norman@a.cs.okstate.edu (Norman Graham) (12/06/89)
In the Monitors cdev, there's a checkbox titled 'Use Special Gamma'.
(To find it, hold down the option key and ckick the 'Options...' button.)
It installs some kind of gamma correction, but what does that mean?
How does it effect my system? I assume that it has something to do
with the color tables.
When using the IIci's onboard video and Apple's protrait monitor, it
lists 2 possible selections when checked: Mac Portrait Std Gamma and
Uncorrected Gamma. I leave mine set to Mac Portrait Std Gamma of course,
but I've not noticed any difference with the other two settings.
Let me summarize this note in one sentence: What is all this Gamma
correction stuff anyways?
--
Norman Graham Oklahoma State University
Internet: norman@a.cs.okstate.edu Computing and Information Sciences
UUCP: {cbosgd, rutgers} 219 Mathematical Sciences Building
!okstate!norman Stillwater, OK USA 74078-0599ftanaka@Apple.COM (Forrest Tanaka) (12/06/89)
In article <5200@okstate.UUCP> norman@a.cs.okstate.edu (Norman Graham) writes: +-------- |In the Monitors cdev, there's a checkbox titled 'Use Special Gamma'. |(To find it, hold down the option key and ckick the 'Options...' button.) |It installs some kind of gamma correction, but what does that mean? |How does it effect my system? I assume that it has something to do |with the color tables. +-------- The human eye doesn't respond to brightness linearly. Dark scenes appear darker to humans than they should be. As a scene becomes brighter, the human eye catches up with reality. Time for graphs: ^ | * | * | * Actual | * Brightness | * | * | ** | ** | *** |**** +-------------------> Apparent Brightness Gamma correction linearizes the response of the human eye by artifically boosting low brightness levels. The effect can be subtle, but you should be able to see some difference. Uncorrected Gamma means that low brightness levels will not be boosted and your eyes will have to fend for itself. Standard Gamma will boost the low brightness levels. You should normally use this setting. If you photograph the screen, you might want to use the uncorrected gamma setting because photographic film's response is much more linear. +-------- |Norman Graham Oklahoma State University | Internet: norman@a.cs.okstate.edu Computing and Information Sciences | UUCP: {cbosgd, rutgers} 219 Mathematical Sciences Building | !okstate!norman Stillwater, OK USA 74078-0599 +-------- -- ******************************************************************************* Forrest Tanaka Macintosh Developer Technical Support Internet: ftanaka@apple.com Apple Computer, Inc. or: Forrest_Tanaka.DTS@gateway.qm.apple.com AppleLink: TANAKA Do extra-terrestrials Phone: (408) 974-1243 believe in God? *******************************************************************************
ftanaka@Apple.COM (Forrest Tanaka) (12/07/89)
Oops! I reversed the axes on the graph I showed. Just switch the labels
around. While I'm here, I might as well mention color tables. Gamma
correction and color tables don't have anything to do with each other. Gamma
correction just alters the brightness of the displayed colors. Gamma
correction works even on direct video devices, which have empty color tables.
--
*******************************************************************************
Forrest Tanaka Macintosh Developer Technical Support
Internet: ftanaka@apple.com Apple Computer, Inc.
or: Forrest_Tanaka.DTS@gateway.qm.apple.com
AppleLink: TANAKA Do extra-terrestrials
Phone: (408) 974-1243 believe in God?
*******************************************************************************pierce@radius.UUCP (Pierce T. Wetter III) (12/07/89)
The hopefully last word on gamma correction:
Monitor phosphors are not linear devices. If you shoot twice as many electrons
at a phosphor you do not get twice as much light. You actually get less light
or more or less the gamma-th root of the number of electrons. You can see this
for yourself by putting a progression of gray bars on the screen: 5% 10% 15% etc.
The values you put in for RG&B will seem to have little relation to the resulting
brightness. The low numbers will be too dark, and there may be a big jump
between the high numbers.
In theory, all you would need to do to correct this fact is do create a table
that indexes the values you send to the display card vs. the screen brightness.
This is called a gamma correction table. All Mac II video cards support gamma
correction tables.
Unfortunately, its not quite that simple. Monitors possess something called
burn-in. The electrons that bombard the phosphors come off a plate inside the
picture tube. As a monitor gets older, there are less and less free electrons.
(This is half simplification/ half analogy. It is not strictly accurate, but is
mostly meant as an illustration.) Because of this the gamma response of a monitor
changes with time.
Most picture tubes start with a gamma of 2.8. Apple has chosen to use a table
which will correct this to 1.8. (A comprimise between T.V.(2.2-2.8) and scanned
images (1.0). Most programs assume that gamma is 1.0, or linear (twice the red
is a red that is twice as bright.).
There are a few problems with this approach. First of all, a gamma of 1.4 means
that everything looks bad. For example, scanned images will look too dark and
video images will look too washed out. Also there may be offsets in the monitor,
0 is supposed to be black but 0 may be 1% brightness or black may range from
0-15. (Circuitry is never exact. Turn up your brightness and contrast all
the way, and look at a black square then turn them down all the way. Black is a
relative term.) Finally, the different colors may age at different rates or
have different gammas.
The facility for changing the gamma table to "Uncorrected" in the Monitors
CDEV prevents the gamma table from loading giving you the base gamma of the
monitor, appoximately 2.8. This is probably a hint that Apple plans to have
some video products soon. The NTSC (USA TV) signal is precorrected to a gamma
of 2.2. Most TV sets have a gamma of 2.8, so if you set your monitor to
Uncorrected and look at some video production it would look like standard TV.
For anything else its pretty useless.
****** Plug for the project I am working on ******
Radius, Inc.'s solution is a product called PrecisionColor. (695 Sugg. Retail)
This is a little ADB device and an associated CDEV. The CDEV talks to the ADB
device, running through all 256 shades of each color and recording the brightness.
Once this is done, you can set your gamma to a CALIBRATED gamma of (1.0, 1.2,
1.4, 1.8 (Apple), 2.2 and 2.8). You can also change the color temperature
(white balence) to: 9300-Weird standard monitor bluish white
7500-Europeon daylight
6500-Old standard graphic arts daylight
5500- Yet Another Choice
5000- Standard Daylight - Paper White -Most useful
4100- Cool white flourescent - Paper under office lights.
Also included is my portion of the project, the PrecisionPantones.
PANTONE Colors* were originally matched on a Mitsubishi monitor with a different
phosphor set from the currently popular Sony Trinitrons. Because of this,
PANTONE Colors on the screen do not match the Pantone Colors in the standard
booklet. What you See is Not What You Print. Because the PrecisionColor
Calibrator removes the gamma, and offset variables from the monitor,
Pantone and Radius have teamed up to improve the display of PANTONE Colors
on Macintoshes. With the calibrator you get a Quark Xtension for Xpress 2.12
which will display Pantone Colors on-screen more accurately than ever before,
a similar thing for Freehand 2.03, and a set of exact values for use in the
"Monitor Calibration" dialog in PageMaker and Illustrator.
The calibrator comes with a site license for the CDEV for 5 different machines
(it is assumed that you only buy one calibrator and use that to calibrate every
five machines in the office), and will ship 1Q '90. Note: it only currently
works with our montors and the Apple RGB 13".
Pierce.
Disclaimers:
Radius, Inc., has no knowledge and takes no responsibility for my postings whatsoever.
I personally make no gauruntee as to their accuracy since anything made with
a unix editor is automatically suspect.
Details: The standard Apple gamma is not actually 1.8 its 1.8 that has been t
tweaked until they liked how it looked.
Note the discrepancy between the TV signal as broadcast (gamma =2.2)
and the TV signal as received (gamma = 2.8)
Since PrecisionColor is not yet shipping some details may change.
Assuming any gamma for any uncalibrated system may be better than nothing,
but make sure that users can turn off that assumption or change it to
something else if they have a calibrated system.