#Gamma?
41 messages in this thread
Hi John. I've got a question, and a problem. I've created a
walkthrough, where the first part is very dark and the lights come up
as the camera moves through an office. The problem is that the
rendered images are very light and low contrast in the framebuffer
during the dark part. And I've got all lights except four set at
zero. The four lights are tight spots, and there are no stray
lights. When I view the image on my computer RGB monitor they look
very dark and contrasty, as I would like. But on NTSC they look
washed out and sh8tty.
I've got the Targa+ initiated with Tmode 11 only, so I would guess
there is no Gamma correction happening there. I pass through a VTR,
but I tried a direct connect between the Targa and my monitor (not a
real monitor, by the way, just a Mitsubishi TV with video in) and it
makes no difference, so I guess it's not affecting gamma. I've got
Framebuffer gamma set at 1.8, but it looks better in this case, the
lower I go. I'm going to pop the frames on an Abekas asap and see
what it looks like there, but I'd like to be able to fix my in house
setup so it looks better. Any ideas?
Thanks,
John Tissavary (La Luna cie)
John – > I'd like to be able to fix my in house setup so it looks better. <
First thing I'd recommend is getting a descent NTSC monitor, one with
a Blue Gun and Cross pulse switch. This is essential for setting up
your monitor to color bars. (Not so much the cross pulse but it can
be real handy for checking horizontal and vertical sync, and it
enables you to see black and video black.)
Each job is a little different and I tweak accordingly. Most of the
time I leave Gamma On (now that I have it <G>). But there are some
circumstances like the one that you describe where I would turn it
OFF, ie set it to 1.0. This makes lighting you scene a little more
complicated but will give you the contrast you require. I would then
light the scene the way you like it with all the lights set at the
values that seem appropriate to you, and then back off for the
opening.
You mention you've got all lights except four set at zero does this
include "ambient"?, if not, try turning ambient off and as the lights
come up in the keyframer adjust it accordingly. Adjusting ambient
light gives you that much more control. If you're going for a "Limbo"
lit scene you're going to need some ambient light, in which case
you're going to get the best contrast turning Gamma OFF and adjusting
ambient accordingly.
One other factor which I think gives you a little more flexibility is that
you're sending your images to the Abekas which is going to be able to handle a
little more contrast than your VTR. A good NTSC monitor however will help you
immensely making these adjustments.
-JE
There's no question that an real monitor would help, and I'm saving to get a
decent one, rather than a cheap one now.
I guess tweaking the gamma is my best bet, and I was going to try that, but it
seemed like cheating <g>.
I have the ambient light set optimally, since I don't want absolute darkness
it's set VERY low, but not to zero.
Thanks again for your invaluable help,
John Tissavary (La Luna cie)
John – Definitely better to save for a good one, one of the most valuable
pieces of equipment you can purchase.
I don't think its cheating to adjust gamma, but I wouldn't recommend it for
everyone as it can have a rippling effect on all your jobs. But for a specific
job with a certain look to it, it can be the way to go. After all for alot of
animations there wasn't any gamma adjustment and it was often compensated for
with good lighting.
Sounds like you've got a good handle on it.
-JE
Thanks for the vote of confidence. All I can say is it looks best with Gamma
at 1.32. Go figure.
John Tissavary (La Luna cie)
John – That's very interesting about the final Gamma value. I'm
curious to hear you assessment of the final result after the tranfer
to the Abekas and Beta Sp ie: whether you notice any difference and
if it comes out the way you expect.
-JE
I didn't reset the file gamma, just the framebuffer gamma. I want the
files to go on the Abekas as they should be. If they suck and can't
be fixed, I'll just re-render them at a lower gamma setting.
>> When I view the image on my computer RGB monitor they look very dark and
contrasty, as I would like. But on NTSC they look washed out and sh8tty.
John, I think this goes back to some earlier discussions about Gamma
correction… It's a problem I've encountered constantly, since I only have a
composite NTSC monitor. That washed out, low-contrast look is double gamma
correction.
GARY?!?
So how the &%*$ do I turn off gamma correction on that peice of everlovin' #%*&
Targa+ (hating it more and more every day)???????!!!!!?????? Man, I'm so fed
up with this.
John Tissavary (La Luna cie)
Wasn't there a big discussion about how to turn gamma off on the Targa+ in
early October? Isn't that technique working for you?
– G
John – Its the same with the Matrox and every other board that's outputting
NTSC. I think the problem you're experiencing with dark images lies elsewhere.
I can say this because I have an old 32 which just has RGB out (no composite
signal and is not Gamma Correceted) and sending the signal through the EVO with
its encoder has the SAME RESULT as sending a 32+ SVHS signal into the EVO. The
internal encoder does not have Gamma correction other than that specified by
NTSC.
It has been determined through Sony that the EVO and CRV do not have gamma
correction other than what is specified for NTSC. NTSC is gamma neutral which
is why the boxes line up at 1.0 when viewed on an NTSC monitor. Gamma set to
1.0 on the Matrox is the same as Gamma set to 1.0 on the Targa which is Gamma
Neutral.
In a previous thread you mentioned that you didn't change your file input gamma
value just your file output. My experience is that for everything to work
correctly they need to match. That is File-Input, Framebuffer Gamma and
File-Output values should match, otherwise things can get really fouled up. But
I didn't want to second guess what you were trying to do so I didn't say
anything.
-JE
At this point I'm so confused by gamma, NTSC, and RGB, that I've lost
confidence in the process. I have a scene that is well lit, and comes out
beautifully in RGB, but looks so lousy in NTSC that I just can't believe it.
The dark areas are totally washed out and there is little contrast. I can't
play with the input output gamma values because it's going to Abekas, and I
just want to preview on my deck. So the issue here is, how do I get a good,
high contrast image to NTSC with the stuff I've got? Lighting is not the key,
because I even set all lights at 10 or 15, and the scene looks like ambient
light was turned up to 50 or 60. The closer to 1.0 I set the framebuffer
gamma, the better it looks, more like on an RGB monitor.
John Tissavary (La Luna cie)
John – No doubt every factor you've described complicates the situation. So
there has to be a starting point upon which you can build. In R2 there was no
gamma correction, so basically what you had to contend with was lighting your
scene in RGB and adjusting the lights to an acceptable level in NTSC.
Basically you want to approach R3 the same way. Most of the time you can leave
your gamma at 1.8 – 2.2, the caveat being that if you have a dark scene its
better to adjust Gamma to 1.0 Input/Output/Framebuffer because this is going to
give you your best contrast.
The down side to this is that your scenes will look colder which can be
somewhat compensated for by the colors you use and how you light the scene. In
video they have a term for lighting called "Color Temperature" which has an
effect on the warmth of a scene. Light in video is not pure white but has a
color temperature of 32k Kelvin. Some math wizards out there can probably
calculate what HLS values light can be adjusted to to achieve this color of
lighting, but the way I do it is to adjust the blue and green components down
about 10 units which adds a little more warmth to the scene.
Gamma correction for the most part has negated the need to do this. My
recommendation is to set Gamma at 1.0 as outlined above and adjust lighting to
achieve the desired NTSC result. Lighting really is the key, just as it was in
R2.
With regard to the Abekas unless they specifically state that you can't adjust
the gamma and that it needs to be set a certain value, I see no reason why you
can't set gamma to 1.0. If this is not the case then I appreciate your dilema
and you will have to speak with Gary to resolve this. -JE
I'm going out to Abekas tommorrow, and I'll post my success/failure. I think
I'll be fine with gamma 1.8, because it looks great on my SVGA monitor, and the
Abekas is more similar to that than my NTSC setup.
They also have a very good component TBC and can probably help out if the image
is not quite on the mark.
John Tissavary (La Luna cie)
Hey John, lets think positive! I'm looking forward to hearing of your success!
<G> I hope you'll be very pleased with the results, and I think 1.8 is a good
compromise. -JE
Everything was a smashing success (in my opinion). Even the guys at the SGI
playground where I do my Abekas stuff said it looked good.
Looks like I'll have to check the cable going to my NTSC TV monitor, then get a
real RGB monitor. There's a cool 8" PVM by Sony that I saw at the service
bureau today with all the needed features, a lovely picture, and it's in my
price range. Maybe Santa will stop by R. E. Snader and pick one up for me…
John Tissavary (La Luna cie)
Congratulations! We used a gamma of 1.8 for the Abekas stuff on our tape and
it worked great. It's reassuring to know that the same thing happened for you.
Good work,
– G
>>We used a gamma of 1.8 for the Abekas stuff <<
NOW you tell me… <g><g> . I'm glad we ended up with the same figure, too,
because now I feel like it was less guesswork than taste and judgment.
John Tissavary (La Luna cie)
John – That's really great news, I'm glad things worked out. I have a
suggestion for a monitor you might like to look into. Its got a great
price/performance ratio. Its the Panasonic BT-H1350Y 13" (as opposed to 8")
which I think is really to small, high grade monitor w/SMPTE Type C Phospher,
750 lines horizontal resolution, underscan, pulse cross, Line A/B/VTR/RGB multi
standard input, S-VIDEO and its around $1100. Maybe someone like Greg P or
Angelo can help you obtain one. If not Computablity has them.
-JE
That sounds like a great monitor, but is currently out of my price range. I
think the 8" is similar in features, and I can get one in a couple of weeks. I
realized I just can't put this off anymore, so I'll use the 8" smooshed up to
my face for color checking, and the larger NTSC for composition.
Thanks for the help,
John Tissavary (La Luna cie)
>>They also have a very good component TBC and can probably help out if the
image is not quite on the mark.
Just curious, do you put a frame of NTSC colorbars when going to a service
bureau? If you did this they would have something to calibrate their TBC/Proc
amp with.
Yes, we load up a set of color bars. But I was pleasantly surprised when the
images went up on the Abekas and down on Beta SP, and looked EXACTLY like I
wanted them to in the first place. Looks like my NTSC setup is suspect, but I
knew that from the get go. Just didn't realize how limiting it is. I'll be
investing in a real RGB monitor VERY soon.
John Tissavary (La Luna cie)
If your images are looking washed out when recorded to NTSC, it means almost
definitely that you're gamma-correcting twice somewhere in the video chain.
The simplest thing for you to do would be to just turn gamma correction OFF in
3DS and do some tests with the Abekas. If you're satisfied with those results,
just go with it.
– G
Washed out images can also be lost termination, Gary. If it is noticeable I'd
look for this first. Put an ohm meter across the cable to the deck and it
should be 75 ohms.
Thanks, Don. I'll check the cable to the VTR.
John Tissavary (La Luna cie)
Do that! Always remember, when the termination load resistance is higher than
75 ohms the picture will be brighter and washed out. When lower than 75ohms
the picture will be darker and low in contrast. I use a WFM scope on line all
the time and a quick glance at the negative sync tip will alert me as to the
line impedance.
I think that at gamma 1.8 I'll be getting good contrast and light levels on the
Abekas. I guess I'll just have to experiment with the "straight to NTSC"
method and see what comes of it. Perhaps Don is right and I've lost
termination on my cable to the VTR.
John Tissavary (La Luna cie)
I'd be very interested to hear if it's a termination problem. I've learned a
lot about video signals from the guys on this forum, and the whole thing
constantly reminds me of how much I hate video.
– G
I will let you know. Now I've got to track down an ohm-meter. I sold mine
when I quit gaffing.
John Tissavary (La Luna cie)
>>That is File-Input, Framebuffer Gamma and File-Output values should match,
I second that. rule of thumb! Unless the output is intended for another
device, such as a printer.
Don – And in the case of output to a printer the consensus seems to be that 1.8
is the best gamma value. But that's another world. <G> -JE
I concur… FI should match FO, and they should usually match FB.
– G
After just finishing our studios big CD-ROM project, I am finally gettting a
chance to try and get R3 working with my video equipment which has set idle
for a few months. In R2 I had spent time experimenting with my Targa Gamma to
get it where I wanted. Now I am having all kinds of problems with R3. (My
Targa is set the same, in other words it is providing the gamma corrections.)
Here is where I am stumped . . . tell me if I am wrong. If you set the file
output to 1.2 and then load the file back into the video post, composite and
output back out with gamma 1.2 to video, is this or is this not adding gamma
correction twice? Right now I have turned 3DS's gamma correction off and am
letting the Targa do it when outputing. Is this best or should I play with
R3's gamma controls.
Andrew McClary
Graphic Alchemy Productions
Andrew – If I understand you correctly, yes indeed you are Gamma correcting
twice.
But for further clarification I'll hi-lite a few points which will help you
make the determination.
The Targa needs to be neutral ie: Use TMODE 11 6, if you have to use Tconfig
set gamma at 1.0 .
In Video post there is an option to again add Gamma correction, which if you
are using is causing double gamma. (But only in the situation you've described,
not necessarily in all cases.)
If you do not use the Gamma option in Video post then you are not double gamma
correcting. ie: if you are using images generated from a previous animation as
a background which are already gamma corrected, these images are not being
gamma corrected again, but the scene you are rendering is.
If you are satisfied with the gamma settings on your Targa, I would suggest
using them to get the job done, because this is what you are most familiar
with. However I would also suggest experimenting with the Gamma in 3DS;
software gamma during the rendering process is much suprerior to hardware gamma
control. As time permits try experimenting with the default values with the
Targa set at 1.0 Then try using the hardware gamma values you've been using 1.2
in 3DS, see which you like best. Ideally you want 2.2 but there's alot of
flexibility here. Hope this helps. -JE
What a great answer! I've saved that one for the next time someone asks about
gamma correction. Thanks!
– G
Gary – You're welcome, glad to help!
-JE
John,
Most of my rendering is out to file, then I use VP to output the entire
animation to tape with transition effect etc. If I am setting the output
display gamma to 1.8 then I don't want to be playing with the output file
gamma? Am I correct?
Thanks
Andrew McClary
Andrew – No, actually this is incorrect if I understand you correctly. If you
have your Display Gamma set at 1.8 and this is your primary display ie: you're
not using a Framebuffer with an RGB monitor connected to it, then you want to
have the same values specified for Input File and Output File.
Display Gamma : 1.8 Framebuffer : 1.8 (If not used this doesn't matter)
Input File Gamma : 1.8 Output File Gamma: 1.8
If you set your Display Gamma to 2.0 then Input would be 2.0 etc.
In the case you describe you don't need to use the Gamma option in the VP
Dialog box when compositing, just send it out to tape. This example is the
default by the way and is working very well for most situations. Just make sure
that your Targa is set to 1.0.
-JE
Thanks,
I plan to play with this a little over the next few weeks.
Andrew McClary
Graphic Alchemy
Andrew
You're welcome. I think once you get a handle on it you'll really appreciate
having the Gamma Correction feature.
Have fun.
-JE
John – Gary pointed out that Gamma correction has a dramatic effect on dark
pixels which is the problem you're dealing with in your particular situation.
Which goes back to what I was saying about putting Gamma at 1.0 and adjusting
the light accordingly. It's in the readme.doc VII.6. So its not the Targa and
its not NTSC.
-JE