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.*

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Google Chrome 59.0.3071.115 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
SoftRAID 5.6.1 - High-quality RAID manag...
SoftRAID allows you to create and manage disk arrays to increase performance and reliability. SoftRAID allows the user to create and manage RAID 4 and 5 volumes, RAID 1+0, and RAID 1 (Mirror) and... Read more
Suitcase Fusion 7 18.2.4 - Font manageme...
Suitcase Fusion 7 is the creative professional's font manager. Every professional font manager should deliver the basics: spectacular previews, powerful search tools, and efficient font organization... Read more
BetterTouchTool 2.25 - Customize Multi-T...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
WhiteCap 6.7.1 - Visual plug-in for iTun...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more
DiskMaker X 6.0 rc5 - Make a bootable OS...
DiskMaker X (was Lion DiskMaker) helps you to build a bootable drive from the official OS X installer app (the one you download from the Mac App Store). It detects the OS X Install program with... Read more
Lyn 1.9 - Lightweight image browser and...
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
WhiteCap 6.7.1 - Visual plug-in for iTun...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more
DiskMaker X 6.0 rc5 - Make a bootable OS...
DiskMaker X (was Lion DiskMaker) helps you to build a bootable drive from the official OS X installer app (the one you download from the Mac App Store). It detects the OS X Install program with... Read more
Lyn 1.9 - Lightweight image browser and...
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

Latest Forum Discussions

See All

This War of Mine gets a new ending and m...
This War of Mine just got a big new update, featuring free DLC that adds a new ending to the game, among other exciting changes. The update is celebrating the game's two-year release anniversary. Apart from the new ending, which will be quite... | Read more »
Summon eight new heroes in Fire Emblem H...
Nintendo keeps coming at us with Fire Emblem Heroes updates, and it doesn't look like that trend is stopping anytime soon. The folks behind the game have just announced the new War of the Clerics Voting Gauntlet, expected to start next Tuesday. [... | Read more »
The best deals on the App Store this wee...
iOS publishers are pulling out all the stops this week -- there's a huge number of seriously great games at discounted prices this week. Let's not waste any time and get right down to business. [Read more] | Read more »
The House of da Vinci (Games)
The House of da Vinci 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Enter The House of Da Vinci, a new must-try 3D puzzle adventure game. Solve mechanical puzzles, discover hidden... | Read more »
Solve the disappearance of history’s gre...
Blue Brain Games invites you to indulge in an immersive hands-on 3D puzzle adventure in similar vein to The Room series, with its debut release The House of Da Vinci. Set during the historic period of the Italian Renaissance (when Leonardo himself... | Read more »
Age of Rivals (Games)
Age of Rivals 3.3 Device: iOS Universal Category: Games Price: $.99, Version: 3.3 (iTunes) Description: Deep civilization-building strategy in a fast-paced card game! | Read more »
Panthera Frontier (Games)
Panthera Frontier 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
Angry Birds Evolution beginner's gu...
Angry Birds changes things up a fair bit in its latest iteration, Angry Birds Evolution. The familiar sling-shot physics mechanics are still there, but the game now features team-based gameplay, RPG elements, and a new top-down view. With all of... | Read more »
Sega Forever is for the retro game fans
Sega is launching a new retro games service titled Sega Forever, in a move that's sure to delight games enthusiasts with a bit of nostalgia. Sega's releasing five classic games for free. The titles include Sonic the Hedgehog, Phantasy Star II,... | Read more »
The Little Acre (Games)
The Little Acre 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

13-inch 1.8GHz/256GB MacBook Air on sale for...
B&H Photo has the updated 2017 13″ 1.8GHz/256GB MacBook Air (MQD42LL/A) in stock and on sale for $1129 including free shipping plus NY & NJ tax only. Their price is $70 off MSRP. Read more
27-inch 3.4GHz iMac on sale for $1699, save $...
B&H Photo has the new 2017 27″ 3.4GHz iMac (MNE92LL/A) in stock and on sale for $1699 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. Read more
21-inch 2.3GHz iMac on sale for $1049, save $...
B&H Photo has the new 2017 21″ 2.3GHz iMac (MMQA2LL/A) in stock and on sale for $1049 including free shipping plus NY & NJ tax only. Their price is $50 off MSRP. Read more
ABBYY TextGrabber 6 for iOS Implements Instan...
ABBYY has announced the release of TextGrabber 6.0.0, an important feature update to the company’s productivity app developed for iOS and Android devices. TextGrabber 6.0 now offers Real-Time... Read more
vPhone, First Smartphone That Can’t Be Lost,...
Austin, Texas based Hypori has introduced the vPhone, a virtual smartphone that affords every business user the benefits of separate work and personal phones, conveniently delivered on a single... Read more
Save this weekend with 2016 refurbished MacBo...
Apple has dropped prices on Certified Refurbished 2016 15″ and 13″ MacBook Pros by as much as $590 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: -... Read more
New 27-inch 3.4GHz iMac on sale for $1699, sa...
MacMall has the new 2017 27″ 3.4GHz iMac (MNE92LL/A) in stock and on sale for $1699 including free shipping. Their price is $100 off MSRP. Read more
Clearance 2016 MacBook Pros available for up...
B&H Photo has clearance 2016 13″ and 15″ MacBook Pros in stock today and on sale for up to $400 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 15″ 2.7GHz... Read more
Apple Ranks 9th In comScore Top 50 U.S. Digit...
comScore, Inc. has released its monthly ranking of U.S. online activity at the top digital media properties for May 2017 based on data from comScore Media Metrix Multi-Platform. * Entity has... Read more
10.5-inch iPad Pros available for up to $20 o...
B&H Photo has the new 2017 10.5″ iPad Pros available for up to $20 off MSRP including free shipping plus NY & NJ sales tax only: - 64GB iPad Pro WiFi: $649 - 256GB iPad Pro WiFi: $749 - 512GB... Read more

Jobs Board

*Apple* News Product Marketing Mgr., Publish...
…organizational consensus on strategy and vision for publisher tools, authoring, and Apple News Format.Carries this strategy and vision across the organization to Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject matter Read more
Lead *Apple* Solutions Consultant - Apple I...
…integrity, and trust.Success Metrics/Key Performance Indicators:Quantitative* Year over Year growth in Apple Product and Beyond the Box sales in the assigned Point of Read more
*Apple* Solutions Consultant till v%u00E5r...
…ethics, integrity, and trust.Success Metrics/Key Performance Indicators:QuantitativeYear over Year growth in Apple Product and Beyond the Box sales in the assigned Point Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.