posted by Jim Lafferty : This is a great idea -- something Rai and Markus talk about in their HD setup which, on a very cursory, theoretical level I understand, but implementing the reality of it in practice is over my head. It's like sampling the noise of a room and using an audio tool to extract those sounds from dialogue -- see, I'm great with a metaphor but terrible with the code/filters for doing so.
- jim -->>>

Do you believe this is nearly the REAL method used to remove audio noise ??? :D
This method is used to remove echoes and reverb.
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Old December 20th, 2004, 09:47 AM   #10
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Actually, that's exactly how noise removal tools work - you sample the noise without any additional sounds and then you substract it. I use it often in my work, it's a great tool.

Echoes and reverb cannot be completely removed without affecting the dry signal because the frequencies of the reverb /echo are essentially the same as those of the dry signal.
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Old December 20th, 2004, 09:53 AM   #11
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<<<--Echoes and reverb cannot be completely removed without affecting the dry signal because the frequencies of the reverb /echo are essentially the same as those of the dry signal. -->>>

A friend of mine, who works as an engineer for yamaha shown me a system that by sampling the reverb in an empty theatre hall created with a known signal (I guess it was something like a sinus) was able to remove all the reverb from a music sample taken in the same hall.
They are studiyng a system that playing (adding) a certain sound in realtime can remove the reverb of a concert hall whatever the building materials are.
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Old December 20th, 2004, 10:02 AM   #12
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Dario,

I know what you mean, but I think that this kind of device works rather as a gate, cutting the unwanted signal when the level of the useful signal falls below a certain mark.

And by the way, I tried to remove static noise from an image using the same pattern and it works very well. Seamlessly, in fact. I believe this could be the saving grace for the static ground glass system.
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Old December 20th, 2004, 10:12 AM   #13
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<<<--
I know what you mean, but I think that this kind of device works rather as a gate, cutting the unwanted signal when the level of the useful signal falls below a certain mark.
-->>>

I'm getting A LOT off topic... :D

But I knew you won't understand that :D

The system was using a 3d model of the arena, then N emitters playing a known sample (le'ts say a 4 kind of sinus sound each one) where placed in some certain points of the arena.
Each one was sampled by the system, that knowing the phisical shape and material of the arena was able to calculate in realtime how the reverb in that place was generated.
in this way they where able to remove the reverb of a (let's say) guitar in realtime.
It was not only a filter... believe me :D

I saw that demonstration because I asked this friend how I could remove reverb from a video I shoot, he told me there was a way, but he neded a model of the set and some time to sample the base signal on stage. of course it was too tricky for my needs.

Anyway... really too much O.T.... :D
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Old December 20th, 2004, 10:53 AM   #14
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Chris Rubin-
So it worked? Did you do a full test using one image of the GG and seeing if it will work for ALL shots under different lighting conditions? If it doesnt work with just one image of the GG then the system isnt practical. You dont want to have to pull a new frame of the GG whenever your lighting changes...hell whenever you pan the camera. Post some links to the frame grabs...even better...5 seconds of full rez video. I want to be proven wrong on this.
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Old December 20th, 2004, 02:50 PM   #15
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Brett,

my adapter is disassembled at the moment so I cannot post a clip right now. But here's what I came up with using what I had:

http://www.liisikoikson.com/ftp/noise_redux_ex.jpg

As far as I understand, different lighting conditions should have no effect here. Just as the camera does not 'know' what's on the other side of the ground glass (it just captures the image on the gg), neither shoud this patter 'care' what the image itself looks like.

I hope someone chimes in and posts a more thorough test.
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