Hi,

In Yves' first Lighting Tutorial (great tut!) he mentioned doing a tut on proper workstation setup talking about proper gamma settings. I've looked around but can't find it. Did this happen?

Thanks!
Rusty

No. It never happened. Partly because of lack of time to do it properly and partly (but closely related) because it is a very intricate subject. Here is the essence of it:

The symptom

The same render that looked correct on a PC monitor look too washed out on a Mac monitor. Or vice-versa, a render that looked perfect on a Mac monitor now looks too dark on a PC monitor.

That's because both computer monitor gamma are setup differently. Traditionally, PC monitor have no gamma adjustments and Mac monitors have a gamma adjustment of 1.8 from the manufacturer.

To further complexify the issue, different peoples may actually have different gamma settings if they have adjusted their brightness and contrast controls on their monitors or if they have installed a gamma corrector on their computer.

Standardizing the gamma setting

A CRT monitor and the human eye response to illumination must be matched in order to reproduce the reality's illumination as best as possible and this is possible by adjusting the monitor gamma curve. For this, a gamma setting of 2.4 would be ideal. From this, it is clear that a Mac monitor is nearer to reproducing real illumination than a PC monitor. Still the Mac monitor was adjusted to match the printing industry. Not to reproduce photos of the reality.

The imaging industry (mainly digital photo equipment and scanners) recognized that there was a problem of consistency and decided that there was a need for a gamma setting standard. So they analysed the market and decided on a standard based on the market analysis. What they discovered from this market analysis is that no monitor is properly setup to reproduce digital photos correctly. They also discovered that there is a lot more PC monitors on the market than Mac monitors. So they decided on a gamma standard that would match the default PC monitors.

This standard is called sRGB. It essentiually means that all digital photos must be gamma adjusted to gamma 2.2 when they come out of the digital camera or the scanner. Most scanners can be gamma adjusted but digital cameras cannot. So, this way, when a digital photo on the web is displayed on the most common CRT monitor around, it should look right.

The mess

Because of the choice of this standard, 3D renders should also be gamma adjusted to 2.2 in order for them to look as real as possible and match the type of illuminations we are accustomed to by looking at digital photos. A non-gamma adjusted render will look too dark, lots of details in the shadows will be lost and the shadow to light transitions will look too hasrsh.

But this gamma 2.2 is a catch 2.2.

Renderer are linear illuminosity calculators. Renderers calculate the lights as it hits the objects around and as it fades away. Not as the eye perceives it. The corrections as the eye perceives it must be done only after all the calculations are completed.

So in order to properly adjust the illuminations and textures in a scene while building the models and the choreographies, one would actually need to adjust the monitor gamma to 2.2. But here is the catch. If the monitor gamma is adjusted to 2.2 and the final render step also adjusts the render to 2.2, this final render will look way too washed out when viewed on this already gamma adjusted monitor.

The setup

To avoid this catch 2.2, here is a setup that is somewhat messy but that works.

Install a gamma adjuster on your computer. Most graphics cards already have one where you can preset different gamma adjustments and you only need to select the one you need. If you have Photoshop, there is a also a Gamma adjuster that is installed in the control pannel. Otherwise, there are some freeware gamma adjusters available on the web.

When you texture your models and light your scenes, set your monitor gamma to 2.2 in order to properly view the texturing and illumination results.

When you render to file, if you add a gamma correction, expect the final render to look washed out. Preferably, however, do not add gamma correction as a render step to the final render file but do it as a post process. This is especially important if you plan to do other post processing steps on the image because doing image processing on gamma corrected images will produce undesirable artifacts. If you plan to do gamma correction as a post process, you should select a file format that is at least 16-bit per channels because Gamma correction is a severe correction steps and will produce quantizing artifacts in the shadows. Or even better, select OpenEXR. Also, the gamma 2.2 is suitable for reproduction on the Web or on a PC monitor but if you produce for film or DVD or for print, you will need to adjust the gamma differently than 2.2 according to the particular device. Another benefit of not adding gamma correction a priori is that the same render can be gamma corrected differently if it must be displayed on different devices.

When you are ready to view the gamma corrected renders, set your monitor back to no gamma correction.

Group projects

When working on a group project, it is especially important for those doing the texturing and lightings, to make sure their monitors are similarly gamma adjusted. There are charts available on the web that can help adjusting the monitors. Those charts are graphics files that are displayed on the monitor and help visually adjust the monitors gamma.

Texturing

Because photos, out from a digital cameras are gamma corrected to 2.2, if those photos are to be used for texturing, they should first be un-gamma corrected. Remember this.



That's all for now. If you have further question, as them here so I can try my best answering them for all.

Yves,

First I want to sincerely thank you for putting the time and effort into your reply. That was very kind of you and you explained things very well. I’m sure everyone can benefit from this info.

Some of this I knew – which is why I was so interested in any kind of a tutorial, hoping for a silver bullet to nail it down or… just or anything to make the muddy waters clearer. The latter is exactly what your reply did to the degree I think anything could by providing many things I did not know. Thank you.

Questions:

I have 3 ways that I know of to calibrate my monitors (a Dell 20’ UltraSharp and a Dell analog flat panel). I have looked for documentation on all of these but have found nothing that really helps me – embarrassingly everything I find is Greek to me. I have PhotoShop’s utility, MS’s applet and the NVIDIA utility. I’m looking for the most reliable way.

I have been adjusting my monitor using the Photoshop utility every 30 days (the MS applet reminds me to do this). Stumbling though it as I do, the utility says to adjust ‘this’ so that ‘so and so’ best ‘does this’. Although I do this in a dark room to the best of my ability, it is very subjective and I do not feel I am really doing it well (I get it slightly different each time?). Also, the utility does not tell you what this procedure adjusts anything to! Any insights on the usage of this utility would be awesome. It’s the utility that comes with PS CS2.

I have another free Color Management utility from Microsoft; the Color Control Panel Applet – one of the power tools from here:

http://www.microsoft.com/windowsxp/downloa...ppowertoys.mspx

It is the program that reminds me to periodically calibrate my display. I have no clue how to use this utility. It does say that my Default Windows color space is sRTB IEC61966-2.1. It provides dozens of ICC profiles I can choose from but I know nothing about these (what are these?).

Finally my graphics card is an NVIDIA Quadro FX 3450/4000 SDI and it has, I believe, the standard NVIDIA utility . If I go to Color Adjustment and adjust the Gamma to 2.2 things look really horrible -- like I had reduced the colors to 256. Perhaps I am double adjusting at this point.

Perhaps to round off what you have already explained, you might pick one calibration utility and focus on how to use it to move back and forth between 0 and 2.2 gamma for the setup you mentioned in your previous post.

Cheers,
Rusty

Yves' covered a lot in his over view and I found it very helpful.

First I would like to toss a few bits for thought:
-Laptop displays (at least the PC I've used) are near impossible to calibrate correctly.
-viewing angle on some LCD screens drastically changes image presentation (C, B and Gamma)
-poor quality monitors or weak CRTs cann not be calibrated correctly.
-any test patterns used, should test the Hi (White), Lo (Black) and preferably Mid (grey).

After 15+ years in Medical Imaging (Cardiac/Angio) I can calibrate high end B/W Monitors to match, but still have major problems adjusting 2 PC's to display matching images.

Here a a couple of pages that help me at least get in the ball park:
http://www.normankoren.com/makingfineprint...html#gammachart
http://www.photoscientia.co.uk/Gamma.htm#menu

Ganthofer,

Cool. I looked over the sites and BM'd them -- both tell you what you are adjusting to (PhotoShops Gamma utility must also tell you but I do not know where yet). It's starting to make more sense. I almost don't know what I'm doing now. :-)

Rusty

Yves,



This is a kick in the head! Scanners too I guess. Okay... how do you 'un-gamma correct' them Yves? Change the gamma to -2.2 using something in PS if possible?

Cheers,
Rusty

Yes. select the one that is easiest to access or with which you feel most comfortable and use it exclusively to the others. Use only one and disable the others. Which one doesn't really matters.

This is using the NVIDIA board settings. Note that the rendered image has been gamma corrected to 2.2 by AM so we are not really seeing it as we should for comparison. Also note that I do not know if I already have the monitor adjusted to 2.2 by something else! I must figure this out.

Default setting

[attachmentid=16342]

This setting seems to adjust it to 2.2. It just uses the color profile shown (ICC - sRGB IEC61966-2.1) which was one of the default files offered to me. I wonder if the 2.1 at the end of the name means anything?

[attachmentid=16343]

There is no easy way to simply adjust the gamma to 2.2 that I can find using the slider -- closest I can get is 2.25. The 'In' and 'Out' I'm not sure what to do with.

It is interesting that I can leave one monitor un-adjusted -- this might make me crazy though.

Rusty



Okay... this looks easy... just never looked at it before.

Unless you really are in a situation where ultra-exact color matching is required (for instance if you needed to do exact pantone color matching for printing), you normally don't need to be neurotic (like the MS utility seems to be) about gamma correction. Although it is necessary to adjust the monitor's gamma, the difference between 2.2 and 2.3 is barely unnoticeable and unless you have a very deficient monitor, you should not need to readjust it every two weeks. I'd say adjust it one every two month is already overkill IMO.

If you plan to output for TV or Video or DVD or anything that will be displayed on a TV (NTSC) monitor, there is an old inside joke among TV technicians and engineers that goes like "NTSC is for Never Twice the Same Color". Although with newer TV sets and HDTV monitors, this tends to be less and less true, one only need to go at the local electronic shop and compare all the TV side by side to conclude that trying to be neurotic about gamma adjustment is just vain.

The same can be concluded when doing work that is destined to be viewed from the Web in a browser window. The vast majority of computer users have no idea of gamma adjustment and just use their computer as they get it from the manufacturer. In addition, a lot of them tweak the brightness and contrast controls without knowing what they are doing. So even if you try to control your own output as precisely as possible, it is still almost guaranteed that the image will look differently on someone else monitor.

So just set your monitor gamma to approximately 2.2 and this should be good enough.


The gamma chart that on the Norman Koren web site that was pointed to by Ganthofer is a very good chart (better than the checkerboard types) for helping calibrating the monitor. Use it to visually calibrate the monitor. This way, even if you have several color managements working together, you can set the proper gamma correctly nevertheless.

What those gamma adjustment applets do is change the luminosity response curve of the monitor so that you get more range in the dark colors than in the bright colors.

Colors in an image file are encoded linearly. With color channel; values going from 0 to 255, a value of 2 is meant to represent twice the luminosity of 1 (which is almost black). But CRT monitors do not respond linearly to the current they get from the graphics card. When a CRT in not gamma adjusted, you can't distinguish a difference between a value of 1 and a value of 2 and thus you loose the dynamic range in the dark portions of the image. The gamma adjustment is meant to correct that. When a monitor is not gamma corrected, the dynamic range in the dark portion is too compressed while the dynamic range in the bright portions is too expanded. Gamma adjustment doesn't have an effect on black and white but on all the colors inbetween. It is more sensitive to the mid colors which gives this impression on a general brightening of the image.

The first thing to do, before adjusting the gamma, when calibrating a monitor is to ensure the Brightness and contrast controls are set optimally. That is you get black blacks and white whites. I don't recall the procedure for that but it should be explained on Koren web site somewhere.

Once the brightness and contast controls are adjusted optimally, then go to that Norman Koren page where you get that Gamma chart and open your gamma adjustment applet and adjust the gamma by looking at the gamma chart. That's it. No need to fuss more than that on Gamma adjustment.


Gamma adjustment is an ultra simplistic color management hack.

Color profiles are more sophisticated color management specifications. Essentially, each CRT tube have a specific set of color characteristics depending on its fabrication and the phosphore it use and the screen mask, and etc. Each CRT model, scanner model, printer model, camera model, have its own color profile. The idea is that if you tell the OS which device you use and what is its color profile, the OS should be able to adjust the color information that travels between those different devices to always get the correct color representation wherever it is viewed or captured. It does that by transforming the color information it receives from one device into a sort of canonical-normalized-standardized-idealized color device and then transforming the color from that idealized device to the color space of the receiving device.

So if your device already have its own color profile or if you can find it an MS list, then it is best to tell MS OS which profile goes with which device. But if you can't find the specific color profile for a specific device, don't try to match an equivalent. It is not worth it.

Even if you specified the color profiles for your devices, you still need to ensure the gamma correction for your CRT mionitor is adjusted correctly though.


Yes. You probably double adjusted the gamma.

Personally, I tend to prefer the gamma applet that comes with the graphics card because it is probably the nearest to the driver circuitry for the CRT. Ideally, a Gamma adjustment applet should adjust the current that drives the CRT cathode. The worst type of gamma correction is the software ones that adjust the binary values of the colors that are sent to the graphics card. The old Macintosh used to do exactly that by changing the values in the color lookup table. So in the end, there were several colors that mapped to the same values and conversely, there were a large quantity of color values that were not used. I'm not sure but I think those types of software based gamma adjusters don't exist anymore and there is a specificly designated set of registers on the graphics cards to allow a hardware adjustment of the gamma settings. But your description of "reduced the colors to 256" made me think of that.


All true.


Several scanners do have a gamma control in their settings pannel. Just set the gamma control to 1 if there is one.

To adjust Gamma in Photoshop, use the "levels" option in the "Image > Adjustments" menu. You adjust the gamma by sliding the middle cursor under the histogram and you see the current gamma value in the middle "Input Levels" edit box.

To un-gamma correct a photo that you know was gamma corrected to 2.2, you set its gamma to 0.45 (0.45 = 1/2.2).

You can be pretty sure the photo was gamma corrected to 2.2 if it comes from a digital camera. Actually, the transfer curves in each brand of digital camera is more complex than a simple gamma curve and are trade secrets but 2.2 is a very good approximation (another reason for not getting too crazy about ultra-precise monitor calibrations). When a paper photo was scanned, then its actual gamma correction can only be roughly guessed.

If you want to take photos for texturing purposes or for enviornment mapping purposes, it is better to use a digital camera that can save the files in RAW format. Those RAW formats are not gamma corrected at all. But if they are loaded in PS CS, the RAW loader will automatically gamma correct them. You will need another application to save them in some other format un-gamma corrected. I like to use BreezeBrowser for that but it is not free. There is a free DOS command line utility, which I don't recall the name, that can do that too.


This is why it is better to adjust the monitor gamma while looking at a gamma chart and not just by looking at the gamma curve.

If you do it by eye, you have to take into account the way the eye & brain adapt to different lighting conditions. So to judge it by eye you should make sure that your monitor's brightness/contrast settings are the same each time; that you monitors are fully warmed up; that the ambient light in the room is the same, and that your eyes have been fully adapted to those conditions (which IIRC should take no more than about half an hour).

And thanks to Yves for such a detailed description of this stuff.

All important aspect of monitor calibration. But the most important one, by far, is controling the ambiant light in the workroom around the workstation. This means no windows but only artificial lights, preferably the white fluorescent ones. The ambiant lighting should not be dark either but diffuse with a uniform coverage of the workspace.

The idea is to get a monitor calibration to reproduces the real world color as well as possible. A simple testing procedure to see if your ambiant lighting is correct is to place a white sheet of paper next to your monitor and slightly overlaping the monitor. Have a white background on your monitor like opening a blank document. If your piece of paper appears darker than the monitor white, then the ambiant lighting is too low. Conversely, if your piece of paper appears brighter than the monitor white, then the ambiant lighting is too strong.

Next comes the "white balance". Several monitors comes with a set of controls for adjusting the horizontal and vertical size and positions and that sort of controls. One of the controls should be the "temperature" stated in degree kelvin. This basically alters the white balance of the monitor. By adjusting the temperature, you will get yellowish whites, bluish whites, redish whites etc. You need to adjust the monitor temperature to best match the white of the piece of paper. The piece of paper perceived white is dependent on the type of lighting you have around your workstation. If you use traditional incandescent lamps, you will normally get yellowish whites on your piece of paper.

BTW, when it comes to adjusting the monitor temperature, this is truely where color profile files can come into play. It does not dispense from adjusting the monitor temperature to match the ambiant lighting conditions but it gives a sort of universal baseline.

So true monitor calibration is much more than gamma adjustment but more like the whole workplace calibration. The only peoples I know that truely do that sort of workplace calibration are those graphics artists who work in the advertisement industry.

Awesome information. I especially like the paper trick to get the white nailed down -- I've been trying to figure someway to do this for ages -- how totally simple!

I suspect that when you've adjusted everything correctly that the desktop will look a bit washed out (I'm on my notebook on the road and can't try anything now). If this is true, is there a way to adjust for this?

Cheers,
Rusty

CRT monitors are very limited in their dynamic range so it is not possible to get the same dynamic range as in the real world. This said, to get maximum dynamic range out of a CRT monitor, requires that contrast and brightness controls on the monitor be set to their optimal value.

Trying to do that sort of calibration with a laptop LCD screen would be illusive to say the least. One day, we will all have plasma monitors on our computers.

Yves,

I think you stated that mac monitors come from the factory with a gamma of 1.8. Did you happen to mention anywhere what gamma PC monitors have from the factory? Or, is this totally random (yikes)?

The reason I ask is; if I reset my main flat screen it stays very close to 2.2 -- looks like 2.0. It looks like I've set the gamma on this one via the color space and visual adjustments so resetting the monitor may not completely do the trick for this one (I don't know what color space would be the default one). If I reset my second flat screen (set the same way), it goes to 1.6. I thought PC monitors had a lower gamma from the factory as a general rule.

Thanks!
Rusty

It is rather random. But most of the time, for utility computer, there is just no attempt to gamma correct from the manufacturer.


What I mean by no gamma correction is to set the gamma to 1.0.