TweetFollow Us on Twitter

Hertzfeld
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.

 
AAPL
$94.24
Apple Inc.
+0.30
MSFT
$44.91
Microsoft Corpora
+0.08
GOOG
$597.95
Google Inc.
+8.48

MacTech Search:
Community Search:

Software Updates via MacUpdate

MacBook Air EFI Firmware Update 2.9 - Fo...
MacBook Air EFI Firmware Update is recommended for MacBook Air (Mid 2011) models. This update addresses an issue where systems may take longer to wake from sleep than expected and fixes a rare issue... Read more
FileZilla 3.9.0.1 - Fast and reliable FT...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface.Version 3.9.0.1: MSW: Fix installation issue with locked DLLs... Read more
OS X Yosemite 10.10 DP4 - Developer Prev...
Note: This is a Developer Preview. You must be a registered Apple Mac Developer to download this update. OS X Yosemite is Apple's newest operating system for Mac. An elegant design that feels... Read more
FinderPop 2.5.6 - Classic Mac utility, n...
FinderPop is a Universal preference pane that extends OS X's contextual menus using a FinderPop Items folder much as the Apple Menu Items folder used to do for the Apple menu. It has other features... Read more
SpiderOak 5.1.7 - Secure cloud backup, s...
SpiderOak is a multi-platform secure online backup, storage, access, and sharing solution engineered for the consumer and small businesses. You must first sign up to use SpiderOak. Running natively... Read more
Espionage 3.6 - Simple, state of the art...
Espionage offers state-of-the-art encryption and plausible deniability for your confidential data. Sometimes, encrypting your data isn't enough to protect it. That's why Espionage 3 goes beyond data... Read more
calibre 1.45.0 - Complete e-library mana...
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... Read more
iFFmpeg 4.3.1 - Convert multimedia files...
iFFmpeg is a graphical front-end for FFmpeg, a command-line tool used to convert multimedia files between formats. The command line instructions can be very hard to master/understand, so iFFmpeg does... Read more
Chromium 36.0.1985.125 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
pwSafe 3.0 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more

Latest Forum Discussions

See All

MTN Review
MTN Review By Jessica Fisher on July 22nd, 2014 Our Rating: :: ADORABLE, SERENE, AND AMUSINGUniversal App - Designed for iPhone and iPad MTN is an adorable, talking pet mountain that is less game and more zen garden.   | Read more »
Fly High with Ninja UP! Now Available o...
Fly High with Ninja UP! Now Available on the App Store Posted by Jessica Fisher on July 22nd, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Bio Inc. Review
Bio Inc. Review By Nadia Oxford on July 22nd, 2014 Our Rating: :: SICKENING - IN A COMPELLING WAYUniversal App - Designed for iPhone and iPad Bio Inc is about orchestrating the medical destruction of a single person. If that doesn’... | Read more »
HELMUT Review
HELMUT Review By Andrew Fisher on July 21st, 2014 Our Rating: :: TRUNDLE SIMULATOR 2014Universal App - Designed for iPhone and iPad HELMUT is a fun, fleeting time-sink that offers a momentary distraction and nothing else.   | Read more »
Walkr Review
Walkr Review By Jennifer Allen on July 21st, 2014 Our Rating: :: ORIGINAL WALKINGiPhone App - Designed for the iPhone, compatible with the iPad Walking is a bit more exciting thanks to this planet building/discovering sim reliant... | Read more »
Zombie Commando Review
Zombie Commando Review By Jennifer Allen on July 21st, 2014 Our Rating: :: MINDLESS SLAUGHTERUniversal App - Designed for iPhone and iPad Briefly fun but ultimately forgettable, Zombie Commando will scratch an itch then be... | Read more »
Swords & Poker Adventures Review
Swords & Poker Adventures Review By Jennifer Allen on July 21st, 2014 Our Rating: :: SOULLESS POKER PLAYUniversal App - Designed for iPhone and iPad Swords & Poker Adventures is a mishmash of Poker and RPGing, but it lacks... | Read more »
Warhammer 40,000: The Horus Heresy: Drop...
Warhammer 40,000: The Horus Heresy: Drop Assault Coming Soon to iOS Posted by Jennifer Allen on July 21st, 2014 [ permalink ] Coming soon to iOS will be an all-new Warhammer 40,000 tactical strategy game by the name of The Horus Heresy: Drop As | Read more »
A Life Worth Dying For Review
A Life Worth Dying For Review By Jordan Minor on July 21st, 2014 Our Rating: :: A BEAUTIFUL MINDUniversal App - Designed for iPhone and iPad A Life Worth Dying For is a fascinating portrait of a serious subject.   | Read more »
Zombie Puzzle Panic Review
Zombie Puzzle Panic Review By Jordan Minor on July 21st, 2014 Our Rating: :: THE MATCHING DEADUniversal App - Designed for iPhone and iPad Zombie Puzzle Panic puts some pretty neat undead twists on Match-3 puzzling.   | Read more »

Price Scanner via MacPrices.net

Twelve South HiRise For MacBook – Height-Adju...
If you use your MacBook as a workhorse desktop substitute, as many of us do, a laptop stand combined with an external keyboard and pointing device are pretty much obligatory if you want to avoid... Read more
Why The Mac Was Not Included In The Apple/IBM...
TUAW’s Yoni Heisler cites Fredrick Paul of Network World whoi blogged last week that the Mac’s conspicuous absence from Apple and IBM’s landmark partnership agreement represents a huge squandered... Read more
Save $100 on 13-inch Retina MacBook Pros, plu...
Adorama has 13″ Retina MacBook Pros on sale for $100 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 13″ 2.4GHz/128GB MacBook Pro with Retina Display: $1199 - 13″ 2.... Read more
Blurr it 2.3 for iOS – Quickly Blurs Selected...
Hyderabad, India based TouchLabs has announced a new update of Blurr it 2.3, their photography app for iOS users. Blurr it allows you to blur part of the image to hide potentially sensitive or... Read more
MacBook Airs on sale for $100 off MSRP, start...
Best Buy has the new 2014 MacBook Airs on sale for up to $100 off MSRP on their online store. Choose free home shipping or free local store pickup (if available). Prices valid for online orders only... Read more
Amazon Announces Kindle Unlimited: Unlimited...
Amazon.com has introduced Kindle Unlimited — a new subscription service which allows customers to freely read as much as they want from over 600,000 Kindle books, and listen as much as they want to... Read more
New Linksys Wireless Range Extenders Boost Wi...
Linksys has announced its new lineup of Linksys Wi-Fi Range Extenders. Consumers often experience a weak wireless signal in some parts of their house or apartment caused by blocking elements such as... Read more
MacBook Airs available starting at $719
The Apple Store has Apple Certified Refurbished 2013 & 2012 MacBook Airs in stock today starting at $719. An Apple one-year warranty is included with each MacBook, and shipping is free: 2013... Read more
Get the best deals on iPad minis with Apple r...
The Apple Store has Certified Refurbished 2nd generation iPad minis with Retina Displays available for up to $130 off the cost of new models, starting at $339. Apple’s one-year warranty is included... Read more
Best Buy’s College Student Deals: $100 off Ma...
Take an additional $100 off all MacBooks and iMacs, $50 off iPad Airs and iPad minis, at Best Buy Online with their College Students Deals Savings, valid through July 25th. Anyone with a valid .EDU... Read more

Jobs Board

*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
Mac Expert - *Apple* Online Store Mexico -...
…MUST be fluent in English and Spanish to be considered for this position At Apple , we believe that hard work, a fun environment, creativity and innovation fuel the Read more
*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
Mac Expert - *Apple* Online Store - Apple (...
**Job Summary** At Apple , we believe that hard work, a fun environment, creativity and innovation fuel the ultimate customer experience. We believe each customer Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.