TweetFollow Us on Twitter

Pictures
Volume Number:2
Issue Number:4
Column Tag:Toolbox Notes

On the nature of Pictures

By Chris Derossi, Chief Wizard, MacTutor Contributing Editor

A Quick Note on Pictures

This article has two puposes. The first is to more fully explain the information contained in Macintosh Technical Note #21; that is, the QuickDraw internal encoded picture format. The second bit of information concerns two problems which I have found relating to QuickDraw pictures.

To begin, let's go over the process of creating a picture. When you call OpenPicture, QuickDraw allocates a new handle, stores some initial data there, sets the picSave field for the current port, calls HidePen, and returns the new handle. The initial data consists of the length, rectangle, version, and clipping region of the picture.

When you then proceed to draw, calling QuickDraw with various artistic requests, QuickDraw notes that there is an open picture because of the valid picSave handle. Because of this, QuickDraw translates your request into its own picture shorthand, and appends this information to the picture data. The length information at the beginning of the picture data is increased to reflect the new data. Since HidePen had been called, none of this shows up on the screen. (Unless, of course, you have called ShowPen without a balancing call to HidePen.)

Finally, you call ClosePicture which calls ShowPen, places an end-of-picture marker in the picture data, and sets the picSave field to NIL.

Now, let's go through some picture data byte by byte. The first two bytes of information are the length, which represent the length of ALL of the data, including the length bytes themselves. Next are eight bytes (4 words/Integers) of rectangle information (top, left, bottom, right). This is the rectangle which was passed as the parameter to OpenPicture. Following this are the picture opcodes and their data.

A picture opcode is a single byte which tells QuickDraw to do something, or how to interpret some amount of additional data. The length and composition of the data following the opcode depends on the opcode itself. The first opcode in a picture is always $11 which represents "Version". This opcode has one byte of data which represents the version number. Whenever QuickDraw encounters $11, it knows to treat the next byte as the version number, and to skip over it to get to the next opcode. Opcodes and their data are packed together. That is, immediately following an opcode and its data is the next opcode.

After the version, there may be any number of QuickDraw opcodes. The last opcode in a picture is always $FF which stands for "End-of-Picture". Data after this opcode is ignored. (Under normal circumstances, there won't be any further data.)

If the picture was created in the usual manner (i.e. with OpenPicture instead of generating the picture data some other way) then the next opcode is $01 which is "ClipRgn". The data following this opcode is a region, the first word of which is the length of the region. Normally, it will look something like the data in Figure 1. When you call OpenPicture, the region is copied from the current port's clipping region.

Most of the opcodes are pretty straightforward. Some, however, could use a little explanation. Most of the verb-object calls have a corresponding verbSameobject call. The latter simply uses the last explicit data (be it a Rect, RRect, etc) from a prior call for its own argument. For example, if you wanted to do a paintRect (opcode $31) on a rectangle with coordinates (0, 0, 45, 45) and then invertRect the same rectangle, the resultant picture data would look like that in Figure 2.

Instead of passing the fill pattern as an argument each time as in the QuickDraw call, pictures use the concept of a 'current' fill pattern which the TechNote calls "thePat". (This is analagous to the current pen pattern.) To fill several Rects with the same pattern, there would be one opcode to set thePat ($0A), and then several fillRect opcodes and their respective rectangles. (Figure 3).

Because simple lines and text are so common, there are several versions of each so that QuickDraw can use the version that takes the least amount of space. Lines come in two sizes: regular and short. Short lines have vertical and horizontal lengths which can be represented by a single byte (-128 to 127). The horizontal and vertical lengths are then used instead of the line endpoint. Both line and short line (opcodes $20 and $22) also include the starting endpoint. The remaining two line opcodes ($21 and $23) start the line from the current pen location, which could have been set by previous line or text drawing.

If text is to be drawn only a short distance from the current pen location, QuickDraw uses opcodes $29, $2A, or $2B to move the pen horizontally, vertically, or both before drawing the text. If the starting location for the text is too distant to represent with single byte offsets, opcode $28 is used, and the actual point at which to draw the text is specified.

The last 'gotcha', and the first problem I'd like to discuss both concern the SetOrigin call, which is opcode $0C. This opcode takes four bytes (two words/Integers) as arguments. BUT, the two words do not represent the actual origin that you specified with the SetOrigin call; Instead, they are the offset from the current origin to the new one.

For example, if you were at origin (0,0) and you made a SetOrigin(15,10) call, the picture data would be $0C $00 $0F $00 $0A. More importantly, if you were in a situation where the origin was unknown, you might be inclined to call SetOrigin(0, 0) before performing any drawing. This can have some unexpected side effects. Consider the following scenario:

Your code does not know where the origin is, so it calls SetOrigin(0, 0). Let us say that the origin was in fact at (15, 10). QuickDraw would record an origin change of -15, -10 ($0C $FF $F1 $FF $F6). Then you do your drawing and close the picture.

Now, since you didn't make any more SetOrigin calls, you know that the origin is at (0, 0), which is where you want it to be. So you call DrawPicture. QuickDraw would then come across the $0C opcode and changes the origin to (-15, -10), and your drawing would be in the wrong place! The way to avoid this problem is to call SetOrigin before you open the picture, and again before you draw the picture.

The technique of changing picture origins has a use, for example, in printing, when you want to draw a very large picture. You can simply originate different areas of the picture over the 'printer paper' port and draw the picture. In this way, you can print your large picture on several pieces of paper. Moving pictures around is also a nice way to show different parts of a picture in response to a scroll bar; You only have to generate the display once, and just draw the picture with offsets based on the scroll bar values.

But a common occurrance while doing this is to have the picture vanish when it is drawn at any location other than the one where it was created. The reason for this is simple but elusive.

Recall that when you call OpenPicture, QuickDraw copies the clipping region of the port into the picture data. Well, a newly created port has a very large clipping region, specifically, a rectangle with coordinates (-32767, -32767, 32767, 32767). In hexadecimal, those coordinates translate to ($8001, $8001, $7FFF, $7FFF).

If you offset the picture rectangle just one pixel to the right, QuickDraw re-calculates all of the coordinates in the picture data, including those of the clipping region. But, if you add 1 to $8001, you get $8002 (which is -32766) and 1 + $7FFF is $8000 (decimal -32768). This leaves you with coordinates of (-32767, -32766, 32767, -32768) for the clipping rectangle. This is an empty rectangle because the right side is less than the left side. The whole picture gets clipped!

The solution to this problem is as easy as the solution to the last one: Set your clipping region before you open your picture. You can keep the clipping region very large, but stay away from the values that come close to hex $8000. Such is life with finite mathematics.

I hope this has helped to shed some light on your use of pictures. Used intelligently, they can save you from redrawing the same things again and again. As a side note, if you are interested in pictures and the LaserWriter, you should see TechNote #27: The MacDraw Picture Format.

Ciao.

________________________________________________________________________________

Macintosh Technical Notes

#21: Quickdraw's Internal Picture Definition

See also: QuickDraw

Programming in Assembly Language

Written by: Ginger Jernigan April 24, 1985

__________________________________________________________________________________

This technote describes the internal format of the QuickDraw picture data structure.

__________________________________________________________________________________

This technote describes the long awaited internal definition of the QuickDraw picture. The information given here is meant for DEBUGGING PURPOSES ONLY. It is NOT useful in writing your own picture bottleneck procedures. The reason is that if we add new objects to the picture definition, your program will not be able to operate on pictures created using standard QuickDraw. Your program will not know the size of the new objects and will, therefore, not be able to proceed past the new objects. (What this ultimately means is that pictures will not be downward compatible; you can't process a new picture with a old bottleneck proc.)

Before listing the opcodes a little information is in order. An "opcode" is a number that DrawPicture, for example, uses to determine how to draw that particular object in the picture, and how much data is associated with it. The following list gives the opcode, the name of the object, the associated data, and the total size, in bytes, of the opcode and associated data. To better interpret the sizes, please refer to page 4 in Programming in Assembly Language. For types not described there, here is a quick list:

opcode 1 byte

point long

0..255 1 byte

-128..127 1 byte

rect 8 bytes

poly 11+ bytes

region 10+ bytes

fixed point number long

pattern 8 bytes

Each picture definition begins with a picsize (word), then a picframe (rect), and then the picture definition, which consists of a combination of the following opcodes:

Opcode Name Additional Data Total Size (bytes)

00 NOP none 1

01 clipRgn rgn 1+rgn

02 bkPat pattern 9

03 txFont font (word) 3

04 txFace face (byte) 2

05 txMode mode (word) 3

06 spExtra extra (fixed point) 5

07 pnSize pnSize (point) 5

08 pnMode mode (word) 3

09 pnPat pattern 9

0A thePat pattern 9

0B ovSize point 5

0C origin dh, dv (word) 5

0D txSize size (word) 3

0E fgColor color (long) 5

0F bkColor color (long) 5

10 txRatio numer (point), denom (point) 9

11 picVersion version (byte) 2

20 line pnLoc ( point ), newPt ( point ) 9

21 line from newPt ( point ) 5

22 short line pnLoc ( point ), dh, dv (-128..127) 7

23 short line from dh, dv (-128..127) 3

28 long text txLoc ( point ), count (0..255), text 6+text

29 DH text dh (0..255), count (0..255), text 3+text

2A DV text dv (0..255), count (0..255), text 3+text

2B DHDV text dh, dv (0..255), count (0..255), text 4+text

30 frameRect rect 9

31 paintRect rect 9

32 eraseRect rect 9

33 invertRect rect 9

34 fillRect rect 9

38 frameSameRect rect 1

39 paintSameRect rect 1

3A eraseSameRect rect 1

3B invertSameRect rect 1

3C fillSameRect rect 1

40 frameRRect rect 9

41 paintRRect rect 9

42 eraseRRect rect 9

43 invertRRect rect 9

44 fillRRect rect 9

48 frameSameRRect rect 1

49 paintSameRRect rect 1

4A eraseSameRRect rect 1

4B invertSameRRect rect 1

4C fillSameRRect rect 1

50 frameOval rect 9

51 paintOval rect 9

52 eraseOval rect 9

53 invertOval rect 9

54 fillOval rect 9

58 frameSameOval rect 1

59 paintSameOval rect 1

5A eraseSameOval rect 1

5B invertSameOval rect 1

5C fillSameOval rect 1

60 frameArc rect 9

61 paintArc rect 9

62 eraseArc rect 9

63 invertArc rect 9

64 fillArc rect 9

68 frameSameArc rect 1

69 paintSameArc rect 1

6A eraseSameArc rect 1

6B invertSameArc rect 1

6C fillSameArc rect 1

70 framePoly poly 1+poly

71 paintPoly poly 1+poly

72 erasePoly poly 1+poly

73 invertPoly poly 1+poly

74 fillPoly poly 1+poly

78 frameSamePoly (not yet implemented) 1

79 paintSamePoly (not yet implemented) 1

7A eraseSamePoly (not yet implemented) 1

7B invertSamePoly (not yet implemented) 1

7C fillSamePoly (not yet implemented) 1

80 frameRgn rgn 1+rgn

81 paintRgn rgn 1+rgn

82 eraseRgn rgn 1+rgn

83 invertRgn rgn 1+rgn

84 fillRgn rgn 1+rgn

88 frameSameRgn (not yet implemented) 1

89 paintSameRgn (not yet implemented) 1

8A eraseSameRgn (not yet implemented) 1

8B invertSameRgn (not yet implemented) 1

8C fillSameRgn (not yet implemented) 1

90 BitsRect rowBytes, bounds, srcRect, dstRect, mode, 30+unpacked

byteCount, unpacked bitData bitData

91 BitsRgn rowBytes, bounds, srcRect, dstRect, mode, 30+rgn+

maskRgn, byteCount, unpacked bitData bitData

98 PackBitsRect rowBytes, bounds, srcRect, dstRect, mode, 30+packed

byteCount, packed bitData bitData

99 PackBitsRgn rowBytes, bounds, srcRect, dstRect, mode, 30+rgn+

maskRgn, byteCount, packed bitData packed bitData

A0 shortComment kind(word) 3

A1 longComment kind(word), size(word), data 5+data

FF EndOfPicture none 1

 
AAPL
$100.96
Apple Inc.
-0.83
MSFT
$47.52
Microsoft Corpora
+0.84
GOOG
$596.08
Google Inc.
+6.81

MacTech Search:
Community Search:

Software Updates via MacUpdate

Airfoil 4.8.9 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
WhatRoute 1.13.0 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the... Read more
Chromium 37.0.2062.122 - 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
Attachment Tamer 3.1.14b9 - Take control...
Attachment Tamer gives you control over attachment handling in Apple Mail. It fixes the most annoying Apple Mail flaws, ensures compatibility with other email software, and allows you to set up how... Read more
Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.2 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Quicken 2015 2.0.4 - Complete personal f...
Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial transactions,... Read more
iMazing 1.0 - Complete iOS device manage...
iMazing (formerly DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and... Read more
Xcode 6.0.1 - Integrated development env...
Apple Xcode is Apple Computer's integrated development environment (IDE) for OS X. The full Xcode package is free to ADC members and includes all the tools you need to create, debug, and optimize... Read more

Latest Forum Discussions

See All

View Source – HTML, JavaScript and CSS...
View Source – HTML, JavaScript and CSS 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: View Source is an app plus an iOS 8 Safari extension that makes it easy to do one key web developer... | Read more »
Avenged Sevenfold’s Hail To The King: De...
Avenged Sevenfold’s Hail To The King: Deathbat is Coming to iOS on October 16th Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Just in time for Halloween, on October 16 Avenged Sevenfold will be launching | Read more »
Talisman Has Gone Universal – Can Now be...
Talisman Has Gone Universal – Can Now be Played on the iPhone Posted by Jessica Fisher on September 19th, 2014 [ permalink ] | Read more »
Tap Army Review
Tap Army Review By Jennifer Allen on September 19th, 2014 Our Rating: :: SHOOT EM ALLUniversal App - Designed for iPhone and iPad Mindless but fun, Tap Army is a lane-based shooter that should help you relieve some stress.   | Read more »
Monsters! Volcanoes! Loot! Epic Island f...
Monsters! Volcanoes! Loot! | Read more »
Plunder Pirates: Tips, Tricks, Strategie...
Ahoy There, Seadogs: Interested in knowing our thoughts on all this plundering and pirating? Check out our Plunder Pirates Review! Have you just downloaded the rather enjoyable pirate-em-up Plunder Pirates and are in need of some assistance? Never... | Read more »
Goat Simulator Review
Goat Simulator Review By Lee Hamlet on September 19th, 2014 Our Rating: :: THE GRUFFEST OF BILLY GOATSUniversal App - Designed for iPhone and iPad Unleash chaos as a grumpy goat in this humorous but short-lived casual game.   | Read more »
A New and Improved Wunderlist is Here fo...
A New and Improved Wunderlist is Here for iOS 8 Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Evernote Update for iOS 8 Adds Web Clipp...
Evernote Update for iOS 8 Adds Web Clipping, Quick Notes, and More Posted by Ellis Spice on September 19th, 2014 [ permalink ] | Read more »
Apple Names Ultimate Productivity Bundl...
Apple Names Ultimate Productivity Bundle by Readdle as the Essential Bundle on the App Store Posted by Jessica Fisher on September 19th, 2014 [ permalink | Read more »

Price Scanner via MacPrices.net

Updated Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Mac Pros available for up to $260 off MSRP
Adorama has Mac Pros on sale for up to $260 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 4-core Mac Pro: $2839.99, $160 off MSRP - 6-core Mac Pro: $3739.99, $260... Read more
13-inch 2.6GHz/256GB Retina MacBook Pros avai...
B&H Photo has the 13″ 2.6GHz/256GB Retina MacBook Pro on sale for $1379 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Previous-generation 15-inch 2.0GHz Retina Mac...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
21″ 2.7GHz iMac available for $1179, save $12...
Adorama has 21″ 2.7GHz Hawell iMacs on sale for $1179.99 including free shipping. Their price is $120 off MSRP. NY and NJ sales tax only. Read more
iOS 8 Adoption Rate Slower than iOS 7, 6, Hit...
Apple began pushing out iOS 8 updates to eligible devices around 1pm ET on September 17, 2014. However, unlike with iOS 7, which boasted a wide variety of differences from its predecessor iOS 6, in... Read more
LIkely Final Definitive OS X 10.9.5 Mavericks...
Apple has released what will almost certainly be the last incremental version number update of OS X 10.9 Mavericks (save for futire security updates) before OS X 10.10 Yosemite is released next month... Read more
Fingerprints, Apple Pay and Identity Theft Wa...
On Sep 9th, CEO Tim Cook unveiled Apple Pay, along with the new iPhone 6 and iWatch. Apple Pay is a newly developed technology that utilizes a near field communication (NFC) to enable customer... Read more
Amazon Introduces Two All-New Kindles
Amazon on Thursday introduced the 7th generation of its Kindle dedicated e-reader device: Kindle Voyage, its top-of-the-line e-reader, and the new $79 Kindle, with a 20% faster processor, twice the... Read more
Save up to $300 on the price of a new Mac wit...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more

Jobs Board

Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*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
*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
*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
*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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.