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Most people who've done any graphics programming on the Macintosh are aware of the Palette Manager, because it's the documented way to control the on-screen color environment, and perhaps because my cohorts and I in Developer Technical Support keep going on about how right the world would be if everyone used it. In an effort to follow the rules as best they can, some people have taken the Palette Manager so much to heart that they use it not only with windows, but with off-screen cGrafPorts as well--something that isn't heard about very much. Some of these people have concluded that all the features of the Palette Manager apply just as well to off-screen cGrafPorts as they do to windows. Logical enough, right?

Well, that's the kick; whether this is logical or not, the truth is that only a small part of the Palette Manager works with off-screen cGrafPorts. Specifically, the pmCourteous usage mode and the pmWhite and pmBlack usage-mode modifiers work fine when they're used in a palette that's attached to an off-screen cGrafPort, but the pmTolerant, pmAnimated, and pmExplicit usage modes do not. In this column, I'll describe how you can take advantage of the Palette Manager features that work off-screen and how you can simulate the features that don't work.

The pmCourteous usage mode seems pretty useless to a lot of people because it has no effect on the current color environment. But in general, making a palette full of pmCourteous colors is a lot better than hard-coding RGBColors into your code. Instead of hard-coding colors, make a palette of courteous colors--as many entries as you need colors--and save it as a 'pltt' resource. When your application runs, call SetPalette to attach this palette to your off-screen cGrafPort. When you need to use a color while drawing into this cGrafPort, pass the desired color's palette index to PmForeColor or PmBackColor, and then draw. This is better than hard-coding colors because you or a software localizer can easily change the colors by changing the 'pltt' resource--no code changes are necessary.

The pmWhite and pmBlack usage-mode modifiers are new with System 7; they let you specify whether you want a particular palette entry to map to white or black in a black-and-white graphics environment. By default, colors whose average color-component value is larger than 32767 are mapped to white and other colors are mapped to black. (If you use RGBForeColor, Color QuickDraw also checks to see whether your specified color is different from your background color but maps to your background color; if so, Color QuickDraw uses the complement of the color you specified so that your drawing is visible over the background.) By specifying that a palette entry is pmCourteous + pmBlack or pmCourteous + pmWhite, you can control which colors map to black and to white when there aren't enough colors available. This applies to palettes attached to off-screen cGrafPorts as well as to palettes attached to windows.

Those are the Palette Manager features that do work off-screen. Now I'll talk about the features that don't and what you can do to get the same effect. The pmExplicit usage mode is handy when you want to draw using a pixel value without knowing or caring what color that pixel value represents. With this mode you can easily show the colors in a screen's color table, and you can also draw into a pixel image with a specific value even though you specify the color for that value elsewhere.

When you have a palette that's attached to an off-screen cGrafPort, pmExplicit colors are interpreted as pmCourteous colors. Instead of using a palette, you should convert your pixel value to an RGBColor and use this as the foreground or background color. Set the current GDevice to your off- screen GDevice so that the color environment is set; then pass your pixel value to Index2Color, which is documented on page 141 ofInside Macintosh Volume V. Index2Color converts your pixel value to the corresponding RGBColor, which you can pass to RGBForeColor or RGBBackColor, and then you can draw. The result is that your pixel value is drawn into the destination pixel image.

Both the pmAnimated and pmTolerant usage modes are used to modify the color environment, and both are interpreted as pmCourteous when they're in a palette that's attached to an off-screen cGrafPort. The most important difference between the two usage modes is in the style of color-table arbitration that they do-- pmTolerant gives the front window the colors it needs, while pmAnimated additionally makes sure that nothing outside the front window is drawn in its colors. Color-table arbitration doesn't apply off screen, so the pmAnimated and pmTolerant usage modes can be unified into "I want to change my off-screen colors."

Changing the colors in an off-screen color environment means changing its color table; the most straightforward way to do this is to modify the contents of the color table directly. That is, get your off-screen color table's handle and then directly assign new values to the rgb fields in its CSpecArray. You could also assign a whole new color table to the off-screen environment by assigning the new one to the pmTable field of the off-screen pixMap. Either way, you have to tell Color QuickDraw what you've done by updating the changed color table's ctSeed field. The next time you draw into your off-screen graphics environment, Color QuickDraw detects your change by comparing the ctSeed of your changed color table against the iTabSeed of the current GDevice's inverse table, and it rebuilds the inverse table according to the changed color table. You can update the ctSeed field by assigning to it the return value of GetCTSeed, which is documented on page 143 ofInside Macintosh Volume V. If the 32-Bit QuickDraw extensions are available, you can update a color table's ctSeed simply by passing the color table to CTabChanged, documented on page 17-26 ofInside Macintosh Volume VI.

If you have a GWorld and you want to replace its color table, you should call UpdateGWorld, passing it a new color table. UpdateGWorld makes sure that all the cached parts of a GWorld are properly updated, which is tough to do any other way. If you don't pass any flags to UpdateGWorld, it's within its rights to destroy your existing GWorld's image. But if you pass the clipPix or stretchPix flag, UpdateGWorld is obligated to keep your existing image, and it tries to reproduce the existing image in the new colors as best it can.

To wrap up, you can use the Palette Manager with off-screen graphics environments, but you'll only be able to use the pmCourteous usage mode and the pmWhite and pmBlack usage-mode modifiers. But that's not to cast aspersions on these features, because they can be very handy for both on-screen and off-screen drawing. The pmExplicit, pmTolerant, and pmAnimated usage modes don't work for off-screen drawing, but there are easy ways to simulate those features without the Palette Manager and without risking future compatibility.

FORREST TANAKA has been playing Developer Technical Support as one of the graphics support people for slightly more than two years. "It amazes me still," he says, "that the more you learn about the Macintosh graphics tools, the farther off total understanding seems to be." Outside of DTS, he likes to ride his bike, and uses it to commute the three blocks to his office ("Hey, it's faster than driving the three blocks!"), and he likes to try getting his radio-controlled car to act as if it's actually controlled.*

PRINT HINTS FROM LUKE & ZZ is in hibernation.*

For more details about changing or replacing off-screen color tables, see the October 1991 version of Macintosh Technical Note #120, "Principia Off-Screen Graphics Environments." *


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