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.*

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Lyn 1.7.2 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Lyn 1.7.2 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Tunnelblick 3.6.7beta02 - GUI for OpenVP...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
jAlbum Pro 13.4 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
calibre 2.65.1 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
jAlbum 13.4 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results - Simply drag and drop photos into groups, choose a design... Read more
Backblaze 4.2.0.966 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Backblaze 4.2.0.966 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Tunnelblick 3.6.7beta02 - GUI for OpenVP...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
calibre 2.65.1 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more

Siralim 2 (RPG / Roguelike) (Games)
Siralim 2 (RPG / Roguelike) 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Siralim 2 is an old-school monster catching RPG. Summon and customize hundreds of creatures to fight for you as... | Read more »
Clean Text (Productivity)
Clean Text 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Gemini - A Journey of Two Stars (Games)
Gemini - A Journey of Two Stars 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: *** SPECIAL LAUNCH SALE: $2.99 (25% off) *** "A mesmerizing and unexpectedly emotional journey." -- Los... | Read more »
How to get four NFL superstars for your...
Even though you're probably well on your way to building a top notch squad for the new season in Madden NFL Mobile, let's say you could beef it up by adding Rob Gronkowski, Antonio Brown, Von Miller, and Todd Gurley to your roster. That's... | Read more »
Cartoon Network Superstar Soccer: Goal!!...
Cartoon Network Superstar Soccer: Goal!!! – Multiplayer Sports Game Starring Your Favorite Characters 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Become a soccer superstar with your... | Read more »
NFL Huddle: What's new in Topps NFL...
Can you smell that? It's the scent of pigskin in the air, which either means that cliches be damned, pigs are flying in your neck of the woods, or the new NFL season is right around the corner. [Read more] | Read more »
FarmVille: Tropic Escape tips, tricks, a...
Maybe farming is passé in mobile games now. Ah, but farming -- and doing a lot of a other things too -- in an island paradise might be a little different. At least you can work on your tan and sip some pina coladas while tending to your crops. [... | Read more »
Become the King of Avalon in FunPlus’ la...
King Arthur is dead. Considering the legend dates back to the 5th century, it would be surprising if he wasn’t. But in the context of real-time MMO game King of Avalon: Dragon Warfare, Arthur’s death plunges the kingdom into chaos. Evil sorceress... | Read more »
Nightgate (Games)
Nightgate 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: *** Launch Sale: 25% OFF for a limited time! *** In the year 2398, after a great war, a network of intelligent computers known as... | Read more »
3 best fantasy football apps to get you...
Last season didn't go the way you wanted it to in fantasy football. You were super happy following your drafts or auctions, convinced you had outsmarted everyone. You were all set to hustle on the waiver wire, work out some sweet trades, and make... | Read more »

Price Scanner via MacPrices.net

MacBook Airs on sale for up to $101 off MSRP
Amazon has 11″ and 13″ MacBook Airs on sale for up to $101 off MSRP for a limited time. Shipping is free: - 11″ 1.6GHz/128GB MacBook Air (model MJVM2LL/A): $798 $101 off MSRP - 11″ 1.6GHz/256GB... Read more
Apple certified refurbished iPad mini 4s avai...
Apple has certified refurbished iPad mini 4s now available for up to $120 off the cost of new models. An Apple one-year warranty is included with each iPad, and shipping is free. The following models... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Global Tablet Shipments Projected to Increase...
Digitimes’ Jim Hsiao reports that global tablet shipments will increase by 16.3 percent sequentially to reach nearly 47 million units in 2016′s third quarter, but that volume will still be down over... Read more
Apple’s 2016 Back to School promotion: Free B...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB Wi-Fi iPad Air 2: $499 - 64GB Wi-Fi iPad... Read more
13-inch 2.5GHz MacBook Pro available for $961...
Overstock has the 13″ 2.5GHz MacBook Pro available for $961.63 including free shipping. Their price is $138 off MSRP. Read more
Clearance 12-inch Retina MacBooks, Apple refu...
Apple has Certified Refurbished 2015 12″ Retina MacBooks available starting at $929. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
BookBook Releases SurfacePad, BookBook &...
BookBook has released three new covers just for iPad Pro: SurfacePad, BookBook and BookBook Rutledge Edition. BookBook for iPad Pro is a gorgeous leather case reminiscent of a vintage sketchbook.... Read more
Clean Text 1.0 for iOS Reduces Text Cleanup a...
Apimac today announced availability of Clean Text for iOS, a tool for webmasters, graphic designers, developers and magazine editors to reduce text cleanup and editing time, and also for any iPhone... Read more

Jobs Board

*Apple* Retail - Multiple Positions Victor,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* /Mac Support Engineer - GFI Digital,...
FI Digital, Inc. is currently seeking candidates for a full time Apple Support Engineer to add to our Maryland Heights, Missouri IT team. Candidates must be dynamic Read more
SW Engineer *Apple* TV - Apple Inc. (United...
The Apple TV team is looking for excellent software engineers with experience in hardware, media management, media playback, content delivery and a passion for Read more
Senior *Apple* Administrator - Pratt Instit...
POSITION SUMMARY: Directs the coordination and standardization of campus-wide Apple systems, including planning, analysis and implementation of Apple -related Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 51218534 Pleasant Hill, California, United States Posted: Aug. 18, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.