Amiga Computing - Commodore Is Awesome

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HE Amiga's Interchange File Format is a wonderful thing. By adopting a workable. intelligent standard the user is assured a relatively easy time when it comes to swapping data between programs and even other computer systems_ So far we have looked at the overall structure of the IFF tree, and described a few of its many possible uses. We now increase the magnification of our microscope a little more and focus on the contents of the IFF chunks themselves. What lies within? A PROPERTY is a chunk which describes other chunks. It is rather like a set of assignments in a program, such as: width.323; height , xxord710; 200: y.cord.10; A data. or non-property, chunk holds the actual data of a FORM. You could think of property chunks as adjectives, and data chunks as nouns, so BMHD and CMAP are properties, and BODY is a data chunk. Some chunks are essential in a particular type of file. An ILBM file, for example, must have a BMHD chunk in order to be valid, so BMHD is described as a required property_ The CMAP chunk, however, is not essential, so we call it an optional property. A universal chunk is one — such as FORM — which is defined in IFF-85, and forms part of generic WE The identifier 'FORM", among others, is reserved for all IFF files, so if I develop a new supplement I can't invent some new type of leaf chunk and call it FORM. A FORM will always be a group chunk holding a data object. A local chunk is one which is reserved only within the supplement in which it is defined. A CMAP, for example, is currently only defined within ILBM. If I come up with a new supplement, say ATLS for an atlas, then there's no reason why I can't use CMAP to hold a nautical navigation chart, so long as I only use it as such within a FORM ATLS. When an IFF supplement is first produced it is asking a bit much to expect the designer to think of every possible use to which the new FORM 116 AMIGA COMPUTING Nommtler 1.9go type will be put. The IFF standard permits developers to invent new types of chunk to go into previously defined forms. One example is CRNG — a chunk introduced by Electronic Arts when they developed Deluxe Paint. It is inserted into a FORM ILBM to describe colour cycling. Because it was introduced after ILBM first came into use, it is known as a nonstandard chunk. As IFF allows nonstandard chunks to be introduced, any program which reads IFF files must be prepared to skip over any chunks it does not recognise. A "CAT" is a group chunk which is used as a mixed bag. It could hold. for example, a slide show, with a FORM SMUS for music. and a LIST of FORM ILBMs to be shown in sequence. A LIST is like a CAT, except that it allows for properties to be shared between chunks inside it. So if a lot of pictures all use the same palette, you can have a LIST containing a PROP with a CMAP inside it, followed by a whole sequence of FORM ILBMs without CMAPs. A PROP is not the same as a property chunk — rather it is a group chunk which is used for holding one or more property chunks. A PROP will only ever appear inside a LIST — not a FORM or CAT. If one or two pictures have a different palette from the majority, you can put a PROP at the top to set a default for FORM ILBMs without CMAP chunks, then different CMAP chunks inside the ones which differ, to override the default. • TO give you a flavour of how IFF files really work, there are two programs on this month's cover disk. IFFanalyse lets you convert IFF files into a human-readable form. You can just examine them, or if you wish edit them with Your favourite text editor. Afterwards you can use the second program. IFFsynthesise, to compile FORM is universally defined, CMAP is not 'FORM' HAM'. W A I 'BODY P 'FORM' BMHD' A PROP inside a LIST sets default properties Delot paigetu cietst4 Usti rts awn pal them back into the IFF format that almost all Amiga applications use. In its simplest form, IFFedit would simply convert binary !FE files into hex listings with chunk names and sizes in text. All that recompilation would then involve would be a hex lookup, plus a bit of backtracking to ensure that chunk sizes were correctly set. This is just what IFFedit does if an IFF file is not of a type that it recognises. In this case I will refer to it as generic translation. IFFedit does more than this though. If a file is of a type that is recognised. such as 8SVX or SMUS, then any standard data or property chunks will be translated in a way that reflects their meaning. Non-standard chunks will still, of course, be translated using the generic method, provided that they conform to IFF-85 syntax. 1- EACH IFF file contains one FORM, LIST. or CAT, inside which may be nested other chunks according to the concrete syntax below (based on "Syntax Definitions", Exec Manual
App,B p.40 - this version sacrifices rigour for readability): Ounk FCRM CAT LIST PROP /D 4( uPYTE* ) [0] ::= 'FORM" 4( Type (Chunk I FORM ::= 'CAT ' 4( Type (FORM I LIST I ::= 'UST 4( Type PROP* (FORM I u. Type Chunk* I Note: [01 represents a pad byte which may be needed. # is a LONG type number stating the number of following {braced) bytes. * means 0 or more instances (it's not a C-language pointer). An equivalent syntax for the text version would read: Chunk FORN and so on... LIST I CAT)* ] CATI* ) LIST CATI* ID 'I' CMAR* 9' q. Type (Chunk I FORM Lisr I CAT(' T Note that the pad bytes and byte counts have gone - alignment is unnecessary, and the delimiting braces now appear in the actual file, not just the syntax statement. Two cosmetic features have also been added - spaces and comments. They don't appear in the syntax diagram because they are skipped over at lexical analysis, before syntax analysis starts. Comments start with a semicolon, and extend to the end of the line on which they appear. This example shows a sample use of IFFanalyse on a generic - in other words unrecognised - form. This is how the example form would appear if listed using "list NBRS.iff opt h" 0000: 464F5240 00000026 4E425253 413594C49 1B1I...iNFIRSKYLI 0010: 00000008 01234567 89ADCDEF 497C0100 0020: 4A41534E 00010005 03122937 5900 - T A S K i n IFFanalyse would show that this file is a FORM of type NBRS containing two chunks of type KYLI and JASN respectively. Chunk sizes are given (in decimal) as comments, and an Ascii representation of chunk contents is also given. FORMI HERS :size=38 :eize=11 01234567 89N3CCIEF 497CD1 rsize=5 03122937 59 If you were to edit this text file you would only need to concern yourself with material before the semicolon in each line. IFFsynthesise will calculate chunk sizes from the amount of actual data between '-{" and ")" braces, so the fact that a size comment such as ;size-38 "or Ascii comment such as' may no longer be correct after editing a file is unimportant. An output like the one above is better ;.4Eq....II. A than nothing, but it suffers from one drawback - it reflects only the structure of the data, not its meaning. If you're working with 8SVX sampled sound files, or SMUS music files, you're in luck. IFFedit recognises these forms, and for each one it has a list of standard chunk types which receive special treatment. There is a separate list for each form type because chunk types are only reserved within a form type. A FORM AUTH chunk happens to be the same as an 8SVX AUTH, but there's no rule saying it has to be. In particular, I might treat an ILBM BODY - if I get round to supporting ILBM - quite differently from 8SVX BODY. Any addition made to IFFanalyse must be made to IFFsynthesise as well, otherwise IFFsynthesise will fail to recognise some mnemonic which has been used to represent data. An 8SVX chunk has eightstandard chunk types - VHDR, NAME, Copyright ("(c) "), AUTH, ANNO, ATAK, RLSE and BODY. Descriptions of these can be found in the Exec manual App.0 pp.63- 68, and are not given here. NAME, Copyright, AUTH and ANNO are treated as text, so a name chunk: 4E414E145 00000006 73636F72 6532 MAME_ •score2 would appear as: NAME i ;size=6 ' s c o r e r [ ATAK, RLSE and BODY are translated generically. A future version could give mnemonics for ATAK and RLSE chunks. VHDR has a special format: 56484452 00000014 00000400 00000001 00000000 , 6E19010 0 0 0 0 1 0 0 0 0 appears as: 'VIER ( :size=20 oneShot 1024 repeat 0 PerRi 0 PerSec 28185 81 octaves 1 comp 0 volune 65536 Mnemonic names, for example. "PerSec" are not case-sensitive, but they must appear in the order given above, and the decimal numbers after them are size checked by IFFsynthesise. VHDR ....n ....... INJ Paul Holmes continues his look at the Amiga's unique filing system, the IFF: This month - properties, chunks and a chance for you to experiment with file formats yourself Next month Paul looks at more examples where IFF can save you time and frustration. More chunks than a jar of orange marmalade! AMIGA comptrutx November 1990 117
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Volume 3 Number 6 Novembert990 E2.9
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Amiga Computing - Commodore Is Awesome

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