FixGamma fixes a bug present in the Macintosh Quadra 660av and 840av. (It isn't useful on any other models of Macintosh that I'm aware of.) Because of this bug, attempts to change the video gamma correction often fail. Running this application at startup causes it to work properly. Just drop the file named ~FixGamma into the Startup Items folder and you're set.
･ Say What?
Okay, okay.... Gamma correction is a feature of all color-capable Macintoshes, originally designed to compensate for color inaccuracies inherent in most monitors. The Macintosh Quadra 660av and 840av have a bug which sometimes prevents changes to the gamma correction from showing up. Though it was designed to make the images on monitors more accurate, gamma correction has since been used by:
- Screen savers, to dim or black out the screen (a very efficient method, BTW)
- Games or other applications to provide smooth fading and create special effects
- Image editing applications such as Photoshop, to preview color corrections
Without FixGamma, the fades and the color correction may not take place. Basically, this means that if you use a screen saver, play games, edit images or video, do color accuracy work, or just generally care about fixing a lurking bug in your 660av or 840av, this program is for you.
･ How Do I Install FixGamma?
Just open your System Folder, locate the Startup Items folder within, and drag the ~FixGamma file into it. Every time you start up after that, the program will run and fix the bug.
･ What Is Gamma Correction?
Gamma Correction attempts to correct for a problem inherent in the way monitors work. CRT (cathode ray tube) monitors use an electron gun to fire streams of electrons at the back of the screen, which is coated with a substance which glows when electrified. As it directs the stream at different parts of the screen, it varies the voltage used. The higher the voltage, the brighter the that part of the screen is. If no voltage hits the screen in a particular spot, that part of the screen will be completely dark. If the maximum possible voltage hits the screen in a particular spot, that part of the screen will be at its brightest. Now, if the voltage was half of the maximum voltage, you would expect the brightness to be half of the maximum brightness, right? Unfortunately, this isn't the case. Look at the following graph:
ﾊ
As you can see, 50% voltage ends up generating only around 20% of the maximum possible brightness. For reasons that would require a few physics lessons to explain, this is a natural part of how monitors work. Here's where the concept of gamma comes in. The graph above is of the voltage raised to the power of 2.2. That exponent is called the gamma, and it is a measure of how "bent" the graph is for a given monitor. 2.2 happens to be a common gamma value for monitors. Monitors with a gamma that is greater than 1.0 will come out with the colors too dark. To correct for this, computer software has been designed to compensate by making the colors brighter. Note that this is not the same as simply turning up the brightness control, since doing that would leave some colors too bright. "Gamma correction" carefully brightens colors in exactly the right proportions to straighten the graph.
The gamma correction on the Mac is normally set and changed from the Options... section of the Monitors Control Panel, and in that context it is used exclusively for overcoming the effects of monitor gamma. However, the gamma correction system used on the Mac is so flexible, it has a number of other applications....
Adjusting all colors so that they are completely black is a neat way to make a screen saver because it doesn't require conflict-prone patches, it doesn't suck up processor time trying to cover up things on the screen, and because everything is still there on the screen, there's no delay waiting for the screen to redraw when you wake it back up.
Many games also use gamma correction as a way to fade in and out, by slowly adjusting the gamma to or from this completely black state. Because you can adjust the red, green, and blue values separately, some games, such as Marathon, use gamma correction to make the screen flash a certain color to indicate that you've been hit or gotten an item.
Image editing applications, such as Photoshop, can show previews of the effects of certain color filtering and adjustment operations by changing the gamma correction, allowing the user to see the effects in real-time, before making a more permanent and time-consuming change to the image itself.
So, as you can see, not having a properly working gamma correction system can become a handicap in many situations.
･ What Exactly Are The Details Of This Bug?
At a low level, I don't know why the bug happens, but I know the conditions under which it will happen. After a 660av or 840av starts up with the screen depth set to Thousands or Millions mode, changes to the gamma tables mysteriously fail to work. Changing the monitor depth fixes the problem. It's easy to switch depths temporarily, but troublesome to remember to do, and though some may try and fix this with a macro package or AppleScript, I find this application to be a fast and efficient solution. It doesn't switch depths unnecessarily if the machine is booted with 256 colors or less, and it attempts to choose the best depth to switch to and from so as to avoid the "screen bounce" of resynchronization associated with switching between certain screen depths.
･ Fine Print
FixGamma is free. It may not be sold for profit or included with software or other products which are sold for profit without the permission of Tyson Boucher. Otherwise, you may distribute it as long as you only distribute the original unmodified archive.
This program should do what I've said. It's not very complicated, and switching a screen's depth twice is hardly likely to cause trouble. However, I'd hate to have that written on my gravestone, so, I'd better say the following:
Tyson Boucher provides FixGamma as is, without any warranty or promise of technical support. Tyson Boucher disclaims any liability of any kind for any damages whatsoever resulting from the use of FixGamma, including, without limitation, incidental, consequential, indirect or special damages of any kind, even if Tyson Boucher is aware of the possibility of such damages. Tyson Boucher makes no warranties, express or implied, with respect to the program, including but not limited to warranties of merchantability or fitness for a particular purpose.
･ Version History
1.0 - Dec, 9, 1995
The first version for use by others, this was an extremely limited release to a few people on the MACAV-L mailing list. It may not even say 1.0 on it, but it's easily identifiable, since it has no icon.
1.1 - Oct, 19, 1996
I added an icon and some code to choose an appropriate depth to switch to so as to make the switch as inconspicuous and fast as possible.
･ Acknowledgements
Thanks to Mason Bliss for allowing me to discover this problem. When trying his screen saver, ScreenDaemon, (which uses the fade to black trick I mentioned earlier) I found that the screen would often fail to dim. Some investigation led to the discovery I mention above. I reported this to Mason, and he sent a program which was a crude start of what I later rewrote as FixGamma. Had Mason never made ScreenDaemon, I'd never have discovered this problem.
Thanks also to all the members of MACAV-L, the Macintosh AV mailing list, for helping me research this bug further and for testing out version 1.0 for me.
･ About The Author
Tyson Boucher is a Computer Science student at Oregon State University, where his unending willingness to help others, his insatiable interest in far too many subjects, and his dreams of someday developing more substantial software for the Macintosh have left him with a million unfinished projects and far too little time to complete any of them. <sigh>
You may reach me via email at boucher@engr.orst.edu and you can visit my web page, at <http://www.engr.orst.edu/~boucher/>.
