TweetFollow Us on Twitter

March 93 - DEVICELOOP MEETS THE INTERFACE

DEVICELOOP MEETS THE INTERFACE

JOHN POWERS

[IMAGE Powers_article_rev1.GIF]

With the ascendancy of multimedia, 3-D shading and elaborate color backgrounds are showing up in an increasing number of interface designs. But what happens when these sophisticated interface elements must be displayed across multiple monitors of different bit depths? This article explains how to use the DeviceLoop function to take care of the device, clipping, and bit-depth logistics involved in multiple-monitor displays.


One of the great things about the Macintosh is its ability to support more than one monitor at a time. You can display windows in any active monitor or split a window -- and the objects in it -- across several monitors at once. What's more, you can make an image adjust to the bit depth and other capabilities of each monitor it's displayed on, so that the visual interface looks as good as it possibly can on each of the devices attached to the computer.

I recently worked on a project in which one of the goals was exactly that -- we wanted our application windows to look really good across multiple monitors and at any bit depth. The task was complicated by the fact that the interface was quite sophisticated graphically. To give our windows a distinctive, three-dimensional look, we used shaded color graphics. We filled the content area with background graphics, text, patterned and colored lines, and 3-D buttons. With the exception of our standard List Manager lists, all the window objects were drawn by our application program. Even the conventional scroll bar, close box, and zoom box were replaced by custom art drawn by the application, not the Window Manager.

Displaying these complex windows across multiple monitors was obviously going to be a challenge. We knew that the Finder, for example, pulled it off -- whenever Finder windows span monitors of different bit depths, the parts of the window on each monitor are drawn to the individual monitor's depth. "If the Finder does it, so can we," I decided, although I actually knew very little about how to solve the problem.

DEVICELOOP TO THE RESCUE

I bit the bullet. The search for ways to draw a window across multiple monitors led in a number of directions, all of them involving visible regions, clipping regions, and region-rect conversions. I asked a lot of people for advice, and while everyone was gracious in offering help, the job was looking complicated. Fortunately, one of the advice givers suggested that I check out the DeviceLoopfunction inInside MacintoshVolume VI. (I found out later that the advice giver was the author of the DeviceLoop function.)

When I looked up DeviceLoop in Volume VI, here's what I found: The DeviceLoop procedure searches all active screen devices, calling your drawing procedure whenever it encounters a screen that intersects your drawing region. You supply a handle to the region in which you wish to draw and a pointer to your drawing procedure. . . . If the DeviceLoop procedure encounters similar devices -- having the same pixel depth, black-and- white/color setting, and matching color table seeds -- it makes only one call to your drawing procedure, pointing to the first such device encountered.

This sounded exactly like what we were looking for. The Window Manager itself uses DeviceLoop to display window components on a variety of monitors. Since we were drawing our own windows, DeviceLoop was clearly what we needed.

Here's what DeviceLoop looks like in C:

pascal void DeviceLoop (RgnHandle drawingRgn,
	DeviceLoopDrawingProcPtr drawingProc,
	long userData, DeviceLoopFlags flags);

The drawingRgn parameter is a handle to the region that will be drawn in (usually a window's visRgn). The drawingProc parameter is a pointer to your drawing routine (see below). The userData parameter is a long that gets passed to your drawing routine. Finally, the flags parameter controls how devices are grouped before your drawing routine is called. (Pass 0 for the default behavior -- grouping similar devices together. See the description inInside Macintoshfor other possible values.)

The drawing routine needs to be declared as follows:

pascal void MyDrawProc (short depth, short deviceFlags,
	GDHandle targetDevice, long userData);

Here the depth parameter is the depth of the device you're currently drawing on. The deviceFlags parameter is a copy of the device's gdFlags, targetDevice is a handle to the device, and userData is whatever you passed to DeviceLoop.

DeviceLoop works like this: Each time your drawing routine is called, the current port's visRgn will have been set to the intersection of your drawing region and some screen device. DeviceLoop passes the drawing characteristics of the particular screen it's working on to the drawing routine, which can then make use of them -- for instance, by drawing to the appropriate bit depth. In short, DeviceLoop takes care of all the device, clipping, and bit-depth logistics, while all you have to do is draw.

USING DEVICELOOP IN AN OBJECT-ORIENTED WORLD

In our application, we had to draw not only the contents of the window, but also the window itself. True to our object-oriented design, we created classes for all the interface objects. These classes included a TArt class for backgrounds, graphics, and 3-D button objects; a TLine class for lines; a TTxt class for black-and-white text; and a TBkg class for backgrounds for the text. Although we used DeviceLoop for drawing objects in every class except the text classes, the heart of the process is best illustrated by our use of DeviceLoop for TArt objects.

The graphics for TArt objects were stored as PICT resources. To give the best possible image, the interface designer created an 8-bit-deep PICT for display depths of 8 bits or deeper. For all other display depths and CPUs without Color QuickDraw, she created a 1-bit-deep, black-and-white PICT. We could have let the Macintosh use the 8-bit PICT for all drawing -- color and black-and- white -- and, with dithering, the results would have been pretty good. But since we had our own hand-designed, 1-bit version of the PICT, DeviceLoop was a better solution. Our window object kept track of all the interface objects that it needed to draw. When an update event was received, the document object told the window object to draw. Specifically, our BeginUpdate/EndUpdate function called a particular drawing routine for each of the objects. Each object, in turn, called DeviceLoop with our DrawProc callback, which contained the actual drawing code for that object. Figure 1 shows this strategy.

[IMAGE Powers_article_rev2.GIF]

Figure 1 An Inefficient Way to Incorporate DeviceLoop

We used this DeviceLoop-within-each-object's-drawing-procedure approach until someone pointed out how inefficient it was to call DeviceLoop for every interface object. We realized that it would be much better to call DeviceLoop once and have the drawing procedure that we passed to it decide which object had to be drawn. We wound up with a single DeviceLoop call in the window's BeginUpdate/EndUpdate function, as shown in Figure 2. The use of a single DeviceLoop call in the window object really streamlined the design.

[IMAGE Powers_article_rev3.GIF]

Figure 2 A Better Way to Call DeviceLoop


One problem we encountered was that the compiler balked whenever we referenced our drawing routine (called DrawProc) in the DeviceLoop parameter list. We even included the scope -- TWin::DrawProc -- and that didn't help. The breakthrough came when we made DrawProc static. Unfortunately, changing it to static caused another problem: the compiler choked when we referencedthis within DrawProc. We forgot that static functions can't reference nonstatic member variables. (You C++ aficionados are probably smiling, but we recent converts must struggle at first.) We couldn't use static variables, however, because each of our objects required its own variables. Thus, to access an object's variables, we had to pass the window object pointer in the userData parameter of the DeviceLoop function.

AN EXAMPLE

The Developer CD Seriesdisc contains a sample application that shows how we used DeviceLoop for TArt objects in our interface. The application, DeviceLoopInDrag, displays a window that can be dragged between monitors of different bit depths. Figure 3 shows this window spanning a grayscale and a black-and-white monitor.

Excerpts from the DeviceLoopInDrag source code follow. First there's the update function that's called whenever the window needs to be redrawn. It just calls the drawing procedure for the window object (TWin).

[IMAGE Powers_article_rev4.GIF] Figure 3 DeviceLoop in Action

// TDoc::DoUpdate
// Document object.
// Entry for update event action.
void
TDoc::DoUpdate()
{
	BeginUpdate(this->fDocWindow);
	this->fWinObj->Draw();
	EndUpdate(this->fDocWindow);
}

The window's drawing procedure does little more than set up and call DeviceLoop. Notice that we're passing the reference to the current window object --this -- in DeviceLoop's userData parameter, as described earlier. Since we want the default DeviceLoop behavior, we set the flags to 0.

// TWin::Draw
// Window object.
// Within BeginUpdate/EndUpdate.
void
TWin::Draw()
{
	// Have DeviceLoop manage the drawing.
	// Pass the window object in userData.
	long					userData = (long)this;
	DeviceLoopFlags	flags = 0;
	GrafPtr				myPort;
	GetPort(&myPort);
	DeviceLoop(myPort->visRgn, TWin::DrawProc, userData, flags);
	// Draw the stuff we don't need DeviceLoop for.
	// We tell the subview to take care of that.
	this->fView->Draw();
};

Next, theTWin drawing procedure is the callback procedure that DeviceLoop invokes to coordinate the drawing of each of the elements on the screen.

// TWin::DrawProc
// Called by DeviceLoop.
// A static function. Must be in a resident segment, locked and
// unpurgeable. Because it's static, it can't access object member 
// variables directly. We use the window object passed in userData 
// to access its member variables.
#pragma segment Main
pascal void
TWin::DrawProc(short depth, short /*deviceFlags*/,
			GDHandle hTargetDevice, long userData)
{
	// Get the window object from userData.
	TWin* theWinObject = (TWin*) userData;
	// Use depth of 1 if we have a computer without Color QuickDraw.
	depth = (hTargetDevice==NULL)?1:depth;
	// Draw our objects.
	theWinObject->fBackground->Draw(depth);
	theWinObject->fLogo->Draw(depth);
	theWinObject->fText->Draw(depth);
	theWinObject->fButton->Draw(depth);
};

Finally, here's the actual TArt::Draw function, used for various items in the window. Based on the bit-depth parameter passed to it, the Draw function decides whether to use the black-and-white or the color version of its PICT.

// TArt::Draw
// All art objects (PICTs) are drawn here. This is where we
// distinguish between B&W or color renderings of TArt objects.
// The B&W rendering has a resource ID that's kBWOffset larger
// than its color counterpart value.
void
TArt::Draw(short depth)
{
	// Don't draw empty art.
	if(this->fPictID==0)
		return;
	PicHandle	hPict;
	if(depth<8)
	{
		// Use B&W PICT.
		hPict = (PicHandle) GetResource('PICT',
		    this->fPictID+kBWOffset);
	}
	else
	{
		// Use color PICT.
		hPict = (PicHandle) GetResource('PICT', this->fPictID);
	}
	if(hPict)
	{
		Rect	theDrawRect;
		this->GetDrawRect(theDrawRect);
		HLock((Handle) hPict);
		DrawPicture(hPict, &theDrawRect);
		HUnlock((Handle) hPict);
	}
};

SUMMING UP

How did we wind up feeling about DeviceLoop? After we first discovered it, our tendency was to use it everywhere. We even used it to call a drawing routine that always drew in black and white, no matter what the bit depth. We later stripped this use out of the interface because it offered no advantage and added extra code.

One concern we had was that performance would degrade to an intolerable level as we added objects to be drawn. To see what would happen, the mischievous test engineer for our project devised a case with 99 separate TArt objects in the same window. Predictably, the 99 objects weren't displayed all at once. While you can expect some lag between the appearance of first object in a window and the last, however, the drawing time when you use DeviceLoop is really very short, well within user tolerance.

All in all, our design team was very pleased with DeviceLoop. We were glad to have found such an easy way to solve the problem of displaying interface objects on monitors of different bit depths. The interface designer got the look she wanted, and we were able to accomplish the job with a minimum of hassle and a minimum of code. This was one challenge that left everyone happy.

JOHN POWERS (AppleLink JOHNPOWERS) started his career as a behavioral scientist, studying how people use computers. He worked his way up the management ladder, and then cofounded a company that developed software for the first home computers. That lead him to Atari, but Atari got weird, so John joined Convergent Technologies to develop the WorkSlate notebook computer, eight years before the PowerBook. That led him to another management ladder and into The Learning Company, where he developed software for children. Locked in his management office, John discovered the Macintosh and decided to become a Macintosh software developer. Now he's at Apple Computer developing Macintosh software that helps people use computers. *

The DeviceLoop call first appears in System 7. If your application will be running under an earlier version of system software, you'll need to implement your own DeviceLoop function. For an example of how to do this, see the column "Graphical Truffles: Multiple Screens Revealed" in Issue 10 of develop.*

THANKS TO OUR TECHNICAL REVIEWERS Edgar Lee and Brigham Stevens. Special thanks to Pat Coleman, the Interface Designer on the project that inspired this article.*

 
AAPL
$524.94
Apple Inc.
+5.93
MSFT
$40.01
Microsoft Corpora
-0.39
GOOG
$536.10
Google Inc.
-20.44

MacTech Search:
Community Search:

Software Updates via MacUpdate

Tweetbot 1.5.1 - Popular iOS twitter cli...
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Mac DVDRipper Pro 4.1.7 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
PDFpenPro 6.2 - Advanced PDF toolkit for...
PDFpenPro allows users to edit PDF's easily. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Create... Read more
PDFpen 6.2 - Edit and annotate PDFs with...
PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Features... Read more
Monolingual 1.5.9 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
Starcraft II: Wings of Liberty 1.1.1.180...
Download the patch by launching the Starcraft II game and downloading it through the Battle.net connection within the app. Starcraft II: Wings of Liberty is a strategy game played in real-time. You... Read more
Sibelius 7.5.0 - Music notation solution...
Sibelius is the world's best-selling music notation software for Mac. It is as intuitive to use as a pen, yet so powerful that it does most things in less than the blink of an eye. The demo includes... Read more
Typinator 5.9 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
MYStuff Pro 2.0.16 - Create inventories...
MYStuff Pro is the most flexible way to create detail-rich inventories for your home or small business. Add items to MYStuff by dragging and dropping existing information, uploading new images, or... Read more

Latest Forum Discussions

See All

Have a Special Dead Trigger 2 Easter Bas...
Have a Special Dead Trigger 2 Easter Basket Full of Goodies, Courtesy of Madfinger Games Posted by Rob Rich on April 18th, 2014 [ permalink ] Dead Trigger 2 | Read more »
Zynga Launches Brand New Farmville Exper...
Zynga Launches Brand New Farmville Experience with Farmville 2: Country Escape Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
David. Review
David. Review By Cata Modorcea on April 18th, 2014 Our Rating: :: MINIMALISTIC IN A DIFFERENT WAYUniversal App - Designed for iPhone and iPad David is a minimalistic game wrapped inside of a soothing atmosphere in which the hero... | Read more »
Eyefi Unveils New Eyefi Cloud Service Th...
Eyefi Unveils New Eyefi Cloud Service That Allows Users to Share Media Across Personal Devices Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
Tales from the Dragon Mountain: The Lair...
Tales from the Dragon Mountain: The Lair Review By Jennifer Allen on April 18th, 2014 Our Rating: :: STEADY ADVENTURINGiPad Only App - Designed for the iPad Treading a safe path, Tales from the Dragon Mountain: The Lair is a... | Read more »
Yahoo Updates Flickr App with Advanced E...
Yahoo Updates Flickr App with Advanced Editing Features and More Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
My Incredible Body - A Kid's App to...
My Incredible Body - A Kid's App to Learn about the Human Body 1.1.00 Device: iOS Universal Category: Education Price: $2.99, Version: 1.1.00 (iTunes) Description: Wouldn’t it be cool to look inside yourself and see what was going on... | Read more »
Trials Frontier Review
Trials Frontier Review By Carter Dotson on April 18th, 2014 Our Rating: :: A ROUGH LANDINGUniversal App - Designed for iPhone and iPad Trials Frontier finally brings the famed stunt racing franchise to mobile, but how much does its... | Read more »
Evernote Business Notebook by Moleskin I...
Evernote Business Notebook by Moleskin Introduced – Support Available in Evernote for iOS Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
Sparkle Unleashed Review
Sparkle Unleashed Review By Jennifer Allen on April 18th, 2014 Our Rating: :: CLASSY MARBLE FLINGINGUniversal App - Designed for iPhone and iPad It’s a concept we’ve seen before, but Sparkle Unleashed is a solidly enjoyable orb... | Read more »

Price Scanner via MacPrices.net

iMacs on sale for up to $160 off MSRP this we...
Best Buy has iMacs on sale for up to $160 off MSRP for a limited time. Choose free home shipping or free instant local store pickup (if available). Prices are valid for online orders only, in-store... Read more
iPad Airs on sale this weekend for up to $100...
Best Buy has WiFi iPad Airs on sale for $50 off MSRP and WiFi + Cellular iPad Airs on sale for $100 off MSRP on their online store for a limited time, with prices now starting at $449. Choose free... Read more
Apple restocks refurbished Mac minis starting...
The Apple Store has restocked Apple Certified Refurbished Mac minis for up to $150 off the cost of new models. Apple’s one-year warranty is included with each mini, and shipping is free: - 2.5GHz Mac... Read more
Hyundai Brings Apple CarPlay To The 2015 Sona...
Hyundai Motor America has announced it will bring Apple CarPlay functionality to the 2015 Sonata. CarPlay is pitched as a smarter, safer and easier way to use iPhone in the car and gives iPhone users... Read more
Updated iPads Coming Sooner Than We Had Thoug...
MacRumors, cites KGI securities analyst Ming Chi Kuo, well-respected as an Apple product prognisticator, saying that Apple will introduce an upgraded iPad Air and iPad mini in 2014/Q3, meaning the... Read more
Toshiba Unveils New High And Low End Laptop M...
Toshiba has announced new laptop models covering both the high-end and low-end of the notebook computer spectrum. Toshiba 4K Ultra HD Laptop Toshiba’s new Satellite P55t features one of the world’s... Read more
Save up to $270 with Apple refurbished 13-inc...
The Apple Store has Apple Certified Refurbished October 2013 13″ Retina MacBook Pros available starting at $1099, with models up to $270 off MSRP. Apple’s one-year warranty is standard, and shipping... Read more
Apple now offering refurbished iPad mini with...
The Apple Store has Certified Refurbished 2nd generation iPad minis with Retina Displays now available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free.... Read more
Microsoft Blinks – Drops Microsoft Office 365...
Microsoft has dropped the annual subscription fee for Microsoft Office 365 Personal – which is needed in order to create and edit documents in Microsoft Office for iPad. However, Apple’s iOS and OS X... Read more
New AVG Vault Apps for iOS and Android Help K...
AVG Technologies N.V. an online security company for 177 million active users, has announced the launch of its latest mobile application, AVG Vault. The free app introduces an innovative user... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Retail - Manager - Holyoke - Apple I...
Job Summary Keeping an Apple Store thriving requires a diverse set of leadership skills, and as a Manager, you’re a master of them all. In the store’s fast-paced, Read more
*Apple* Retail - Manager - Apple (United Sta...
Job SummaryKeeping an Apple Store thriving requires a diverse set of leadership skills, and as a Manager, you're a master of them all. In the store's fast-paced, dynamic Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Retail - Market Leader - Cincinnati...
…challenges of developing individuals, building teams, and affecting growth across Apple Stores. You demonstrate successful leadership ability - focusing on excellence Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.