TweetFollow Us on Twitter

Volume Number:4
Issue Number:6
Column Tag:Programmer's Spotlight

Andy Hertzfeld on QuickerDraw

By Chester Peterson Jr., Reporter-At-Large, Lindsborg, Kansas

The Story of QuickerDraw

QuickDraw, the imaging program used on the Mac, doesn’t always live up to its name when used on the Mac II.

Incredibly fast and wonderfully crisp at one bit per pixel, it bogs down to something that could be more aptly described as SlowDraw at eight bits per pixel. The normal subtle responsiveness of the Mac II suffers.

“Actually, in the eight bit per pixel mode it almost feels like you’re using your Mac II under water,” is how Andy Hertzfeld describes the action--or lack thereof.

Hertzfeld is, of course famous in Mac circles as the man responsible for much of the Mac’s Operating System and design of the Toolbox.

So, in late December he decided to satisfy his curiosity about the QuickDraw graphics routines and how they were coded. This is really easy to do, he says.

Just get the Mac II to do the graphics operation in which you’re interested, and then randomly hit the interrupt button. This will interrupt it statistically in the place it’s executing the most--the inner loop.

What Hertzfeld discovered was that the inner loops weren’t optimally coded. His initial strategy was to move the entire QuickDraw into RAM. He wrote an INIT that moved 60k of the ROM out into RAM where he could patch it.

And, although there were some problems with that, Hertzfeld got it working. But, as he progressed, disassembling to the bottom of the system, he saw this wasn’t really necessary.

The reason: Apple had the foresight to have this low-level jump table that all the inner loops are bottle-necked through. All he had to do was replace addresses in the little memory jump table to take over the inner loops in a clean way.

“So, once I saw that, I thought, ‘Hey, this could be a project worth looking into’,” Hertzfeld recalls.

“And, the more I got into it, the more I was able to find ways to increase the speed of QuickDraw. I ended up improving the speed of some important operations by a factor of three or so, ending up with QuickerDraw.”, or as Apple has called it in release 6.0 of the operating system, QuickerGraf.

Something that confuses people and which he thinks is important is that the performance increases are anything but flat. Instead it’s a spiky curve, with some things speeding up a whole lot and others not at all.

The explanation is that the speed-ups are both case dependent and also data dependent. Depending on exactly what you’re doing, you’ll get different responses.

“My point is that the speed-ups aren’t uniform,” Hertzfeld points out. “Apple has some of the code, such as when you say either EraseRect or PaintRect with black, that are already fairly well optimized. I wasn’t able to improve them only because they’re already about as good as they can be.

“But, if you take PaintRect with a color that isn’t black or white, then it goes to a different loop that wasn’t well done. Here’s where I was able to improve speed by that factor of three.

Hertzfeld believes that the most important item in the graphical programmer’s bag of tricks is special casing. In other words, certain instances of a particular problem are easier to handle than are other instances.

So, he thinks that when speed isn’t important that a programmer should try to fold his cases to write as little code as possible to handle the entire situation.

But, when speed is essential, as it is in the QuickDraw routines, the opposite approach must be used, he says. This involves picking off all the different cases and seeing if you can handle each case a little faster.

A compromise Apple made on its standard graphics card was that it has to support one bit, two bits, four bits, and eight bits per pixel.

A lot of the QuickDraw routines were coded in such a way that they were common for four different screen formats, according to Hertzfeld.

“I was able to special case the eight bit per pixel case , because that’s the only one that’s really important from a performance point of view,” he says.

“While Apple used rather slow bit-field instructions, I used special cache code to take advantage of the faster addressing modes in the 68020 to do things faster.”

“I also saved some registers doing that, registers that the Apple code uses just for maintaining which bits per pixel are to be used. Freeing up these for other things allowed me to go faster.”

Hertzfeld also took advantage of the “principal of locality”. He defines this as meaning whatever you’re doing, it’s pretty likely you just did the same thing a short time ago.

He exploited this in producing QuickerDraw through the use of caches. In the computationally intensive parts of QuickDraw like the arithmetic transfer modes, he put in caches that say, “Hey, this is just the same as what I saw before--I don’t have to do all the work again, because I’ve already figured out the answer.”

Hertzfeld used this technique in the instance of copy bits to two different pix maps that have different color look-up tables, a common thing on the Mac II with digitized images.

Each digitized image would have its own color look-up table that wouldn’t be identical to the one on the screen.

When you do a copy bits, it has to do a mapping operation, taking each pixel and looking it up in a table to find the correct pixel in destination bit map.

Hertzfeld changed this so that long word maps are remembered, short- circuiting the memory references involved in doing the look-up. He used similar techniques in many places to gain significant speed-ups.

“You want to hit memory as little as possible,” he advises. “A lot of the Apple loops were doing essentially one memory reference per pixel.

“My routines always do one memory reference per long word. Why? Because the 68020 is capable of pulling in 32 bits just as quickly as it can pull in eight bits at a time.”

The Apple routines makes it a little easier to code just accessing memory eight bits at a time, while Hertzfeld accesses memory 32 bits at a time, spinning it around in the registers and mailing it faster.

“You just attempt to be as clever as possible when you’re trying to code,” he says. “This is interesting code to write, because it has an unusual sort of design criterium.”

With most code in normal circumstances you’re always balancing the twin trade-offs between speed and space, or as Hertzfeld puts it, trying to serve two masters while producing the nicest code possible.

But, the interesting thing about the QuickerDraw code he wrote is that space isn’t a consideration. He says the system spends so much time in the QuickDraw inner loops that he did everything to make them go faster. He used a different coding style that also made it a little more interesting and fun.

“Like, for example, I did everything possible to avoid a subroutine call in the inner loops,” Hertzfeld explains. “You copy 50 in-line instructions, because it’s worth it in the context of the inner loop.”

Hertzfeld also devised another creative and interesting technique to speed up QuickDraw, something he calls region counting.

“As I was speeding up QuickDraw, I was just a little bit disappointed that I wasn’t getting as much speed-up as I would have liked when I was clipping to regions,” he says.

“What I then realized is that the region mask doesn’t change much from scan line to scan line.”

“The other thing to notice is an eight bit per pixel region mask is eight times as long as it would be in one bit per pixel, or eight times as likely to be homogeneous,” Hertzfeld observes.

If you pick up a long word of the region mask it’s extremely likely that it will be all ones or all zeros. Hertzfeld started special casing the region mask.

He found that normally when masking you have to do something like a seven-instruction sequence that involves three memory references to plot a long word with a mask. But, if it turns out the mask is all zeros you don’t have to do anything, because it’s all going to be masked out.

You don’t even have to hit memory at all, just skip over it. If the mask is all ones, you can just use one store instead of having to read it back and do the coding in order to accomplish the masking.

So, he began special casing that way. And, even though the tests cost him a little, he still won enough to make it worthwhile, because the region mask does tend to be homogeneous. The result: A 40 percent speed increase from that special casing of the region mask.

Then as Hertzfeld was looking at the region mask as it went by, he began counting up runs in it so it could remember how many successive long words in a row were all zeros or how many successive long words in a row were all ones.

“If the region mask doesn’t change from scan line to scan line, which it doesn’t more than 90 percent of the time, I don’t have to fetch it. As a matter of fact, if it’s all masked out at the beginning I can just skip over it,” he observes.

Where the Apple routines were pulling a long word from memory, then sticking the same long word back, Hertzfeld just skipped over all that.

He’s proud of this original technique of region counting for obtaining a tremendous speed increase when things are heavily clipped.

Contrary to a misconception about its size, the QuickerDraw memory resident code is only approximately 10k. And, half of that is devoted to the arithmetic transfer modes that aren’t used too often.

The QuickerDraw file is 27k, but that includes logo resources. The nice colored picture that it comes up with is 12k alone.

Incidentally, the arithmetic transfer modes were introduced with the Mac II and are only relevant to color. Most applications don’t use them yet.

Hertzfeld accomplished his QuickerDraw core work in a two-week period between this last December 22 and January 7. It then became apparent that Apple was interested in his acceleration of QuickDraw.

Hertzfeld realized that if he was truly producing a speed-up, then he’d also have to address the arithmetic transfer modes. A second two-week burst of work got these speed-ups implemented, too.

The bottom line: QuickerDraw involves no change in the architecture of QuickDraw. Instead, view it as implementing a high performance tune-up of Apple’s standard.

Hertzfeld signed a non-exclusive contract with Apple for QuickerDraw in February, accepting less money so he could upload it to CompuServe and distribute it on his own.

Apple will incorporate QuickerDraw in its next release file 6.0, due out at the end of May.

“Although there are a few cases that I didn’t handle, I do think I’m pretty close to the optimal plotting speed of QuickDraw,” Hertzfeld comments. “I basically just re-implemented the inner loops so they were more efficient.”

There will be no need to further refine QuickerDraw for the 68030. This is because it has an instruction set identical to the 68020’s.

“The things that will make Apple change QuickDraw next are the architectural issues such as scaleable fonts and resolution independent display routines--basically catching up with Display Postscript,” Hertzfeld thinks.

He’d like to see Apple offer both an enhanced QuickDraw and Postscript so applications programmers could select their choice for both screens and printers.

“The Macintosh would be better off if it could have both. And, I also think it would be a little less risky for Apple than to continue trying to develop on their own all the things that Postscript does so well”, Hertzfeld says.

“In the meantime, my QuickerDraw ‘tune-up’ will make graphics production easier and faster on the Mac II.”

Hertzfeld on Creativity

Is computer programming creative, creative in the sense as producing a masterpiece painting or writing a best-seller?

“Absolutely!” Hertzfeld states.

“There are two different types of programming creativity, though,” he advises, “and both are equally important in a good programmer.”

The first sort of creativity is involved in initially picking the right area and then the right problem on which to work. This involves thinking about what the users really need that will help them the most.

Then there’s the actual writing of code and choosing instructions which can be as individualistic as any painting or writing style, he says.


Community Search:
MacTech Search:

Software Updates via MacUpdate

Between 2 Taps - Tap for Tap interview M...
Hello, and welcome back to Between 2 Taps, Tap for Tap’s Indie Dev interview series. [Read more] | Read more »
Facility 47 (Games)
Facility 47 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: You wake up alone and freezing in an icy cell. You try the cell door but it’s locked, it seems that you are stuck with no... | Read more »
The best Photoshop alternative on iPad
Instagram and Lightroom are great and all, but sometimes people need to get extra creative with their image editing.Like, Photoshop creative. If you're one of these people, take a look at our pick for the best mobile Photoshop experience on iPad... | Read more »
The Walking Dead: No Man’s Land guide -...
A new update for The Walking Dead: No Man’s Land was released last week, making it the perfect time for you to head back to your base and take out some walkers. Here’s the lowdown on what’s new to the game, and how to take advantage. [Read more] | Read more »
Goat Rider guide - Tips and tricks to st...
We've all been there. One second, we're riding high on a crazed goat, and the next, we've been tossed off it like someone who's no good at goat ridin'. [Read more] | Read more »
Real Boxing 2 CREED: How to become a gre...
Just in time for Rocky fans who can’t wait to see CREED, the latest movie, we have the official tie-in game,Real Boxing 2 CREED. It builds on the success of its predecessor and there’s lots to take in so we at 148apps thought we’d run you through... | Read more »
CoinOp Heroes 2 guide - How to build an...
CoinOp Heroes 2 justlaunched and, like all clickers, it's dangerously addictive stuff. You have to furiously tap your screen to defeat wave after wave of foes and earn an insane amount of cash to spend on character upgrades and an army of minions... | Read more »
Dr. Panda Firefighters (Education)
Dr. Panda Firefighters 1.0.1 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0.1 (iTunes) Description: FIGHT FIRES AND SAVE THE DAY!Work together with Dr. Panda and his firefighting team to rescue his trapped... | Read more »
Puddle + (Games)
Puddle + 1.0 Device: iOS iPhone Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Puddle is back in a new "+" edition featuring enhanced graphics, new videos and Apple TV support ! No IAP and No Ads. Dive into Puddle... | Read more »
Football Manager Mobile 2016 (Games)
Football Manager Mobile 2016 7.0.0 Device: iOS Universal Category: Games Price: $8.99, Version: 7.0.0 (iTunes) Description: Football Manager Mobile 2016 is designed to be played on the move and is the quickest way to manage your... | Read more »

Price Scanner via

Adorama Black Friday deals: Up to $400 off Ma...
Adorama has released their Black Friday deals for 2015. Save up to $400 on MacBook Pros, $200 on MacBooks and MacBook Airs, and $270 on iMacs. Use code RYBFDEAL during checkout to see these prices.... Read more
B&H Photo Deals: $200 off 12-inch 1.2GHz...
In addition to the B&H Photo Black Friday week sales we posted yesterday, B&H has lowered their price on two products to $200 off MSRP: - 12″ 1.2GHz Gray Retina MacBook: $1399 save $200 - 13... Read more
Best Buy Early Access: Today only, Up to $125...
Best Buy has iPad Air 2s on sale for up to $125 off MSRP and Apple Watch models on sale on their online store for up to $100 off MSRP with special codes through midnight CT tonight. Choose free... Read more
UPPERCASE DESIGNS Premium Ultra Thin Keyboard...
UPPERCASE Designs today announced its new Premium Ultra Thin Keyboard Protector and its Palm Rest Protector Set for the 12-inch MacBook. The accessories provide durable protection for the 12-inch... Read more
Al Jazeera Launches New iOS And Android Mobil...
Doha, Qatar based Al Jazeera has launched new mobile and tablet apps on the iOS and Android systems bringing the latest Al Jazeera news and programmes live together with on-demand personalisation.... Read more
B&H Photo Holiday Sale: Up to $250 off Ma...
B&H Photo has all new Macs on sale for up to $500 off MSRP as part of their Holiday sale including free shipping plus NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off - 15″ 2.... Read more
Free Aura ‘Ultimate’ Mac App For Gmail Update...
Miami, Florida based Crosscoded has announced Aura 1.2.0, an update to the Mac app for Gmail. Aura mixes the power of a native client with the flexibility of the Gmail web app with support for up to... Read more
Apple Will Edge Closer to Samsung in Smartpho...
Total smartphone shipments for 2015 are projected to decline by 9.7% to 1.286 billion units, according to the latest report from global market research firm TrendForce. Though Chinese vendors have... Read more
Sidefari – Split Screen Multitasking In Safar...
Francisco Cantu’s Sidefari is a simple web browser designed to act as a companion to Safari on the iPad. With multitasking in iOS 9, Sidefari uses the new Safari View Controller to show an extra... Read more
12-inch MacBooks in stock for up to $120 off,...
Adorama has 12″ Retina MacBooks in stock for up to $120 off MSRP including free shipping plus NY & NJ sales tax only. For a limited time, Adorama will include a free Apple USB-C to USB Adapter,... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Merchant Operations Manager: *Apple* Pay -...
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 than Read more
*Apple* Pay QA Manager - Apple Inc. (United...
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 than Read more
Sr Software Engineer *Apple* Pay - Apple In...
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 than Read more
Hardware Systems Architect - *Apple* Watch...
# Hardware Systems Architect - Apple Watch Job Number: 38449977 Santa Clara Valley, Califo ia, United States Posted: Apr. 16, 2015 Weekly Hours: **Job Summary** The Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.