TweetFollow Us on Twitter

May 92 - GRAPHICAL TRUFFLES

GRAPHICAL TRUFFLES

MULTIPLE SCREENS REVEALED

FORREST TANAKA AND BILL GUSCHWAN

[IMAGE Tanaka_final_draft_rev1.GIF]

One very neat feature of the Macintosh is that you can connect more than one screen to the computer and use them as if they were one big screen. Better still, applications take advantage of multiple screens automatically. But the screens that are attached to your system can have different sizes, depths, and color tables, and you might want to optimize your application for each screen, or you might want to find the best screen to display something on. Both these things are easy to do, but not necessarily in the ways that you might think at first. In this column, we'll uncover a few important truths about QuickDraw's handling of multiple screens, and we'll talk about a few ways to deal with multiple screens if you want to go beyond what QuickDraw gives you for free.

It's important to understand that if you're just drawing items to a window and want to stay completely above the specifics of different screens, don't do anything special--just draw to your window as if there were one screen. QuickDraw was designed to make multiple screens look like one, so you should take advantage of this valuable abstraction if you can. Note too that machines with original QuickDraw can also have multiple screens, but we don't describe that here.

Truth #1: Windows don't change their depth or color table when they're moved to different screens.

One of the most common misconceptions about multiple screens is that a window's pixMap holds the size, depth, and color table of the screen that the window is on. That seems logical enough at first glance, especially considering that each screen has its own pixMap. But it's not true, because a window can cross more than one screen. Instead, the pixMap of a window always holds the depth, color table, and bounds rectangle of the main screen (the one with the menu bar) even if the window is nowhere near the main screen. The pixMap of a window is, in essence, a copy of the main screen's pixMap, except for one detail: the bounds rectangle of a window's pixMap is in the local coordinates of the window while the bounds rectangle of the main screen's pixMap is in global coordinates. In fact, any screen's pixMap has a bounds rectangle that's in global coordinates, indicating that screen's position relative to the main screen.

To find the sizes, depths, or color tables of the screens your window is on, you should use the list of GDevices that the system maintains (usually called thedevice list ), which gives you the pixMap of each screen. We'll describe a method of using the device list later.

Truth #2: There are exactly two coordinate systems.

With multiple screens, it's easy to get confused by what looks like many coordinate systems, but there are only two: the local coordinate system of the current port and the global coordinate system. QuickDraw has no concept of a coordinate system for each screen. Global screen coordinates are always relative to the main screen--the global coordinate (0,0) is always at the extreme upper left corner of the menu bar. All coordinates in a graphics port are local coordinates, including the bounds rectangle of the port's pixMap. This bounds rectangle has two purposes. First, it defines the area of a pixel image that QuickDraw can draw into. Second, the top left point of the bounds rectangle is the horizontal and vertical distance from the origin of the local coordinate system to the origin of the global coordinate system. Specifically, if you subtract the coordinate of the top left corner of the bounds rectangle from all the other coordinates in a port, you convert those coordinates into the equivalent global coordinates.

An example of the relationship between the portRect of a window and the bounds rectangle of its pixMap is shown in the following figure. The two screens in the example are next to each other and are both 640 pixels across and 480 pixels down, with the main screen on the left, and the window is contained entirely on the second screen. Global coordinates are marked around the corners of the screens and the portRect and bounds rectangle are marked with a dashed outline. Notice that the bounds rectangle circumscribes the main screen, and it's in the local coordinates of the window. If you subtract the components of the bounds rectangle's top left corner from the coordinates of the portRect, you get the rectangle [T:25 L:660 B:325 R:1160], which is the portRect in global coordinates.

[IMAGE TanakaFig1.gif]

Truth #3: QuickDraw switches to the GDevice of each screen your drawing crosses as it's drawn.

When you draw something to a window, QuickDraw searches the device list for every GDevice whose gdRect intersects your drawing. For each intersecting GDevice, QuickDraw makes it the current GDevice and then draws the intersecting part of your drawing. Switching GDevices is important because the current GDevice provides the current color environment, which tells the system what color corresponds to each pixel value and vice versa. As QuickDraw draws across your screens, it keeps switching the current GDevice to the one for the screen it's actively drawing to.

Color environments are specific to each screen. Compare this with grafPorts and cGrafPorts, which provide the screen-independent drawing environment that tells the system things like the pattern, pen size, and color to use when drawing something. Each window gets its own drawing environment, but has to share the color environments with other windows.

Therefore, you should never switch GDevices to have QuickDraw draw to a specific screen-- QuickDraw switches GDevices as appropriate. Whenever you have QuickDraw draw to any screen, the current GDevice should be the main screen's GDevice, which it is by default. The only time that you should switch GDevices explicitly is to switch between on-screen and off-screen drawing.

Truth #4: On- and off-screen drawing are different.

QuickDraw distinguishes between on-screen and off-screen drawing for a couple of reasons. Starting with 32-Bit QuickDraw 1.0, video memory can only be reached in 32-bit addressing mode. If QuickDraw detects that it's drawing to a screen, it switches to 32-bit addressing mode, writes to video memory, and then switches back to the native addressing mode. QuickDraw stays in the native addressing mode for the entire operation when it draws off-screen unless bit 2 of the pmVersion field of the destination pixMap is set or unless it draws into a GWorld that's cached on a QuickDraw accelerator board. In those two cases, QuickDraw switches to 32-bit addressing mode even though it's drawing off-screen. Another important difference between on-screen and off-screen drawing is that on-screen drawing makes QuickDraw go through the additional work of using the gdRects of the screens to determine which GDevices you're drawing to. We described this in Truth #3. When QuickDraw draws off- screen, it just uses the current GDevice.

QuickDraw senses whether it's drawing on-screen or off-screen by comparing the baseAddr field of the current graphics port's pixMap against the baseAddr of the main screen's pixMap. If they're equal, QuickDraw assumes that it's drawing on-screen (not necessarily the main screen!). Otherwise, QuickDraw assumes that it's drawing off-screen.

To avoid confusing QuickDraw regarding whether it's drawing on-screen or off-screen, make sure that you always draw to a window for any on-screen drawing. The pixMap of any window is a lot like the main screen's pixMap, as we described in Truth #1, so the baseAddr of a window's pixMap is always the same as the baseAddr of the main screen's pixMap.

TRUTH IN ACTION
There are several ways to use these truths so that your applications optimize their displays for the sizes, depths, and color tables of each of the screens that are attached to the systems your application runs on. What follows are a few ways to do this.

If your window is completely contained on one screen, you might want to optimize your window's image for the screen it appears on. Usually, this means finding out the depth and color table of the screen your window is on. The device list, introduced in Truth #1, is invaluable for getting this information. For each GDevice in the list (remember, each GDevice represents a screen), compare the rectangle of its gdRect field against the rectangle of your window. The gdRect is in global coordinates while your window's portRect is in local coordinates, so you'll have to convert one or the other before doing the comparison. Once you've found the GDevice whose gdRect encompasses your window, get the GDevice's pixMap from the gdPMap field. Within this pixMap, the pixelSize field tells you the depth of the screen, and the pmTable field gives you a handle to the screen's color table. The device list is a linked list; you can get the first GDevice in the list with GetDeviceList, and you can go to the next GDevice with GetNextDevice.

What if your window intersects more than one screen? A common way to deal with this is to compromise by choosing a screen based on some criterion. You might want to choose the deepest screen that your window crosses, or the screen that intersects most of your window. The program listing at the end of this column shows a routine called FindScreenGDevice that takes a rectangle in global coordinates and a criterion, and returns the GDevice of the screen that satisfies the criterion. From this GDevice, you can get the information you need from the pixMap in the gdPMap field. If you pass kDeepestScreen for the criterion, FindScreenGDevice returns the GDevice of the deepest screen that intersects the rectangle. If you instead pass kLargestAreaScreen, the GDevice of the screen that has the largest intersection area is returned. Normally, you'd convert your window's portRect to global coordinates with the LocalToGlobal QuickDraw routine, and pass the resulting rectangle to FindScreenGDevice.

If your window displays an off-screen image and GWorlds are available, you can use GWorlds to make an off-screen image with the best depth and color table for the screens your window is on. If you pass 0 as the pixel depth to NewGWorld or UpdateGWorld and pass a rectangle defining the part of your window that displays the off-screen image in global coordinates, NewGWorld and UpdateGWorld set up an off-screen graphics environment that has the same depth and color table as the deepest screen your rectangle intersects, even if the area of intersection is as small as one pixel.

In some cases, you might want to display an image specifically to one screen, maybe for a presentations application or a game. To choose a screen, use a routine like FindScreenGDevice. Once you've chosen a screen, set up a window that fills that entire screen. Then draw to the window normally. In other words, you should again pretend that there's only one screen available, except that you have a little bit of insider information about where to put a window on that screen to make your images look or act best. System 7 introduced the DeviceLoop routine, which is the recommended method for drawing images that are optimized for every screen they cross. For example, the highlight color can be drawn in black on a 1-bit screen, but in magenta on a deeper screen. If your application is running on a pre-7.0 system, you can simulate DeviceLoop by using a routine like DeviceLoopSim, as we show below. But to maintain future compatibility, DeviceLoop should be used if it is available.

You don't have to do anything special to let your applications work with multiple screens; QuickDraw makes multiple screens look like one screen. Use this abstraction even if you want to take advantage of specific screens. Keep using QuickDraw at a high level, and multiple-screen compatibility comes for free.



void DeviceLoopSim(
    RgnHandle                 drawingRgn,     /* Region to draw to */
    DeviceLoopDrawingProcPtr  drawingProc,
                                        /* Routine to call to draw */
    long                      userData,     /* User-definable data */
    DeviceLoopFlags           flags)   /* Options; not implemented */
{
    GDHandle   aGDevice;           /* GDevice of each screen */
    RgnHandle  screenRgn;
                     /* Intersection of screen area and drawingRgn */
    RgnHandle  savedClip;
                       /* Saves the current port's clipping region */
    Rect       screenRect;
                      /* Rectangle of screen in global coordinates */

    /* Save the current port's clipping region */
    savedClip = NewRgn();
    GetClip( savedClip );

    /* Loop through every GDevice in the device list */
    screenRgn = NewRgn();
    aGDevice = GetDeviceList();
    while (aGDevice != nil)
    {
        /* Find region of intersection between screen and */
        /* drawingRgn */
        screenRect = (**aGDevice).gdRect;
        GlobalToLocal( &topLeft( screenRect ) ); 
        GlobalToLocal( &botRight( screenRect ) );
        RectRgn( screenRgn, &screenRect );
        SectRgn( screenRgn, drawingRgn, screenRgn );

        /* If there is an area of intersection, call drawing proc */
        if (!EmptyRgn( screenRgn ))
        {
            SetClip( screenRgn );
            (*drawingProc)( (**(**aGDevice).gdPMap).pixelSize,
                (**aGDevice).gdFlags, aGDevice, userData );
        }
        /* Go to the next GDevice in the device list */
        aGDevice = GetNextDevice( aGDevice );
    }
    SetClip( savedClip );
    DisposeRgn( savedClip );
    DisposeRgn( screenRgn );
}
enum { kDeepestScreen, kLargestAreaScreen };

GDHandle FindScreenGDevice(
    Rect    *   bounds,
                    /* Global rectangle of part of screen to check */
    short   screenOption)
                /* Use deepest or largest intersection area screen */
{
    GDHandle  baseGDevice;     /* GDevice that satisfies criterion */
    GDHandle  aGDevice;
                     /* Handle to each GDevice in the GDevice list */
    long      maxArea;          /* Largest intersection area found */
    long      area;           /* Area of rectangle of intersection */
    Rect      commonRect;             /* Rectangle of intersection */

    /* Different screen options require different algorithms */
    if (screenOption == kDeepestScreen)
        /* Graphics Devices Manager tells us the deepest */
        /* intersecting screen */
        baseGDevice = GetMaxDevice( bounds );
    else if (screenOption == kLargestAreaScreen)
    {
        /* Get a handle to the first GDevice in the device list */
        aGDevice = GetDeviceList();

        /* Keep looping until all GDevices have been checked */
        maxArea = 0;
        baseGDevice = nil;
        while (aGDevice != nil)
        {
            /* Check to see whether screen rectangle and bounds */
            /* intersect */
            if (SectRect( &(**aGDevice).gdRect, bounds,
                    &commonRect ))
            {
                /* Calculate area of intersection */
                area = (long)(commonRect.bottom - commonRect.top) *
                         (long)(commonRect.right - commonRect.left);

                /* Keep track of largest area of intersection */
                /* found so far */
                if (area > maxArea)
                {
                    maxArea = area;
                    baseGDevice = aGDevice;
                }
            }
            /* Go to the next GDevice in the device list */
            aGDevice = GetNextDevice( aGDevice );
        }
    }
    return baseGDevice;
}

FORREST TANAKA Just before fastening that buckle on his bike helmet and snapping into those pedals, Forrest whispered, "Howdy, my name is Forrest; I don't drink, and I hate nicknames and terms of endearment. But I firmly believe that real life is more exciting and fantastic than the best fiction, except for Antoine de Saint-Exupéry's The Little Prince ."*

BILL ("ANGUS") GUSCHWAN Stopping between moguls after some maney fakey shredding on his snowboard, Bill borrowed a few clock hands to say "Hi, my name is Angus; I like tacos, '71 Cabernet, and my favorite color is magenta." His favorite philosopher, Ludwig Wittgenstein, would be proud of his brevity. *


The device list is documented in the section "The Graphics Device Record" in Chapter 21 ofInside Macintosh Volume VI. *

DeviceLoop is described in Chapter 21 of Inside Macintosh Volume VI. *

For information about using the Picture Utilities Package to find colors that are optimized for different screen depths, see the article "In Search of the Optimal Palette" later in this issue. *

Thanks to Edgar Lee, Guillermo Ortiz, and John Wang for reviewing this column. *

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Viber 6.8.6 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device, so... Read more
Carbon Copy Cloner 4.1.17 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
EtreCheck 3.4.2 - For troubleshooting yo...
EtreCheck is an app that displays the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to... Read more
Hopper Disassembler 4.2.10- - Binary dis...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more
VueScan 9.5.81 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
iFFmpeg 6.4.2 - Convert multimedia files...
iFFmpeg is a comprehensive media tool to convert movie, audio and media files between formats. The FFmpeg command line instructions can be very hard to master/understand, so iFFmpeg does all the hard... Read more
Fantastical 2.4.1 - Create calendar even...
Fantastical 2 is the Mac calendar you'll actually enjoy using. Creating an event with Fantastical is quick, easy, and fun: Open Fantastical with a single click or keystroke Type in your event... Read more
Fantastical 2.4.1 - Create calendar even...
Fantastical 2 is the Mac calendar you'll actually enjoy using. Creating an event with Fantastical is quick, easy, and fun: Open Fantastical with a single click or keystroke Type in your event... Read more
Live Home 3D Pro 3.2.2 - $69.99
Live Home 3D Pro, a successor of Live Interior 3D, is the powerful yet intuitive home design software that lets you build the house of your dreams right on your Mac. It has every feature of Live Home... Read more
Live Home 3D Pro 3.2.2 - $69.99
Live Home 3D Pro, a successor of Live Interior 3D, is the powerful yet intuitive home design software that lets you build the house of your dreams right on your Mac. It has every feature of Live Home... Read more

Latest Forum Discussions

See All

The best deals on the App Store this wee...
There are quite a few truly superb games on sale on the App Store this week. If you haven't played some of these, many of which are true classics, now's the time to jump on the bandwagon. Here are the deals you need to know about. [Read more] | Read more »
Realpolitiks Mobile (Games)
Realpolitiks Mobile 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: PLEASE NOTE: The game might not work properly on discontinued 1GB of RAM devices (iPhone 5s, iPhone 6, iPhone 6 Plus, iPad... | Read more »
Layton’s Mystery Journey (Games)
Layton’s Mystery Journey 1.0.0 Device: iOS Universal Category: Games Price: $15.99, Version: 1.0.0 (iTunes) Description: THE MUCH-LOVED LAYTON SERIES IS BACK WITH A 10TH ANNIVERSARY INSTALLMENT! Developed by LEVEL-5, LAYTON’S... | Read more »
Full Throttle Remastered (Games)
Full Throttle Remastered 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Originally released by LucasArts in 1995, Full Throttle is a classic graphic adventure game from industry legend Tim... | Read more »
Stunning shooter Morphite gets a new tra...
Morphite is officially landing on iOS in September. The game looks like the space shooter we've been needing on mobile, and we're going to see if it fits the bill quite shortly. The game's a collaborative effort between Blowfish Studios, We're Five... | Read more »
Layton's Mystery Journey arrives to...
As you might recall, Layton's Mystery Journey is headed to iOS and Android -- tomorrow! To celebrate the impending launch, Level-5's released a new trailer, complete with an adorable hamster. [Read more] | Read more »
Sidewords (Games)
Sidewords 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Grab a cup of coffee and relax with Sidewords. Sidewords is part logic puzzle, part word game, all original. No timers. No... | Read more »
Noodlecake Games' 'Leap On!...
Noodlecake Games is always good for some light-hearted arcade fun, and its latest project, Leap On! could carry on that tradition. It's a bit like high stakes tetherball in a way. Your job is to guide a cute little blob around a series of floating... | Read more »
RuneScape goes mobile later this year
Yes, RuneScape still exists. In fact, it's coming to iOS and Android in just a few short months. Jagex, creators of the hit fantasy MMORPG of yesteryear, is releasing RuneScape Mobile and Old School RuneScape for mobile devices, complete with... | Read more »
Crash of Cars wants you to capture the c...
Crash of Cars is going full on medieval in its latest update, introducing castles and all manner of new cars and skins fresh from the Dark Ages. The update introduces a new castle-themed map (complete with catapults) and a gladiator-style battle... | Read more »

Price Scanner via MacPrices.net

Apple Move Away from White Label Event Apps C...
DoubleDutch, Inc., a global provider of Live Engagement Marketing (LEM) solutions, has made a statement in the light of a game-changing announcement from Apple at this year’s WWDC conference.... Read more
70 Year Old Artist Creates Art Tools for the...
New Hampshire-based developer Pirate’s Moon has announced MyArtTools 1.1.3, the update to their precision drawing app, designed by artist Richard Hoeper exclusively for use with the 12.9-inch iPad... Read more
Sale! New 2017 13-inch 2.3GHz MacBook Pros fo...
Amazon has new 2017 13″ 2.3GHz/128GB MacBook Pros on sale today for $150 off MSRP including free shipping. Their prices are the lowest available for these models from any reseller: – 13″ 2.3GHz/128GB... Read more
13″ 2.3GHz/128GB Space Gray MacBook Pro on sa...
MacMall has the 13″ 2.3GHz/128GB Space Gray MacBook Pro (MPXQ2LL/A) on sale for $1219 including free shipping. Their price is $80 off MSRP. Read more
Clearance 2016 12-inch Retina MacBooks, Apple...
Apple recently dropped prices on Certified Refurbished 2016 12″ Retina MacBooks, with models now available starting at $1019. Apple will include a standard one-year warranty with each MacBook, and... Read more
Save or Share
FotoJet Designer, is a simple but powerful new graphic design apps available on both Mac and Windows. With FotoJet Designer’s 900+ templates, thousands of resources, and powerful editing tools you... Read more
Logo Maker Shop iOS App Lets Businesses Get C...
A newly released app is designed to help business owners to get creative with their branding by designing their own logos. With more than 1,000 editable templates, Logo Maker Shop 1.0 provides the... Read more
Sale! New 15-inch MacBook Pros for up to $150...
Amazon has the new 2017 15″ MacBook Pros on sale for up to $150 off MSRP including free shipping: – 15″ 2.8GHz MacBook Pro Space Gray: $2249 $150 off MSRP – 15″ 2.89Hz MacBook Pro Space Gray: $2779 $... Read more
DEVONthink To Go 2.1.7 For iOS Brings Usabili...
DEVONtechnologies has updated DEVONthink To Go, the iOS companion to DEVONthink for Mac, with enhancements and bug fixes. Version 2.1.7 adds an option to clear the Global Inbox and makes the grid... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more

Jobs Board

Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
Product Manager - *Apple* Pay on the *Appl...
Job Summary Apple is looking for a talented product manager to drive the expansion of Apple Pay on the Apple Online Store. This position includes a unique Read more
*Apple* Retail - Multiple Positions - Apple...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions - Apple...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Payments Architect - *Apple* Pay - A...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.