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In Developer Technical Support, we're asked from time to time how to make a GWorld so that its pixel image uses memory on a NuBusTM card rather than memory in the application's heap. The idea is to create a GWorld, put the address of the card into the GWorld, use QuickDraw to draw into the GWorld, which effectively draws into the NuBus card's memory, and display the resulting image on the screen. Doing this in a way that works well with the 8*24 GC version of QuickDraw and with whatever QuickDraw brews up in the future isn't possible without breaking a few guidelines. We'll talk about the reasons for this and what you can do instead. For the more cavalier among you, we'll also talk about the least offensive method of coercing a GWorld so that it uses memory on your NuBus card.

NewGWorld allocates off-screen buffers simply by using the same Memory Manager calls that you can make. To allocate the memory, NewGWorld simply calls NewHandle to allocate the buffer in your application's heap unless you have the useTempMem bit set, in which case it allocates the buffer in temporary memory. It then tries to move the buffer as high in your heap as possible by calling MoveHHi. That's really all there is to it. The GWorld's pixMap, GDevice, and CGrafPort are allocated similarly-- they're all allocated in your heap using regular Memory Manager calls with no special options, patches, or other nefarious tricks.

None of this changes when you have the 8*24 GC software active--all memory is still allocated out of your application's heap. Once you start drawing into the GWorld, though, the GC software can copy the parts of the GWorld to the 8*24 GC memory. The GWorld and its parts still occupy your heap's memory though, regardless of whether it's cached on the 8*24 GC card.

If you have a NuBus card with gobs of memory, NewGWorld can't take advantage of it because the Memory Manager calls that it uses can't allocate memory on NuBus memory cards. There are no options to NewGWorld or any other GWorld calls that let you say, "There's lots of memory over on this NuBus card, all for you." While GWorlds are absolutely fantastic for creating off-screen drawing environments for most of the usual kinds of situations, they're just not appropriate if you want complete control over where or how the parts of a GWorld are allocated.

QuickDraw is the only thing that's supposed to know how GWorlds are constructed. We know that they're CGrafPorts and we can get their pixMap, GDevice, and off-screen buffer, but we shouldn't make any assumptions about how they were allocated and where they are. For example, we know that the off-screen buffer is allocated as a handle now, but that won't necessarily be the case in the future. There's no guaranteed way to tell which way it was allocated, or even if NewGWorld uses the Memory Manager to allocate it at all (which it always does currently, of course). Even the GWorld's CGrafPort is allocated as a handle that just happens to be always locked. If you try to dispose of a GWorld in which you've modified the baseAddr, you'll need DisposeGWorld to make sure everything is deallocated properly, but its behavior is undefined when it tries to deallocate the off- screen buffer. So if you want to use the memory on your NuBus memory card and feel comfortable that you're not relying on something that could change, you're going to have to create your own off-screen drawing environment by creating an off-screen pixMap, a color table if your off-screen drawing environment uses indexed colors, a GDevice, and a CGrafPort. The April 1989 edition of Macintosh Technical Note #120, "Drawing Into an Off-Screen Pixel Map," covers creating your own off-screen pixMap, CGrafPort, and color table, but it requires you to have the same depth and the equivalent color table that the screen has, so it just steals a screen's GDevice. We think it's always a good idea to create your own GDevice when you draw off screen. If you use a screen's GDevice for drawing off screen, you have to depend on that GDevice's depth and color table. By creating your own GDevice, your off-screen drawing environment can use any depth and color table you want at any time and still be insulated from whatever changes the user makes with the Monitors control panel.

To create your own GDevice, it's better not to use NewGDevice because it always creates the GDevice in the system heap; it's better to keep your data structures in your own heap so that they don't get orphaned if your application quits unexpectedly and that precious system heap space is preserved. Here's what you should set each of your GDevice's fields to be:

gdRefNumYour GDevice has no driver, so just set this to 0.
gdIDIt doesn't matter what you set this to; you might as well set it to 0.
gdTypeSet to 2 if your off-screen pixMap uses direct colors (16 or 32 bits per pixel) or 0 if it uses a color table (1 through 8 bits per pixel).
gdITableAllocate a small (maybe just 2-byte) handle for this field. After you're done setting up this GDevice and your off-screen pixMap, color table (if any), and CGrafPort, set this GDevice as the current GDevice by calling SetGDevice, and then call MakeITable, passing it nil for both the color table and inverse table parameters, and 0 for the preferred inverse table resolution.
gdResPrefWe reckon that more than 99.9% of all inverse tables out there have a resolution of 4. Unless you have some reason not to, we'd recommend the same here.
gdSearchProcSet to nil. Use AddSearch if you want to use a SearchProc.
gdCompProcSet to nil. Use AddComp if you want to use a CompProc.
gdFlagsSet to 0 initially, and then use SetDeviceAttribute after you've set up the rest of this GDevice.
gdPMapSet to be a handle to your off-screen pixMap.
gdRefConSet to whatever you want.
gdNextGDSet to nil.
gdRectSet to be equal to your off-screen pixMap's bounds.
gdModeSet to -1. Why? We're not sure. This is intended for GDevices with drivers anyway.
gdCCBytesSet to 0.
gdCCDepthSet to 0.
gdCCXDataSet to 0.
gdCCXMaskSet to 0.
gdReservedSet to 0.

For gdFlags, you should use SetDeviceAttribute to set the noDriver bit. You should also set the gDevType bit to 1 if you're using two bits per pixel or more, but it can be left at 0 if you're using only one bit per pixel.

The other big difference from the technique shown in Technical Note #120 is that the off-screen pixel image shouldn't be allocated. Instead, just point the baseAddr field of your off-screen pixMap at your NuBus card's memory. You should also set the pmVersion field of your off-screen pixMap to be the constant baseAddr32 (equal to 4). That tells Color QuickDraw to use 32-bit addressing mode to access your off-screen buffer, and that's a requirement if your off-screen pixel image is located on a NuBus card.

When you want to draw into your off-screen pixMap, save the current port with a call to GetPort and the current GDevice with a call to GetGDevice. Then set the current port to the off-screen CGrafPort with a call to SetPort, and set the current GDevice to the off-screen GDevice with a call to SetGDevice. Now all QuickDraw commands are drawn off screen and the resulting images are in your NuBus card's memory. To switch back to drawing on screen, set the current port and GDevice back to the port and GDevice that you saved earlier. Easy!

Even with all this, there might still be a reason to use GWorlds to draw into a NuBus memory card. You might just want some quick and dirty way to get an off-screen drawing environment that uses your NuBus memory card and don't care whether it works with future system software releases or not. We'll talk about that next and also discuss the issues that you have to be careful about when you do this.

First, create a GWorld using NewGWorld as usual. If you want to, pass it a color table, or you can just pass it nil if you want it to make the default color table. For the GWorld flags, make sure you pass only the keepLocal flag. This makes sure that all the pieces of the GWorld are kept in your own heap rather than being cached into the 8*24 GC card, even when you draw into it. That way, you avoid running into any conceivable conflicts with GC QuickDraw over where the GWorld really is. There's no way to tell NewGWorld not to allocate the pixel image, so you might want to make the bounds rectangle small and then make it bigger later so that your heap isn't hit up for a lot of memory that you don't even want. Don't pass it an empty rectangle because NewGWorld just gives you a paramErr in that case. Call GetGWorldDevice to get a handle to your GWorld's GDevice and save it for later.

Now it's time to have the new GWorld use your NuBus card's memory. The baseAddr of your GWorld's pixMap is allocated as a handle, and it has to be thrown out. Call GetPixBaseAddr with a handle to your GWorld's pixMap to get a pointer to the pixel image that NewGWorld allocated for you. Call RecoverHandle with that pointer to get a handle to the pixel image, and then call DisposHandle to get rid of it. Now put the address of your NuBus board into the baseAddr of your GWorld's pixMap. Then set the pmVersion field of your GWorld's pixMap to the constant baseAddr32. That tells Color QuickDraw that the baseAddr of the pixMap is a 32-bit address and so it should switch to 32-bit addressing mode whenever it draws into your GWorld.

If you passed NewGWorld a rectangle that's smaller than you actually want, you can now set it to the real size. Set the bounds rectangle of your GWorld's pixMap and the portRect rectangle of your GWorld's CGrafPort to the rectangle that you really wanted. Also, set the visRgn of the CGrafPort and the gdRect field of your GWorld's GDevice to that same rectangle. Your GWorld is ready for use!

Now the bad news. Many of the GWorld routines assume that the baseAddr field is either a real handle or a copy of the handle's master pointer. Because the pointer in the baseAddr field isn't a master pointer, those routines can crash when they expect one. Setting the pmVersion field doesn't help in most cases; these routines just assume that the GWorld's pixel image was allocated by NewGWorld, which is a reasonable assumption. What this implies is that you can no longer call many of the GWorld routines to maintain your GWorld without a risk of crashing. When you call SetGWorld for your GWorld, you should pass it the GWorld's GDevice instead of nil (that's why we recommended that you save the GWorld's GDevice after calling NewGWorld). For safety's sake, don't call any of the following:


You can call DisposeGWorld because it won't get hung up trying to deallocate the pixel image on your NuBus card; setting your pmVersion to baseAddr32 makes this possible. Of course, since all these GWorld routines are off limits, almost all the benefits of having a GWorld at all are gone as well.

Another piece of bad news is that this doesn't take advantage of the speed benefits of using GWorlds with an 8*24 GC card. Most of the speed benefit of using GWorlds with GC QuickDraw is that the GWorld's pixel image is allocated on the 8*24 GC card itself, and so the image data doesn't have to take the time to move across NuBus. If your GWorld draws into a NuBus memory card, the image data has to be moved across NuBus, and so that speed benefit is gone.

The last bit of bad news is that even if you follow all of this, you're still not guaranteed that it will still work in future system software or future video card releases. As we said earlier, this should only be done if you don't care whether it works on future system software releases or not. The description above breaks a lot of rules: don't assume that the pixel image is allocated as a handle; don't set the baseAddr of a GWorld; don't change the dimensions of a GWorld without UpdateGWorld; and don't set the pmVersion field of a GWorld.

You have your choices when you want to use QuickDraw to draw off screen into the memory of a NuBus video card. You can be safe for future compatibility by creating your own off-screen drawing environment from scratch, or you can modify a GWorld so that it uses your NuBus card's memory at the risk of breaking on future systems and at the cost of losing most of the benefits of GWorlds. If you choose the first method and you have no existing routines to create off-screen drawing environments, it's worth it to take a look at Skippy White's Famous High-Level Off-Screen Map Routines in DTS Sample Code #15 on theDeveloper CD Series disc. You can see these routines in action in DTS Sample Code #16. These routines are GWorld-like to some extent, except this time you have the great benefit of source code!


  • "About 32-Bit Addressing," Konstantin Othmer, develop Issue 6, Spring 1991, pp. 36-37.
  • "Deaccelerated _CopyBits & 8*24 GC QuickDraw," Guillermo Ortiz, Macintosh Technical Note #289, January 1991.
  • "Drawing Into an Off-Screen Pixel Map," Jim Friedlander, Rick Blair, and Rich Collyer, Macintosh Technical Note #120, April 1989.
  • Inside Macintosh Volume VI, Graphics Devices Manager chapter, Addison-Wesley, 1991.
  • Inside Macintosh Volume V, Color Manager chapter, Addison-Wesley, 1988.

FORREST TANAKA has been in Developer Technical Support just shy of two years after a stint with unemployment and trying to get a job at Apple. Before that, he got a BSCS while writing Macintosh device drivers for scanners and writing utility software for a PBX. Now he's working with anything that makes images appear on the Macintosh's screen while avoiding anything that makes images appear on paper. Whenever he's not working, eating, sleeping, watching TV, reading, or watching a movie, he's out riding his bike and wondering whether he should shave his legs. *

PAUL SNIVELY, formerly of Apple's DTS group, came to Apple from ICOM Simulations, Inc., the land of the TMON debugger. He wrote the TMON 2.8 User's Guide and has written for MacTutor magazine. His interests include natural- language processing, knowledge representation, adventure-game programming, horror and suspense, hiking, camping, spelunking, and other things better left unsaid.*

For information about inverse tables, see pages 137 through 139 in the Color Manager chapter of Inside Macintosh Volume V.*

Thanks to Guillermo Ortiz for reviewing this column.*

We welcome guest columns from readers who have something interesting or useful to say. Send your column idea or draft to Caroline Rose at Apple Computer, Inc., 20525 Mariani Avenue, M/S 75-2B, Cupertino, CA 95014 (AppleLink: CROSE).*


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