TweetFollow Us on Twitter

June 96 - Graphical Truffles: Dynamic Display Dilemmas

Graphical Truffles: Dynamic Display Dilemmas

CAMERON ESFAHANI and KENT "LIDBOY" MILLER

In the Dark Ages, an application could examine the graphics environment once and gather all the information it needed to know. After the System 7.1.2 Renaissance, the Display Manager made the graphics environment dynamic, which provided many new features (and introduced a few implementation issues). In Issue 24 of develop, the Graphical Truffles column described some important features of the Display Manager. Here we'll discuss some common pitfalls that can cause an application to fail in a dynamic display environment -- and ways you can overcome them.

If you use QuickDraw routines in an existing application, your application may already support some aspects of the Display Manager without requiring any extra work on your part. An example we touch on in this column is the graphics mirroring feature, which allows users to make two graphics devices display the same image. QuickDraw, whose routines have already been updated to support the Display Manager, accomplishes graphics mirroring by overlapping the gdRects (global bounds) of the graphics devices. QuickDraw's internal version of DeviceLoop behaves correctly by detecting when devices overlap, then rendering the image properly for each device. This allows overlapping devices to have different color tables or bit depths and still be imaged correctly.

On this issue's CD, we've included a sample application, SuperFly, which illustrates several techniques you can use to support a dynamic environment in your application. Some of the sample code in this column is excerpted from SuperFly.

COMMON ERRORS

When we were integrating the Display Manager into new system software releases, we encountered some common problems that existing applications had when running in a dynamic display environment. Here's a list of suggestions regarding things that might have worked in the past but won't work now; we'll examine each error in turn and suggest a solution.

  • Don't forget to account for mirrored graphics devices when walking the device list.

  • Don't assume that just because your application uses only one logical display, it's drawing on only one physical device.

  • Don't cache the graphics devices and their state on application startup.

  • Don't assume that the menu bar will never move.

  • Don't assume that menus will be drawn on only one display.

  • Don't draw directly to the screen and bypass QuickDraw without checking for the mirrored case.

  • Don't assume that certain 680x0 registers will contain the same values inside a DeviceLoop drawing procedure as when DeviceLoop was called.

Don't forget to account for mirrored graphics devices when walking the device list. When writing applications in the past, some programmers assumed that graphics devices would never overlap. For example, you might assume that if a certain rectangle is fully contained within a gdRect, it isn't on any other device. To implement highlighting, your application might walk the device list and invert the selection if the global rectangle of what you want to highlight intersects the gdRect of that device. However, when there are two displays with the same gdRect in the device list, the first inversion accomplishes the highlighting but the second inversion restores the highlighted area to the original -- unhighlighted -- state.

Solution: Use DeviceLoop for your drawing. If you want to write your own version of DeviceLoop for some reason, make sure that it handles overlapping displays. You could solve the inverting problem by designing an algorithm to guarantee that each rectangle in global space is highlighted only once. The MyHiliteRect routine in Listing 1 is an example of a suitable algorithm.

The code in Listing 1 solves the highlighting problem by keeping track of the global area that has been highlighted. When DMGetNextScreenDevice returns a mirrored device (which will already have been highlighted by the first QuickDraw call), the SectRgn will fail and that device will not be highlighted again.

Listing 1. Highlighting a global rectangle only once

OSErr MyHiliteRect(Rect* hiliteRect)
{
   RgnHandle    hiliteRgn, gdRectRgn, tempRgn;
   GDHandle    theGDevice;
   
   hiliteRgn = NewRgn();
   if (hiliteRgn == nil) 
      return (QDError());
   gdRectRgn = NewRgn();
   if (gdRectRgn == nil) 
      return (QDError());
   tempRgn = NewRgn()
   if (tempRgn == nil)
      return (QDError());
      
   // Make hiliteRect into a region.
   RectRgn(hiliteRgn, hiliteRect);
   
   // Get the first screen device from the Display Manager.
   // Tell it to return only active screen devices so that we won't
   // have to check here.
   theGDevice = DMGetFirstScreenDevice(true);
   
   // Loop until we run out of hiliteRgn or GDevices.
   while ((theGDevice) && (!EmptyRgn(hiliteRgn)) {
      // Does this device's rect intersect hiliteRgn?
      RectRgn(&(**theGDevice).gdRect, gdRectRgn);
      SectRgn(hiliteRgn, gdRectRgn, &tempRgn);
      // If it does, highlight it.
      if (!EmptyRgn(tempRgn)) {
         // Highlight the area described by tempRgn.
         ...
         // Take the area we just highlighted out of the region to
         // highlight. 
         DiffRgn(hiliteRgn, tempRgn, hiliteRgn);
      }
      theGDevice = DMGetNextScreenDevice(theGDevice, true);
   }   
   DisposeRgn(hiliteRgn);
   DisposeRgn(gdRectRgn);
   DisposeRgn(tempRgn);
}
Another solution is given in the sample code in the GDeviceUtilities.cp file on this issue's CD. The function BuildAListOfUniqueDevices builds a list of all graphics devices but eliminates mirrored devices. An application could cache this list and use it for highlighting. However, the list could be invalidated if the user changes the device configuration. The application should register with the Display Manager so that it will be notified (through a notification callback or an Apple event) if the graphics world has changed.

Don't assume that just because your application uses only one logical display, it's drawing on only one physical device. Some applications assume that they're using only one piece of graphics hardware when they're actually using multiple physical devices. An example of this is a multimedia player that searches through graphics devices and uses the first device it finds that meets its criteria for bit depth or size. This technique causes a problem when the application uses Toolbox calls specific to one physical graphics device, such as using SetEntries to animate the color table. If mirroring is turned on, this changes the color table of only one device; the second physical device still has the old color table.

Solution: If you use Toolbox calls specific to one physical graphics device, make sure you do it for all devices that overlap your application's windows and not just the first one you find. As shown in Listing 2, you could use DeviceLoop to accomplish this by calling SetEntries in your DeviceLoop drawing procedure. Or you could use the Palette Manager instead of SetEntries.

Listing 2. Calling SetEntries for overlapping devices

OSErr MySavvySetEntries(WindowRef aWindow, CTabHandle newColorTable)
{
   RgnHandle              tempWindowStructRgn;
   DeviceLoopDrawingUPP   setEntriesDeviceLoopRD;
   OSErr                  theErr = noErr;

   tempWindowStructRgn = NewRgn();
   // Was there a problem making the region?
   if ((theErr = QDError()) != noErr)
      return (theErr);
   GetWindowStructureRgn(aWindow, tempWindowStructRgn);
   
   // We want to get called for every display that intersects our
   // window.
   setEntriesDeviceLoopRD =
       NewDeviceLoopDrawingProc(SetEntriesDeviceLoop);
   DeviceLoop(tempWindowStructRgn, setEntriesDeviceLoopRD,
       (long) newColorTable, singleDevices);
   DisposeRoutineDescriptor(setEntriesDeviceLoopRD);
   DisposeRgn(tempWindowStructRgn);
   return (theErr);
}

static pascal void SetEntriesDeviceLoop(short depth,
      short deviceFlags, GDHandle targetDevice, long userData)
{
#pragma unused(depth, deviceFlags)
   CTabHandle   newColorTable = (CTabHandle) userData;
   GDHandle      savedCurrentGDevice;

   // Since we'll be changing the current GDevice, we need to save
   // and restore it.
   savedCurrentGDevice = GetGDevice();

   // SetEntries applies to the current GDevice, so make targetDevice
   // the current GDevice.
   SetGDevice(targetDevice);

   // Insert the entire table into targetDevice. Do it in indexed
   // mode.
   SetEntries(-1, 255, &(**newColorTable).ctTable[0]);

   // Restore the old current GDevice.
   SetGDevice(savedCurrentGDevice);
}
Don't cache the graphics devices and their state on application startup. With the Display Manager, many things about the graphics world can change, such as the following:

  • Users can change resolutions on multiple-scan displays.

  • Users can deactivate displays with the Monitors & Sound control panel (system software version 7.5.2 and later).

  • Graphics mirroring could be off when the application is launched and turned on while it's running.

  • Users can put their PowerBooks to sleep and attach or remove an external display.
If your application doesn't respond to these changes, it might present an inconsistent interface to the user. For example, the MPW Shell used to cache the gdRect of the main display and pass it to SizeWindow and MoveWindow. So if users changed the resolution of a display, they couldn't grow or move windows beyond the previous size of the display.

Solution: If the Display Manager is present, you should watch for Display Manager notifications to detect changes in the graphics world. (See "Receiving and Responding to Display Manager Events.") Specialized pieces of code, such as extensions or components, can register a callback procedure with the Display Manager. To avoid problems, you should use this method instead of patching to determine when the display environment changes. (Display Manager 2.0 and later even notifies you when the bit depth changes.) Better yet, don't cache device information at all unless your code absolutely needs the extra ounce of performance.


    RECEIVING AND RESPONDING TO DISPLAY MANAGER EVENTS

    The Display Manager has been available since System 7 and is built into system software version 7.1.2 and later. If the Display Manager isn't around, the application can count on devices not moving around and changing sizes.

    An application must set the isDisplayManagerAware flag in its 'SIZE' resource to receive Display Manager events in its main event loop. If your application sets this flag, it's responsible for moving its windows if the graphics world changes, making sure that the windows all remain visible. If it doesn't set this flag, the Display Manager will automatically move windows for your application. Even if your application lets the Display Manager handle its windows automatically, it can still register for Display Manager events by using a callback procedure. The callback procedure is passed an Apple event that the application has to parse.

    The Graphical Truffles column in develop Issue 25 describes this in more detail.


The Display Manager passes all the information about the old and new display configurations to the application when the world changes. The Apple event-handling code on this issue's CD shows some ways to handle Display Manager events. For instance, to detect whether any of the graphics devices have moved, we parse the Apple event and compare the old and new gdRects:
AEGetNthDesc(&DisplayID, 1, typeWildCard, 
   &tempWord, &OldConfig);
AEGetKeyPtr(&OldConfig, keyDeviceRect,
   typeWildCard, (UInt32 *) &returnType,
   &oldRect, sizeof(Rect), nil);
AEGetNthDesc(&DisplayID, 2, typeWildCard,
   &tempWord, &NewConfig);
AEGetKeyPtr(&NewConfig, keyDeviceRect,
   typeWildCard, (UInt32 *) &returnType, 
   &newRect, sizeof(Rect), nil);      
if (!EqualRect(&oldRect, &newRect))
   HandleGDevicesMoved();
Don't assume that the menu bar will never move. This is especially a problem with games or multimedia applications that would like to hide the menu bar. Most applications hide the menu bar by adding the menu bar area to the gray region. (The pointer to the gray region is stored in the GrayRgn global variable and can be retrieved with the Window Manager function GetGrayRgn.) When the user moves the menu bar from one display to another (using the Monitors or Monitors & Sound control panel), the Display Manager reconstructs the gray region. If the application wants to show the menu bar again, it removes the old menu bar area from the rebuilt gray region, but this will not accurately reflect the available screen real estate. This top strip, where the menu bar was, would be lost.

As shown in Figure 1, the menu bar was on the display on the left before the application was launched. The application saved the menu bar location and added it to the gray region so that it could draw there. As shown in Figure 2, when the application quit, it subtracted the old menu bar area back out of the gray region. Since the menu bar was moved, this part of the gray region is now lost. No desktop is drawn where the menu bar used to be (the black rectangle).

Figure 1. Original location of menu bar

Figure 2. Gray region lost because menu bar movedSolution: Become Display Manager-aware and get notified by the Display Manager when the menu bar moves.

Don't assume that menus will be drawn on only one display. In the past, it was a safe assumption that the menu bar would be drawn on only one device at a time, so anything that wanted to draw in the menu bar (such as an MDEF) needed to know about only one display and a single bit depth. Since developers typically don't use DeviceLoop to draw menus or draw in the menu bar, sometimes images are drawn incorrectly when there are overlapped displays, especially on displays with different bit depths. An example would be a menu that contains color, like the Label menu in the Finder.

Solution: If you draw directly to the menu bar or to your menus and bypass QuickDraw, use DeviceLoop. Note that drawing in the menu bar with standard QuickDraw calls works fine in the mirrored case because QuickDraw takes care of the overlapping case for you.

Don't draw directly to the screen and bypass QuickDraw without checking for the mirrored case. Obviously, if an application draws directly to the screen and mirroring is on, the other displays will not reflect any of the drawing.

Solution: Always allow the user to go back to a more "compatible" mode that uses QuickDraw. If the application detects that mirroring is on (by calling DMIsMirroringOn), consider falling back to CopyBits to get your data to the screen.

Don't assume that certain 680x0 registers will contain the same values inside a DeviceLoop drawing procedure as when DeviceLoop was called. We discovered this bug when we made DeviceLoop PowerPC-native for mirroring performance. Some developers who write their DeviceLoop drawing procedure in 680x0 assembly language rely on the fact that the value of A6 when the drawing procedure was called is the same as when DeviceLoop was called. Application developers relied on this to enable them to share stack frames between the caller of DeviceLoop and the DeviceLoop drawing procedure. This will not work if a PowerPC-only version of DeviceLoop is present, so it will not work under Mac OS 8. If you write your DeviceLoop drawing procedure in a high-level language like C, however, you don't have to worry about this problem.

Solution: If you want to share data with your DeviceLoop drawing procedure, use the userData refCon supplied with DeviceLoop. If you need to rely on A6 remaining constant, one solution would be to pass in the A6 of the DeviceLoop caller in the userData parameter and set that to the A6 of the DeviceLoop drawing procedure. Be sure to save and restore the original A6 of the DeviceLoop drawing procedure.

SUCCEEDING IN A CHANGING GRAPHICS WORLD

The Display Manager offers many new features that enable users to configure their graphics devices dynamically. However, this dynamic display environment invalidates certain assumptions that developers might have made when programming in a static graphics environment. This column should start you thinking about these issues. Although the Display Manager does attempt to preserve compatibility with existing applications by moving windows around and preserving graphics device information, it can't fix everything. Your application needs to be able to function in a changing graphics world.

KENT "LIDBOY" MILLER, in a recent attempt to reshape the course of history, has renounced the use of caffeine on a by-minute basis. Lidboy hails from the halls of Apple, where he can be seen pacing in his fuzzy bear slippers. He considers the pinnacle of western culture to have been achieved by the rock group known as Rancid, although he occasionally reads from literary quarterlies on the sly. Were Lidboy to be granted one wish, a side of rice from Taco Bell would no doubt be involved. The single most used word in his vocabulary is "Salsa!"*

CAMERON ESFAHANI (cameron_esfahani@powertalk.apple.com, AppleLink DIRTY) began his career in the spaghetti westerns of Sergio Leone. Industry analysts felt that by going off into that area he would cut himself off from the mainstream and ruin his career, but Cameron felt it was more important to follow his dreams. Now at Apple Computer and looking back, he feels that his spaghetti western period was one of the most exciting and rewarding of his life. He therefore dedicates this column to the memory of Sergio Leone.*

Thanks to Tom Dowdy, Ian Hendry, Mike Marinkovich, and Mike Puckett for reviewing this column.*

 
AAPL
$101.79
Apple Inc.
+0.21
MSFT
$46.68
Microsoft Corpora
+0.16
GOOG
$589.27
Google Inc.
+4.50

MacTech Search:
Community Search:

Software Updates via MacUpdate

iMazing 1.0 - Complete iOS device manage...
iMazing (formerly DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and... Read more
Xcode 6.0.1 - Integrated development env...
Apple Xcode is Apple Computer's integrated development environment (IDE) for OS X. The full Xcode package is free to ADC members and includes all the tools you need to create, debug, and optimize... Read more
Apple Safari 7.1 - Apple's Web brow...
Apple Safari in OS X Mavericks brings you all-new ways to find and enjoy the best of the web. It works with iCloud to give you a seamless browsing experience across all your devices. It looks out for... Read more
Delivery Status 6.1.2 - Check delivery s...
Delivery Status displays delivery status of packages for a variety of shipment services. Can't wait for your packages to arrive? Don't waste your time checking the site constantly, just open this all... Read more
Mavericks Cache Cleaner 8.0.9 - Clear ca...
Mavericks Cache Cleaner is an award-winning general purpose tool for OS X. MCC makes system maintenance simple with an easy point-and-click interface to many OS X functions. Novice and expert users... Read more
OneNote 15.2.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Apple Configurator 1.6 - Configure and d...
Apple Configurator makes it easy for anyone to mass configure and deploy iPhone, iPad, and iPod touch in a school, business, or institution. Three simple workflows let you prepare new iOS devices... Read more
SpamSieve 2.9.16 - Robust spam filter fo...
SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering. SpamSieve understands what your spam looks like in order to block it all, but also learns what... Read more
OS X Server 3.2.1 - For OS X 10.9.5 Mave...
OS X Server is the next generation of Apple's award winning server software. Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, host... Read more
Apple Security Update 2014-004 - For OS...
Apple Security Update is recommended for all users and improves the security of Mac OS X. For information on the security content of this update, please visit this website: http://support.apple.com/... Read more

Latest Forum Discussions

See All

Down Among the Dead Men (Games)
Down Among the Dead Men 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Avast! Take to the high seas in a fully interactive piratical tale of broadsides and buccaneers. From author Dave... | Read more »
Sling Adds Chromecast Support Through Sl...
Sling Adds Chromecast Support Through Slingplaye​r Mobile Apps Posted by Jessica Fisher on September 18th, 2014 [ permalink ] | Read more »
How to Completely Delete Your iPhone’s C...
The iPhone 6 is out tomorrow, and plenty of people are excited about it. So much so that they’re planning to – or already have – traded in their old iPhone to go towards it. The thing about trading in hardware is it’s very important to make sure... | Read more »
Dragon Quest I Review
Dragon Quest I Review By Andrew Fisher on September 18th, 2014 Our Rating: :: THINE QUEST AWAITETHUniversal App - Designed for iPhone and iPad Its historical significance aside, Dragon Quest 1 is a fun, campy, difficult, thoroughly... | Read more »
It Came From Canada: Overkill 3
Overkill 3 is like every trope of big modern gaming rolled into one. It’s a sequel to an action-packed military shooter. It’s flashy and scripted and flaunts its sophisticated graphics. And it’s a mobile game with a heavy emphasis on in-app... | Read more »
New Modes and Leader Boards in Update fo...
New Modes and Leader Boards in Update for Rules! Posted by Jessica Fisher on September 18th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
TwistedRun Review
TwistedRun Review By Rob Thomas on September 18th, 2014 Our Rating: :: DON'T TWIST YOUR ANKLE!Universal App - Designed for iPhone and iPad TwistedRun is kind of like running up a giant curly fry into the sky. Or maybe that was just... | Read more »
Scope Review
Scope Review By Jennifer Allen on September 18th, 2014 Our Rating: :: LOCATION AWAREiPhone App - Designed for the iPhone, compatible with the iPad Want to easily find photos from around the world based on their location? Scope is a... | Read more »
HipstaFox Review
HipstaFox Review By Jordan Minor on September 18th, 2014 Our Rating: :: FANTASTIC MR. FOXUniversal App - Designed for iPhone and iPad HipstaFox is a great single that makes players long for the whole album.   | Read more »
Ninja Raft (Games)
Ninja Raft 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: ** Special Launch Price ** "Ninja Raft is definitely the game to play if you’re into Tower Defense games and want to play something... | Read more »

Price Scanner via MacPrices.net

Save up to $300 on the price of a new Mac wit...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
13-inch 2.8GHz Retina MacBook Pro available f...
B&H Photo has the new 2014 13″ 2.8GHz Retina MacBook Pro on sale for $1699.99 including free shipping plus NY sales tax only. They’ll also include free copies of Parallels Desktop and LoJack for... Read more
16GB iPad Air on sale for $449, save $50
Walmart has the 16GB iPad Air WiFi on sale for $449 on their online store for a limited time. Choose free home shipping or free local store pickup. Their price represents a $50 savings over standard... Read more
13-inch 256GB MacBook Air on sale for $1099,...
B&H Photo has the 2014 13″ 1.4GHz 256GB MacBook Air on sale for $1099.99. Shipping is free, and B&H charges NY sales tax only. Their price is $100 off MSRP. Read more
Toshiba Introduces TransMemory ID High-Speed...
Toshiba’s Digital Products Division (DPD), a division of Toshiba America Information Systems, Inc., today introduced the TransMemory ID USB 3.0 Flash Drive, a simpler storage solution for people who... Read more
New iPads and OS X Yosemite Release Coming Oc...
The DailyDot’s Micah Singleton reports that Apple is planning to hold its next product announcement event on Oct. 21, at which it will unveil the iPad Air 2 and iPad mini 3 and release a final build... Read more
Logitech Bluetooth Multi-Device Cross-Platfor...
Logitech has an enviable track record of making some of the best computer keyboards and mice. At least in my estimation, the best freestanding keyboards I’ve ever used have been Logitech units,... Read more
Roundup of Apple refurbished iPad Airs and iP...
Apple is offering Certified Refurbished iPad Airs for up to $140 off MSRP. Apple’s one-year warranty is included with each model, and shipping is free. Stock tends to come and go with some of these... Read more
Sprint offers 16GB iPad mini for $199.99 with...
Sprint is offering 1st generation 16GB iPad minis for $199.99 with a 2-year service agreement. Standard MSRP for this iPad is $429. Their price is the lowest available for this model. Read more
2.5GHz Mac mini remains on sale for $549, sav...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more

Jobs Board

Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.