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Discussion Starter · #1 ·
I would like this to be an educational opportunity, if possible.


I realize that different people have different techniques, and I am hoping to illustrate what I feel are some different techniques with regards to calibrating the D-ILA projectors.


Please, let's see if we can have an intelligent discussion of calibration philosophies here.


I have read some positive comments about the William Phelps calibration, and how it reduces posterization. This is good, of course. I also have some positive comments of my own regarding the Richard Martin calibrations, as the gamma "curve" is just about ideal for film-based sources.


Anyway, this discussion began on another thread, but was buried and so far off topic, that I wanted to dedicate a new thread to it.


On to the pictures.


Here is the typical look of a Richard Martin gamma curve:
http://www.dilard.com/dilard/images/...x/gamma/rm.jpg


This is from my own projector, but the graphs from RM projectors all appear similar to this one. The important note is the positive Gamma Factor (calculated as ~1.43 here).


Here are two uncalibrated (factory) projectors:
http://www.dilard.com/dilard/images/...amma/uncal.jpg

http://www.dilard.com/dilard/images/...mma/uncal2.jpg


Note that they do not contain a gamma "curve" correction appropriate for film/video (these are business projectors, after all).


Here is the gamma curve from one of William's calibrations (posted with owner's permission):
http://www.dilard.com/dilard/images/...x/gamma/wm.jpg


The point I was making on the other thread was that the gamma "curve" does not seem to compensate for film/video based sources the way I expected based on the positive feedback.


I realize that flattening the gamma will definitely fix any posterization issues, however, and I think that may be the reason behind this technique. I did see Brian's pictures of his G11, and they looked quite good.


Anyway, I just wanted to make some observations, hopefully in an educational/non-threatening way. I am interested in the different techniques and philosophies, and would love to hear other opinions on this.


Since this has the potential to be a "hot" topic, let's try to keep it educational, please...
 

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Discussion Starter · #2 ·
By the way, anyone can graph their own gamma with the Dilard Back Up Wizard. Simply make a back-up and then click on "Create Report" from the "Edit" menu.


You do not need to own a license to back-up your projector with Dilard, or to create a report of the settings.
 

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All of these graphs represent the gamma curves that are used within the projector to drive the D-ILA panels. They are NOT the gamma curve of the projector itself as a video display device. For NTSC video there is a very specific definition of what this should be; a slope of 4.5 until a specific input voltage and then a gamma value above that. I would not expect the projector's INTERNAL gamma curves, which have to compensate for non-linearity in the D-ILA panels and the electronics driving them, to have a curve which indicates what the EXTERNAL gamma characteristics of the projector are. I expect the internal curves to vary from projector to projector, as we have seen that they do. Even small amounts of shading can change the characteristics of the internal gamma curve quite a bit; JVC even seems to be using shading to compensate for errors in the midrange of their calibrations.


William
 

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I would have expected the response of the DILA panels to be highly linear, except with a small amount of manufacturing 'noise'?


Similarly, the electronics should be highly linear. Theres no good reason to process the input data in a non-linear fashion.


Andy K.
 

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Discussion Starter · #6 ·
Hi Hugo,


I used my avatar to show an example of extreme posterization.

http://www.avsforum.com/ubb/avatars/custom/milori.jpg http://www.dilard.com/dilard/images/...mma/poster.jpg


The picture on the left is normal. The one on the right has the same coloring, but exhibits posterization. You will see that what should normally be smooth gradients have become been "splotchy". That's posterization.


Richard's steep climb in the black of the gamma curve may have contributed to a posterization effect (although I do not have it myself). The "flatness" of William's gamma curve fixes the effect.


See the graphs above for examples.
 

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Good job mark.


It should now be obvious to even the casual observer that

there is no magic in any of this.


The line that followed has been removed at the

request of william phelps.



As for variations in the gamma curves, some of this

is like comparing audio cables. Are the water cables

better than the silver cables. All you can really

say is that the two are different. Which one you like

depends on your own tastes and opinions.


You may even want to have one set of calibrations

for film, another for ntsc, and yet a different

one for hdtv.


[This message has been edited by kevin gilmore (edited 05-20-2001).]
 

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Thanks mark for taking the time to explain.


I see what you mean.



Hugo


Update did the 8 Bar wizard works great never had better picture. Noticeble in dark scenes.


Thanks Again Mark and everyone that help develope this product.




[This message has been edited by Hugomed (edited 05-19-2001).]
 

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Milori,


thanks for your excellent comments, we certainly all appriciate what you are doing. Allow me one more question: What would be the difference between posterization and banding? Both phenomena seem to be quite similar to me.


Thanks


Christoph
 

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Mark,


Thanks for the great demo of these curves. The RM curves, pulling sharply out of black, seem similar to the analysis of the JVC Digital calibration which appeared in this forum last fall. My understanding is that this steep rise out of black will improve low level shadow detail--the negative is that it may cause posterization, especially on TV sources.


It does make sense to me that the gamma curve is the internal reflection of an external measurement, as Wm has stated. However, given that these projectors are all probably pretty much alike, can we generally assume that since the RM and JVC digital calibrations seem to have similar curves, they are similar calibrations?


------------------

ham
 

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Discussion Starter · #12 ·
Quote:
What would be the difference between posterization and banding?
Hi Christoph,


Posterization is the more 'technical' term for banding. There are really the same phenomena, but I usually use the term "banding" when referring to posterization of landscapes, clouds...anything where long gradients are involved that turn into "bands" when posterized.


I wouldn't use the term "banding" when looking at posterization of a person's face, but I don't really think it would be wrong to do so.


For example, pretend that this is a blue/white cloud sky that exhibits posterization...I would call that "banding".

http://www.dilard.com/dilard/images/...ma/poster2.jpg


The correct gradient is on the left, and the image with "banding" (posterization) is on the right.

Chuck & Ham,


You could swap out the gamma curve for different sources, if you wanted to. However, I think that we are starting to contribute to the perceived complexity of D-ILA when we tell folks that they can do such things (and encourage it).


I would recommend creating one gamma curve that you are happy with and keeping it in.


Having said that, I am pretty confident that the upcoming Dilard Wizard will offer a control to select the Gamma Factor. That way you can decide if you like the brighter mid-tones of a Richard Martin/JVC style calibration or the more linear, slightly darker mid-tones of a William calibration.


I tried a more "linear" gamma on my projector this morning. Very different!


What I did was to start with a totally flat gamma and then used my colorimeter (and some prototype Gamma Wizard code) to get a D65 response from 0 to 100 IRE. However, I did not add back any gamma factor at all. It was basically a color-corrected linear gamma "curve" (I use that term loosely here).


Although the picture was really very good, it was undeniably different. It was slightly more "moody", and not quite as "lively" as I was used to. The mid-tones did not have the same jump that they had when I had the 1.43 gamma boost applied.


I can definitely see liking either of them, but I can't really see going through the work to swap them out with different source material, although there is no technical reason you couldn't do this.


I took another look at Brian's pictures (recently calibrated by William), and they show pretty well this idea. The Gamma setting does not change the white or black points, only the mid-tones. In this picture, the brights are good and bright, the darks are dark, and the mid-tones are slightly dark, too (no gamma boost applied).

http://www.jet-x.com/brian/CE3K2.jpg


I wish that I owned the same material for a comparison picture.


By the way, the difference of philosophy may be due to the fact that William calibrates a lot of CRT projectors. Just a thought.


In the end, I used the Back Up Wizard to restore my original boosted curve, as I like the overall "feel" of the picture. That is just my opinion, and I believe that each person should be able to make that choice for themselves. Having said that, I know that I could definitely like and get used to the more linear gamma as well.


(PS - I did keep the linear curve, though...who knows, I may want to swap it in someday after all...)
 

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The sense I'm getting from this thread is that the steep curve is probably less suitable for poor sources (Cable, VHS) because bringing out low-level detail is also likely to accentuate the noise on the image.


Would that be an accurate statement, Mark?


Andy K.
 

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Discussion Starter · #14 ·
Hi Andy,


That's pretty accurate, although with too much gamma boost, you can posterize even high quality sources.


Here is a much better example that my "explanation" above:


I used Brian's picture again (I hope that he doesn't mind!), which is an unretouched picture of his G11. I cropped it, and gave it a gamma of 1.6 in Paint Shop Pro (this is higher than my 1.43 on my actual projector, but I wanted to dramatize).


Here are the pictures:

http://www.dilard.com/dilard/images/...gamma/cctk.jpg http://www.dilard.com/dilard/images/...mma/cctk_2.jpg


The picture on the left has a gamma of about 1.0 (linear). The one on the right has a gamma of 1.6.


Look closely, and you should see these phenomena:
  • White point and black point are not affected.
  • The points farthest from BOTH black and white are affected most.
  • The pattern on the back of the boy's shirt is visible with the gamma boost. You can see more of the pattern of the curtains on the wall.
  • The over-corrected image on the right displays some posterization in areas, especially in very dark areas (the front of the table)


Which one is "better"? I think that it is personal preference. The one of the left appears more "moody" to me (especially when I did this to my actual projector), and the one on the right seems more "lively".


I would not tolerate the posterization, but I do like the gamma boost otherwise. My personal opinion.
 

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My goodness! A lot has been said here, proposed as an "educational opportunity", about what my software does, by people who have never seen it and did not work on it. A single internal gamma curve from a projector has been displayed and a LOT of conclusions drawn based on this. The work we did to eliminate posterization has been attributed to "flattening" of the gamma curve.


As Leonard Nimoy used to say, fascinating!


Here is a gamma curve from a recently calibrated JVC projector, submitted for the audience's consideration.

http://www.meier-phelps.com/temp/gamma.07510735.jpg


Does this curve look "flat" to you? It is certainly not as steep out of the black as the RM curve, but I would not call it "flat" either.


The removal of the posterization did not come from flattening the curve.


William
 

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Discussion Starter · #16 ·
Hi William,


Thanks for posting. I think that this thread is quite educational, actually. For me as well.


I see what you mean about that recent gamma curve. It certainly isn't flat. Thanks for posting the picture.


Mark
 

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Hey Mark

Can we swap files between Dila owners.

Lets say if I wanted to try out your gama setting can you email it to me and go threw the backup wizard

and try it out?

Hugo


Just wondering
 

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A well informed audience is certainly better than the

magic produced by the wizard of oz.


William, Live long and Prosper.


And since i am a total trekie, seeing absolutely every

episode of every series including the miserable cartoon

series many times i mean that with the deepest respect.
 

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Discussion Starter · #19 ·
Hi Hugo,


Technically, that is possible. Realistically, it will probably not produce the outcome you are looking for, since it does not take your individual projector's differences into account.


I would suggest waiting a short while for the upcoming Dilard Gamma Correction Wizard (and Shading Wizard) or talking to one of the calibrators instead of attempting that. YMMV.


Before trying anything "out of the ordinary" with Dilard, it's always a good idea to e-mail me first. I'd be happy to let you know how it will work before you try it, or give you some pointers on what to expect if you do want to attempt it.


(PS - Hugo, having said that, I have to let you know that this very thing is being attempted by someone now...I warned him, and he is giving a shot anyway. Stay tuned to the forum for his review of the gamma "operation").
 

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I think the gamma curve William posted is from my projector.

It looks very much the same as what Dilard shows me in the

report. Dilard says the Gamma Factor is 1.08.

Here it is if anyone wants to take a look. (Let me know if this link doesn't work.)


I can't comment on the theory behind all of this, but I can

say that the picture looks great. Definitely no posterization

and great shadow detail. Of course I've only watched one

movie all the way through so far since the calibration, but

if it was any indication, my movie-watching is going to be

much more enjoyable from now on.


- Chris

 
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