Apple has specified optional video driver calls called SetGamma and GetGamma to set and retrieve gamma correction tables. Although this is well documented for developers, there is no easy way for non-programmers to retrieve, adjust, and save the gamma correction for the video board and monitor being used.
The RasterOps Gamma Utility is a desk accessory that allows you to create the desired gamma correction and save it in (1) the System file, (2) a RasterOps Monitors Extension file, or (3) a resource file readable by ResEdit™.
When the gamma correction is saved in the System file, it is permanently saved and applied to the selected monitor(s) every time the machine is booted.
When the gamma correction is saved on a RasterOps Monitors Extension file, that gamma correction is added to a list of gamma corrections that can be selected and permanently applied using the Monitors cdev.
Gamma corrections saved in a resource file can be used later by an experienced Macintosh user to modify the System file, a RasterOps Monitors Extension file or a custom application that uses 'gama' resources.
See instructions below for more details.
SYSTEM REQUIREMENTS
This desk accessory runs on any Macintosh II Family computer with System 6.0.4 or later.
Monitors cdev 4.0 or later is recommended. At least one video board in your system must support the optional video driver call SetGamma.
INSTRUCTIONS FOR THE IMPATIENT
1. Run the Gamma desk accessory.
2. Using the Monitor pop-menu select the monitor you want to adjust.
3. Adjust the brightness, contrast, or gamma arrows or type in the gamma value.
4. Click the Save button.
5. Your gamma correction will be saved in the System file as a 'gama' resource with the name 'Untitled' and remembered when you reboot.
INSTRUCTIONS (MORE DETAILED)
When the Gamma Utility is launched it tests every video board in the machine to see if it supports SetGamma. Starting with the menubar screen, it then reads the gamma correction from the first gamma correction-capable board and displays the correction ramps. You are now ready to make adjustments and save the results.
What follows is brief explanation of the controls in the RasterOps Gamma desk accessory window.
UNCORRECTED: Loads a linear (uncorrected) table into the active ramps.
INVERT: Inverts the active ramps.
APPLE: Loads the standard Apple gamma correction into the active ramps.
MIN and MAX: Sets the active ramps to either the minimum or maximum value respectively. These buttons are disabled when all three ramps are active to prevent you from creating a completely black or completely white screen.
GAMMA ARROWS: Increases or decreases the gamma correction depending on the arrow depressed. Increasing gamma darkens the screen; decreasing gamma brightens the screen. The current gamma value is displayed in the gamma edit text field.
CONTRAST ARROWS: Increases or decreases the contrast depending on the arrow depressed. Changing contrast changes the slope of the gamma curve in the active ramps. The current gamma value is not displayed when contrast is modified.
BRIGHTNESS ARROWS: Increases or decreases the brightness depending on the arrow depressed. Changing brightness shifts the gamma curve up or down in the active ramps. The current gamma value is not displayed when brightness is modified.
GAMMA EDIT FIELD: Type in the desired gamma correction and hit the RETURN or ENTER key. The valid range is 0.005 to 99.0. If you enter an invalid value, the program resets it to 1.0. Typical monitor gamma correction is about 0.45.
MONITOR POP-UP MENU: Use this to select the video board and monitor you want to adjust. The monitors are numbered as in Apple's Monitors control panel, that is, in increasing slot order. When you select a new monitor, its gamma correction table is loaded into the ramps.
RED, GREEN, and BLUE CHECK BOXES: Click these to select the active channels (ramps). By default, all three channels are active. You can, for example, make changes only to the red channel by turning off the green and blue.
MAKE CHANGES TO: Click the appropriate radio button to indicate where changes are to be made. See OPEN, SAVE, and DELETE below. By default, changes are made to the System file.
GAMMA NAME EDIT FIELD: Type in the desired name for your gamma correction. This name will be used to identify the 'gama' resource being saved, deleted, or opened. The default name is "Untitled". You should give meaningful names for future reference, such as "Sony 0.45" or "Frank's 19" screen".
OPEN: Load a 'gama' resource with the current gamma name from either the System, RasterOps Monitors Extension, or a resource file. The gamma correction table will be loaded into the correction ramps. If the 'gama' resource of the current name cannot be found, the correction ramps are not modified.
SAVE: Save the current gamma correction as a 'gama' resource with the current gamma name in either the System, RasterOps Monitors Extension, or a resource file.
When the gamma correction is saved in the System file, it is permanently saved and applied to the selected monitor every time the machine is booted.
When the gamma correction is saved on a RasterOps Monitors Extension file, that gamma correction is added to a list of gamma corrections that can be selected and permanently applied using the Monitors cdev. To do this after saving the current gamma correction:
1. Run the Control Panel desk accessory.
2. Run the Monitors cdev.
3. Select the monitor you want to use by clicking on the Monitors icon if it isn't already highlighted.
4. Hold down the Option key and double click on the selected Monitors icon.
5. Click the "Use Special Gamma" checkbox.
6. Select the gamma correction you want to apply. The name of the gamma correction you saved with Gamma desk accessory should appear in the list. Scroll the list, if necessary, to find the desired gamma correction.
7. Click the OK button. The gamma correction you selected will be always applied when you reboot until you change it.
(What really happens here is the 'gama' resource that you saved in the RasterOps Monitors Extension file is copied to the System file and the System's 'scrn' resource is modified to use this 'gama' resource for the monitor you selected.)
DELETE: Delete a 'gama' resource with the current gamma name from either the System, RasterOps Monitors Extension, or a resource file. If you delete a 'gama' resource from the System or from the RasterOps Monitors Extension file that was being used to correct your monitor, the default Apple gamma correction table will be applied when you reboot.
ABOUT...: Click this to see the effects of changing gamma values and changing contrast. Click again to get a boring dialog.
CLOSE BOX: Click the close box in the upper left corner of the window to quit the RasterOps Gamma Utility.
BUGS, HINTS, & MISFEATURES
1. If you have more than one 'gama' resource with the same name in a resource file, you are able to open only the "first" one. An improved version of Gamma Desk Accessory will allow you to select which resource with a unique id you wish to open.
2. If you create a weird table that is not a exponential gamma curve, hit the RETURN or ENTER key, and you will get an uncorrected table (gamma = 1.0).
3. If you move video boards to different slots or if you remove video boards, any gamma corrections you saved in the System file (either directly or through the RasterOps Monitors Extensions and Monitors cdev) will not be used. The default Apple gamma table will be used.
4. If your Macintosh, such as the Macintosh IIsi or the Macintosh LC, does not have a Floating Point Unit (FPU) the Gamma DA will perform much slower since floating point calculation are used to compute the gamma curve.
If you have any questions or problems, contact RasterOps Technical Support at 408-562-4200 (voice), 408-562-4065 (FAX), AppleLink address RASTERHELP, or Connect address RASTERHELP.
