TweetFollow Us on Twitter

LaserWriter CopyBits Support

Volume Number: 13 (1997)
Issue Number: 12
Column Tag: Develop

LaserWriter 8.5.1 Additional CopyBits Support: Transparent and Clipped Images

by David Gelphman, RBI Software Systems Inc. and Ingrid Kelly, Apple Computer Inc.

Overview

QuickDraw provides several APIs for copying bitmaps from one graphics port into another, the most flexible of which is CopyBits(). Most developers use CopyBits() because it can move offscreen images into an onscreen window, change colors in the pixmap, and resize images. This is a useful API, however, unfortunately, LaserWriter 8.4.x and earlier PostScript printer drivers from Apple have not supported all of the capabilities of the CopyBits() call to draw images. This TechNote describes where Apple has enhanced its handling of the CopyBits() call in LaserWriter 8.5.1 so that developers and users may take advantage of the improvements.

CopyBits()

The prototype for CopyBits() is:

void CopyBits(const BitMap *srcBits, const BitMap *dstBits, 
const Rect *srcRect, const Rect *dstRect, short mode, RgnHandle 
maskRgn);

In the mode parameter, many developers have tried to use the transparent transfer mode to achieve transparency; the LaserWriter driver, however, never supported this mode but instead treated it as a srcCopy mode. The LaserWriter driver also has not supported clips in the maskRgn parameter. This has all changed with the LaserWriter 8.5.1 release.

Transparent Mode

One transfer mode previously unsupported by the LaserWriter driver is the transparent transfer mode. This mode allows drawing of images other than 1-bit deep so that any sample in the source which is the transparent color (defined as the QuickDraw background color) will not be painted to the destination. The advantage of this mode is that it lets a deep (<> 1-bit) image have holes in it by coloring the holes in the image with the transparent color. Whereas previously the LaserWriter driver would paint these images as if the transfer mode were srcCopy, LaserWriter 8.5.1's new imaging code now handles the transparent mode.

maskRgn Clipping

Another previously unsupported feature of CopyBits() was the ability to pass in a mask (via the maskRgn field) to clip out portions of the destination image. Previous LaserWriter drivers completely ignored the maskRgn passed to CopyBits(). The new imaging code in LaserWriter 8.5.1 now supports the supplied maskRgn.

Note: While the LaserWriter 8.5.1 driver now handles a supplied maskRgn to a CopyBits call, we do not do any special handling of the clip provided in the grafPort. GrafPort clipping is still handled by clipping only to the bounding rectangle of the port's clipRgn, both for CopyBits calls as well as line art and text drawing. Apple considered more generalized support for the port clipRgn but has rejected it to date mainly due to compatibility concerns.

Limitations

There are some known limitations with the added CopyBits() support that you should be aware of. They are:

  • Extremely complex clipping regions may occasionally fail on Level 1 or Level 2 devices in low printer memory conditions. The result is that the output will not preserve the clip, and the image will print as if the maskRgn parameter is ignored.

    Specifically, for PostScript Level 1 printers, there is a hard coded limit to how complex a clip can be, regardless of the printer's installed memory. If the clip is more complex than this limit, the LaserWriter driver ignores the maskRgn parameter. For Level 2 printers, the limits are completely memory-based and generally much more flexible. However, once again, in low-memory conditions, the clip may still fail. On PostScript Level 3 printers, all clips should always print correctly regardless of the complexity of the clip since the driver uses the PostScript Level 3 masked image to support this feature.

  • Since the LaserWriter driver implements the transparent mode via clipping, complex transparent regions may occasionally fail on Level 1 or Level 2 devices in low-memory conditions as described above. The result is that the output does not preserve the transparency, and the pixels image with the background color instead.
  • For 2, 4, and 8-bit images with a color look-up table (CLUT), the LaserWriter 8.5.1 driver only supports one index which maps to the background color. Although the CLUT may contain many index values which have the background color and are therefore, in principle, transparent when displayed onscreen, the driver searches the look-up table for the first index which corresponds to the background color. Index samples with that index value are printed as transparent. Other index values which correspond to the background color are not treated as transparent but are printed as the background color.
  • In order to maintain compatibility with many applications, the LaserWriter 8.5.1 driver will ignore a maskRgn which is an empty region under some circumstances (e.g. some rotated text and graphics). Some applications have used the fact that previous drivers always ignored the maskRgn parameter to clip out certain data when printing to QuickDraw printers. Relying on this 'feature' is not recommended, and the fact that there are some edge cases where a zero clip is ignored for compatibility reasons may be a temporary 'feature' of the driver. To avoid accidentally encountering these cases, when you really want a zero clip, either do not draw the image (preferred) or use grafPort clipping to clip it.

Summary

New imaging code in the LaserWriter has opened up some new functionality for the QuickDraw CopyBits() call. We encourage you to try LaserWriter 8.5.1 with your application and any PostScript device.

Further References

Acknowledgements

Thanks to John Blanchard, Tim Carroll and Paul Danbold.


David Gelphman seems to specialize in backwards reading programming languages. From programming in FORTH at the Stanford Linear Accelerator Center, he moved into PostScript at Adobe Systems and then to Telescript at General Magic. He does do most other things in a more or less forward direction, although he has been known to fall off a horse backwards. David, together with his colleague Richard Blanchard, co-designed Apple's LaserWriter 8 PostScript printer driver while working at Adobe Systems. After a stint at General Magic, David now works at RBI Software Systems (http://www.rbi.com) as a contractor to Apple and Adobe on their PostScript printer drivers. He does other contracting work as well, primarily in the area of PostScript printing.

Ingrid Kelly always dreamed of working for Apple Computer while she was growing up in Silicon Valley. What she did not realize back then was that all of her experience printing out school papers on an ImageWriter would turn into a job supporting printing and printer drivers for Apple's Developer Technical Support (DTS) group. After a few years in DTS, Ingrid has now learned to appreciate the black magic involved in printing.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

The best scanner app on mobile
People always say that the best camera is the one you have with you. Well, the same is true with scanners, and your phone can be a pretty great tool for scanning receipts and other documents while you're on the go. [Read more] | Read more »
MARVEL Avengers Academy guide - How to g...
MARVEL Avengers Academy lets you build your own superhero school and fill it with heroes from the Marvel universe. It can be a little slow going to get your school's attendance up though, so we've gathered together somesome tips to help you do this... | Read more »
Shadow Blade: Reload guide - How to hack...
Shadow Blade: Reload is the kind of action-platformer that would have happily sucked up hours of your time on a console a few years back.Now, you can take it with you wherever you go, and its mobile conversion is not too shabby at all. To help you... | Read more »
Tomb of the Mask guide - How to increase...
Tomb of the Mask is a great endless arcade game from Happymagenta in which quick reflexes and a persistent attitude can go a long way toward earning a top score. Check out these tips to see if you can give yourself an edge on the leaderboards. [... | Read more »
Smooth Operator! (Games)
Smooth Operator! 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Smooth Operator is a weird, weird two-player kissing game. Squeeze in for 2 player fun on a single iPad, creating awkward... | Read more »
Sinless: Remastered (Games)
Sinless: Remastered 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: | Read more »
_PRISM Guide - How to solve those puzzle...
_PRISM is a rather delightful puzzle game that’s been tailor made for touch screens. While part of the fun is figuring things out as you go along, we thought we’d offer you a helping hand at getting in the right mindset. Don’t worry about messing... | Read more »
Fractal Space (Games)
Fractal Space 1.3.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.3.1 (iTunes) Description: Live the memorable experience of Fractal Space, a unique first person adventure & puzzle game by Haze Games! Will you... | Read more »
Set off on an adventure through the Cand...
Like match three puzzlers? If so, Jelly Blast, the innovative iOS and Android game which launched last year, is worth a look. Jelly Blast sees you head off on an epic adventure through the Candy Kingdom with your friends Lily, Mr. Hare, and Mr.... | Read more »
Ellipsis - Touch. Explore. Survive. (...
Ellipsis - Touch. Explore. Survive. 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

cb Hardcase – Handmade and Premium Protective...
Baden-Baden, Germany based company cb innovations has introduced the new cb Hardcase for iPhone. Featuring fine Italian Premium leather that makes for a unique look and feel, the cb Hardcase... Read more
Sale! B&H Photo offers 12-inch Retina Mac...
B&H Photo has 12″ Retina MacBooks on sale for $300 off MSRP for a limited time. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook: $999 $300 off MSRP - 12″ 1.... Read more
App Annie Reveals Future of the App Economy:...
App Annie, a San Francisco based mobile app data and insights platform, has launched its first comprehensive app economy forecast. This new offering will provide brands, agencies, investors and app... Read more
Apple restocks Certified Refurbished Mac mini...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more
What iPad Pro Still Needs To Make It Truly Pr...
I love my iPad Air 2. So much that I’m grudgingly willing to put up with its compromises and limitations as a production tool in order to take advantage of its virtues. However, since a computer for... Read more
21-inch 3.1GHz 4K on sale for $1399, $100 off...
B&H Photo has the 21″ 3.1GHz 4K iMac on sale $1399 for a limited time. Shipping is free, and B&H charges NY sales tax only. Their price is $100 off MSRP: - 21″ 3.1GHz 4K iMac (MK452LL/A): $... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Save up to $240 with Apple Certified Refurbis...
Apple is now offering Certified Refurbished 12″ Retina MacBooks for up to $240 off the cost of new models. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more
Apple refurbished Time Capsules available for...
Apple has certified refurbished Time Capsules available for $120 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $179, $120 off - 3TB... 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
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
*Apple* Reporter - Business Insider, Inc. (U...
Business Insider is looking for a reporter to cover Apple , the biggest and arguably most important company in tech. As our primary Apple reporter, you will: * Read more
Infrastructure Engineer - *Apple* /Mac - Rem...
…part of a team Requires proven problem solving skills Preferred Additional: Apple Certified System Administrator (ACSA) Apple Certified Technical Coordinator (ACTC) Read more
Lead Engineer *Apple* OSX & Hardware -...
Lead Engineer Apple OSX & Hardware **Job ID:** 3125919 **Full/Part\-Time:** Full\-time **Regular/Temporary:** Regular **Listed:** 2016\-02\-10 **Location:** Cary, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.