Blorb: A Z-Machine Resource Collection Format Standard

Version 1.0

Andrew Plotkin <erkyrath@netcom.com>

This is a formal proposal for a common format for storing resources
associated with a Z-code file. Resources are data which the Z-machine can
invoke, such as sounds (in the Z-machine versions 3 and up) and pictures (in
the V6 Z-machine.) Future versions of the Z-machine may understand other
kinds of resources.

In addition, a Z-code file may itself be a resource in a resource file. This
is a convenient way to package a Z-code game and all its resources together
in one file.

[[Text in this document is liberally stolen from Martin Frost
<mdf@doc.ic.ac.uk>'s proposal for a common save file format. Ideas are
liberally stolen from my own PICKLE format proposal, which is now withdrawn
in favor of this proposal.]]

This format is named "Blorb" because it wraps your possessions up in a box,
and because the common save file format was at one point named "Gnusto".
That seems to have changed to "Quetzal", but I'm not going to let that stop
me.

This proposal is longer than I would have liked. However, a large percentage
of it is optional stuff -- optional for either the interpreter writer, the
game author, or both. That may make you feel better. I've also put in lots
of examples, explication, and self-justification.

0: Overall Structure

The overall format will be a new IFF type. The FORM type is 'IFRS'.

The first chunk in the FORM must be a resource index (chunk type 'RIdx'.)
This lists all the resources stored in the IFRS FORM.

The resources are stored in the FORM as chunks; each resource is one chunk.
They do not need to be in any particular order, since the resource index
contains all the information necessary to find a particular resource.

There are five optional chunks which may appear in the file: the color
palette (chunk type 'Plte'), the resolution chunk (chunk type 'Reso'), the
loop chunk (chunk type 'Loop'), the release number (chunk type 'RelN'), and
the game identifier (chunk type 'IFhd'). They may occur anywhere in the file
after the resource index.

Several optional chunks may also appear by convention in any IFF FORM: 
'(c) ', 'AUTH', and 'ANNO'. These may also appear anywhere in the file
after the resource index.

1: Contents of the Resource Index Chunk

    4 bytes         'RIdx'          chunk ID
    4 bytes         n               chunk length (4 + num*12)
    4 bytes         num             number of resources
    num*12 bytes    ...             index entries

There is one index entry for each resource. (But not for the optional
chunks.) Each index entry is 12 bytes long:

    4 bytes         usage           resource usage
    4 bytes         number          number of resource
    4 bytes         start           starting position of resource

The index entries should be in the same order as the resource chunks in the
file.

The usage field tells what kind of resource is being described. This must be
one of three values:

    * 'Pict': Picture resource (to be used by the @draw_picture opcode, etc)
    * 'Snd ': Sound resource (to be used by the @sound_effect opcode, etc)
    * 'Exec': Z-code resource (to be loaded and executed by the
interpreter.)

The number field tells which resource is being described, from the game's
point of view. For example, when the game calls @draw_picture with an
argument of 3, the interpreter would find the index entry whose usage is
'Pict' and whose number is 3. For Z-code chunks (usage 'Exec'), the number
should contain 0.

The start field tells where the resource chunk begins. This value is an
offset, in bytes, from the start of the IFRS FORM (that is, from the start
of the resource file.)

2: Picture Resource Chunks

Each picture is stored as one chunk, whose content is a PNG file. The chunk
type is 'PNG '. The PNG file format is available at

    http://www.boutell.com/boutell/png/

3: Sound Resource Chunks

Each sound is stored as one chunk, whose content is either an AIFF file, a
MOD file, or a song file. (Note that these are three possible formats for a
single resource. It is not possible to have an AIFF sound and a MOD sound
with the same sound resource number.)

An AIFF (Audio IFF) file has chunk type 'FORM', and formtype 'AIFF'. The
AIFF format is available at

    http://www.mediatel.lu/workshop/audio/fileformat/aiff13/h_aiff13.html

A MOD file has chunk type 'MOD '. This is the Amiga-originated format for
music synthesized from note samples. The specification, such as it is, is
available in the files 

    http://hyperarchive.lcs.mit.edu/HyperArchive/Archive/gst/snd/mod-form.txt
    http://hyperarchive.lcs.mit.edu/HyperArchive/Archive/gst/snd/mod-info.txt

For generality, MOD files in Blorb are limited to the ProTracker 2.0 format:
31 note samples, up to 128 note patterns. The magic number at byte 1080
should be 'M.K.' or 'M!K!'.

A song file has chunk type 'SONG'. This is similar to a MOD file, but with
no built-in sample data. The samples are instead taken from AIFF sounds in
the resource file. For each sample, the 22-byte sample-name field in the
song should contain the string "SND1" to refer to sound resource 1, "SND953"
to refer to sound resource 953, and so on. Any sound so referred to must be
an AIFF, not a MOD or song. (You can experiment with fractal recursive music
on your own time.)

Each sample record in a MOD or song contains six fields: sample name, sample
length, finetune value, volume, repeat start, repeat length. In a MOD file,
the sample name is ignored by Blorb (it is traditionally used to store a
banner or comments from the author.) In a song file, the sample name
contains a resource reference as described above; but the sample length,
repeat start, and repeat length fields are ignored. These values are
inferred from the AIFF resource. (The repeat start and repeat length are
taken from the sustainLoop of the AIFF's instrument chunk. If there is no
instrument chunk, or if sustainLoop.playMode is NoLooping, there is no
repeat; the repeat start and length values are then considered zero.)

Note that an AIFF need not contain 8-bit sound samples, as a sound built
into a MOD would. A clever sound engine may take advantage of this to
generate higher-quality music. An unclever one can trim (or pad) the AIFF's
data to 8 bits before playing the song. In the worst case, it is always
possible to trim the AIFF data to 8 bits, append it to the song data, fill
in the song's sample records (with the appropriate lengths, etc, from the
AIFF data); the result is a complete MOD file, which can then be played by a
standard MOD engine.

The intent of allowing song files is both to allow higher quality, and to
save space. Note samples are the largest part of a MOD file, and if the
samples are stored in resources, they can be shared between songs.
(Typically note samples will be given high resource numbers, so that they do
not conflict with sounds used directly by the game. However, it is legal for
the game to use a note sample as a sampled-sound effect, if it wants.)

There are also the problems of how the game knows the interpreter can play
music, and how sampled sounds are played over music. See the section
"Z-Machine Compatibility Issues" later in this document.

4: Executable Resource Chunks

There should at most one chunk with usage 'Exec'. If present, its number
must be zero; its content is a Z-code game file, and its chunk type is
'ZCOD'.

A resource file which contains a Z-code chunk contains everything needed to
run the Z-code. An interpreter can begin interpreting when handed such a
resource file; it sees that there is a Z-code chunk, loads it, and runs it.

A resource file which does not contain a Z-code chunk can only be used in
tandem with a Z-code file. The interpreter must be handed both the resource
file and the Z-code file in order to begin interpreting.

If an interpreter is handed inconsistent arguments -- that is, a resource
file with no Z-code chunk, or a resource file with a Z-code chunk plus a
Z-code file -- it should complain righteously to the user.

5: The Release Number Chunk

This chunk is used to tell the interpreter the release number of the
resource file. (The interpreter passes this information to the game when the
@picture_data opcode is executed with an argument of 0.) The release number
is a 16-bit value. The chunk format is:

    4 bytes         'RelN'          chunk ID
    4 bytes         2               chunk length
    2 bytes         num             release number

This chunk is optional. If it is not present, the interpreter should assume
a release number of 0.

6: The Game Identifier Chunk

This identifies which Z-code game the resources are associated with. The
chunk type is 'IFhd'.

This chunk is optional. If it is present, and the interpreter is given a
Z-code file along with a resource file, the interpreter can check that the
game matches the IFhd chunk. If they don't, the interpreter should display
an error. The interpreter may want to provide a way for the user to ignore
or skip this error (for example, if the user is a game author testing
changes to the game file.)

If the resource file contains a Z-code chunk, there's not much point in
putting in the IFhd chunk.

The contents of the game identifier chunk are defined in the common save
file format specification, section 5. This spec can be found at

    ftp://ftp.gmd.de/if-
archive/infocom/interpreters/specification/savefile_13.txt

The "Initial PC" field of the IFhd chunk (bytes 10 through 12) has no
meaning for resource files. It should be set to zero.

7: The Color Palette Chunk

This contains information about which colors are used by picture resources
in the file. The chunk type is 'Plte'. It is optional, and should not appear
if there are no 'Pict' resources in the file.

The format is:

    4 bytes         'Plte'          chunk ID
    4 bytes         n               chunk length
    n bytes         ...             color data

There are two possibilities for the color data format. The first is an
explicit list of colors. In this case, the data consists of 1 to 256 color
entries. Each entry is three bytes, of the form:

    1 byte         red value (0 = black, 255 = red)
    1 byte         green value (0 = black, 255 = green)
    1 byte         blue value (0 = black, 255 = blue)

The second case is a single byte, which may have either the value 16 or 32
(decimal). 16 indicates that the picture resources are best displayed on a
direct-color display which has 16 or more bits per pixel (5 or more bits per
color component.) 32 indicates that the resources are best displayed with 32
or more bits per pixel (8 or more bits per color component.)

The two cases are differentiated by checking the chunk length (n). If n is
1, it's a direct color value; if it's a positive multiple of 3, it's a color
list, and the number of entries is the length divided by 3. Any other length
is illegal.

This chunk is only a hint; there is no guarantee about what the interpreter
will do with it. A color list will most likely be useful if the
interpreter's display can only display a limited number of colors (for
example, an 8-bit indexed color device). The interpreter may set the display
to the colors listed in the palette. Or it may set the display to just some
of the colors listed (for example, if it wishes to reserve some colors for
text display, or if it just doesn't have enough colors available.) Or the
interpreter may ignore the palette chunk, or do something else.

Similarly, if the interpreter finds a "16" or "32" value, it may set the
display to the appropriate bit depth. Or it may set the display to an 8-bit
color cube, and dither the images for display. Or, again, it may ignore the
palette chunk entirely, or do something else.

It is not required that the palette chunk list every color used in the
'Pict' resources. It is not required that the colors in the palette all be
different, or that they all are actually used by 'Pict' resources. It is not
required that the palette have anything to do with the game art at all. Of
course, if you give the interpreter misleading hints, you deserve whatever
you get.

8: The Resolution Chunk

This chunk contains information used to scale images. The chunk type is
'Reso'. It is optional.

A scalable image is one which the author says should be larger when more
space is available, and smaller when less space is available. (Note that the
Z-code game does *not* directly control the scaling of images. The
interpreter controls the scaling of images, in response to the information
in the resolution chunk. The interpreter then provides the scaled size in
response to @picture_data queries, and the game draws its display based on
those queries.)

It is also possible to create images that have a fixed scaling ratio; they
are always scaled up or down by a particular amount, regardless of window
size.

Not all images have to be scalable. Unless the resolution chunk gives
scaling data for an image, that image is assumed to be non-scalable.
Non-scalable images are always displayed at their actual size. (One image
pixel per screen pixel.)

This chunk is optional; if it is not present, then all of the images in this
file are non-scalable.

    4 bytes         'Reso'          chunk ID
    4 bytes         num*28+24       chunk length
    4 bytes         px              standard window width
    4 bytes         py              standard window height
    4 bytes         minx            minimum window width
    4 bytes         miny            minimum window height
    4 bytes         maxx            maximum window width
    4 bytes         maxy            maximum window height
    num*28 bytes    ...             image resolution entries

The "standard window size" is the normal size, the author's original chosen
size, for the Z-machine window. It is not the only possible size; a good V6
game should be prepared for any window the interpreter chooses to create.
The idea is that when the Z-machine window is exactly the standard size,
scalable images are presented at their original size. When the Z-machine
window is larger than the standard size, scalable images are scaled up; when
it is smaller, scalable images are scaled down.

The minimum and maximum window sizes are provided as a hint to the
interpreter, when it is choosing a window size. It may also use the standard
window size as a hint for this purpose. (If the interpreter lacks the
ability to choose its own window size, of course, it will ignore these
hints.) The idea is that the minimum and maximum sizes define the range in
which the game can draw itself successfully.

Any or all of minx, miny, maxx, maxy can indicate "no limit in this
direction" by containing a value of zero. However, px and py must contain
non-zero values. Unless the min or max values are zero, it must be true that
minx <= px <= maxx, miny <= py <= maxy.

*Important* note: The standard, minimum, and maximum window size values are
measured in *screen pixels*. Furthermore, unscaled pictures should be drawn
in screen pixels -- one image pixel per screen pixel. (This may seem dumb as
rocks, and maybe it is, but my rationale is presented at the end of this
document.)

Also note that I have not mentioned Z-pixels. This standard does not concern
itself with Z-pixels.

On with the show.

The standard, minimum, and maximum window sizes are followed by a set of
image entries, one for each scalable image. (Non-scalable images do not have
an entry in this table; that's how they are declared to be non-scalable.)
Each image entry is 28 bytes, of the form:

    4 bytes         number          image resource number
    4 bytes         ratnum          numerator of standard ratio
    4 bytes         ratden          denominator of standard ratio
    4 bytes         minnum          numerator of minimum ratio
    4 bytes         minden          denominator of minimum ratio
    4 bytes         maxnum          numerator of maximum ratio
    4 bytes         maxden          denominator of maximum ratio

The number is the picture number; in other words, this entry applies to the
resource whose usage is 'Pict' and whose number matches this value.

The entry then contains a standard, minimum, and maximum image scaling
ratio. Each ratio is a real number, represented by two integers: 

    * stdratio = ratnum / ratden
    * minratio = minnum / minden, 
    * maxratio = maxnum / maxden.

Minratio can indicate zero ("no minimum limit") by having both minnum and
minden equal to zero. Similarly, maxratio can indicate infinity ("no maximum
limit") by having maxnum and maxden equal to zero. It is illegal to have
only half of a fraction be zero.

To compute the actual scaling ratio for this image, the interpreter must
first compute the overall game scaling ratio, or Elbow Room Factor (ERF). If
the actual game window size is (wx,wy), and the standard window size is
(px,py), then

    * ERF = (wx/px) or (wy/py), whichever is smaller.

(Note that if the game's window is exactly its standard size, ERF = 1.0. If
the window is twice the standard size, ERF = 2.0. If the window is three
times the standard width and four times the standard height, then ERF = 3.0,
because there's really only enough room for the game's standard layout to be
tripled before it overflows horizontally.)

The scaling ratio R for this image is then determined:

    * If ERF*stdratio < minratio, then R = minratio.
    * If ERF*sdtratio > maxratio, then R = maxratio.
    * If minratio <= ERF*stdratio <= maxratio, then R = ERF*stdratio.

If minratio and maxratio are the same value, then R will always be this
value; ERF and stdratio are ignored in this case. (This indicates a scalable
image with a fixed scaling ratio.)

The interpreter then knows that this image should be drawn at a scale of R
screen pixels per image pixel, both vertically and horizontally. The
interpreter should report this scaled size to the game if queried with
@picture_data (as opposed to the original image size).

Yes, this is an ornate system. The author is free to ignore it by not
including a resolution chunk. If the author wants scaled images, or
variably-scalable images, this system should suffice.

Here are some examples. They're not necessarily examples of good art design,
but they do demonstrate how a given set of desires translate into images and
resolution values. All are for a game with a standard size of (600,400).

The game wishes a title image that covers the entire window, and all the
resolution should be visible at the standard size. (So if the window is
twice the standard size, the image will be stretched and coarse-looking; if
the window is half the standard size, the image will be squashed and lose
detail.)

    * Image size (600,400); stdratio 1.0; minratio zero; maxratio infinity.

The game has a background image of a cave, made from a scanned photograph.
At standard window size, this should cover the entire window, but not all
the detail needs to be visible. If the window is larger, the image should
still cover the entire window; more detail will be visible, up to twice the
standard size (at which point all the resolution should be visible.) If the
window is larger than twice the standard size, the image should not be
stretched farther; instead the game will center it and have blank space
around the edges.

    * Image size (1200,800); stdratio 0.5; minratio zero; maxratio 1.0.

The game has small monochrome icons indicating different magical
perceptions, which it will draw interspersed with the text. The icons should
always be drawn at double size, two screen pixels per image pixel,
regardless of the window size.

    * Image size (20,20); stdratio 1.0; minratio 2.0; maxratio 2.0. (In this
case, remember, the stdratio value is ignored.)

The game has a graphical compass rose which it will draw in the top left
corner. This should be 1/4 of the window size in the standard case, and
shrink proportionally if the window is smaller. However, if the window is
larger than standard, the rose should not grow; all the extra space can be
allotted for text. All detail (image pixels) should be visible in the
standard case.

    * Image size (150,100); stdratio 1.0; minratio zero; maxratio 1.0.

The same compass rose, still to be 1/4 of the window size -- but this time
it is critical that all the image detail be visible when the window is as
small as half-standard (that is, when the rose is 75 by 50 pixels). At
standard scale (150 by 100), it will therefore appear stretched and coarse.
If the window is smaller than half the standard size, the rose should not
shrink beyond 75x50, so that pixels are never lost.

    * Image size (75,50); stdratio 2.0; minratio 1.0; maxratio 2.0.

End of verbose examples.

9: The Looping Chunk

This chunk contains information about which sounds are looped, in a V3 game.
The chunk type is 'Loop'. It is optional.

Note that in V5 and later, the @sound_effect opcode determines whether a
sound loops. The looping chunk is ignored. Therefore, this chunk should not
be used at all in Blorb files intended for games which are not V3.

The format is:

    4 bytes         'Loop'          chunk ID
    4 bytes         num*8           chunk length
    num*8 bytes     ...             sound looping entries

Each entry is 8 bytes, of the form:

    4 bytes         number          sound resource number
    4 bytes         value           repeats

The repeats flag is one if the sound is to be played once; it is zero if the
sound is to repeat indefinitely (until it is stopped or another sound
started.) If there is no entry for a particular sound resource, or if the
looping chunk is absent, the V3 interpreter should assume the flag is one,
and play the sound exactly once.

10: Other Optional Chunks

A resource file can contain extra user-level information in 'AUTH', '(c) ',
and 'ANNO' chunks. These are all optional. An interpreter should not do
anything with these other than ignore them or (optionally) display them.

These chunks all contain simple ASCII text (all characters in the range 0x20
to 0x7E). The only indication of the length of this text is the chunk length
(there is no zero byte termination as in C, for example).

The 'AUTH' chunk, if present, contains the name of the author or creator of
the file. This could be a login name on multi-user systems, for example.
There should only be one such chunk per file.

The '(c) ' chunk contains the copyright message (date and holder, without
the actual copyright symbol). There should only be one such chunk per file.

The 'ANNO' chunk contains any textual annotation that the user or writing
program sees fit to include.

11: Z-Machine Compatibility Issues

The image system presented in this document is fully backwards-compatible
with Infocom's interpreters. Infocom V6 games, such as Arthur, Journey, and
Zork Zero, contain only non-scalable image resources. The game files are
written to deal with both variations in window size and variations in image
size (since the interpreters for different platforms had different window
sizes and different art.) Therefore, if you construct a Blorb file
containing the images from a particular platform (say, the Mac) and give it
the suggested window size of the Infocom Mac interpreter, the game file will
deal with it correctly.

The image system is also sort of forwards-compatible, in the following
sense. If you take a Blorb file whose standard (intended) window size is the
same as the Infocom interpreter's, and break it out into Infocom image
files, the Infocom interpreter should display it correctly. The interpreter
will not scale images, but since the window size is equal to the standard
size, the Blorb rules require the images to be displayed unscaled anyway. 

Also, of course, if you take a Blorb file which contains only non-scalable
images, an Infocom interpreter will act correctly, since it will not scale
the images regardless of the standard size.

The sound system is slightly more problematic. A game file can announce that
it uses sound, by setting a header bit; the interpreter can announce that it
does not support sound, by clearing that bit. But there is no way to
distinguish a game that uses sampled sound only, from one that uses sampled
sound and music. (And similarly for the interpreter's support of samples
versus music.) This may be addressed in a future revision of the Z-machine.
In the meantime, games should set that header bit if any kind of sound is
used (samples or music or both.) And interpreters should clear that bit only
if *no* sound support is available. If the interpreter supports sampled
sound but not music, it should leave the header bit set, announcing that it
does "support sound." It should then ignore any request to play a music
resource.

There is also the question of overlapping sounds. The Z-Spec (9.4.2) says
that starting a new sound effect automatically stops any current one. But it
is not desirable that a sound effect such as footsteps should interrupt the
playing of background music. Therefore, the interpreter should amend this
rule, and consider sampled sounds and music to be in seperate "channels".
Samples interrupt samples, and music interrupts music, but one form of sound
does not interrupt the other.

This is an actual variance in the behavior of the Z-machine, and worse, a
variance which depends on data format. (One sound will either stop another,
or not, depending on whether the sound is stored in AIFF or MOD format.) We
apologize for the ugliness.

Again, future versions of the Z-machine may address this issue, and allow a
more general system where any sound can be overlaid on any other sound, or
interrupt it, as the game desires and regardless of storage format. (After
all, there can be background *sounds* as well as background *music*.) Such a
system would also allow the interpreter to announce its limitations and
capabilities -- whether it can play music, whether it can play two pieces of
music at once, how many sampled sounds it can play at once, etc.

A final, ah, note: The remark at the end of Z-Spec chapter 9, about
sequencing sound effects to simulate the slow Amiga version of "The Lurking
Horror", should not be applied to music sounds. New music should interrupt
old music immediately, regardless of whether keyboard input has occurred
since the old music started.

12: The IFF Format

A description of the IFF format can be found at

    http://www.cica.indiana.edu/graphics/image_specs/ilbm.format.txt

In the interests of simplicity, this proposal does not use IFF LISTs or
CATs, even though its purpose is to contain concatenated lists of data.
Therefore, the format can be quickly described as follows:

    4 bytes         'FORM'          Magic number indicating IFF
    4 bytes         n               FORM length (file length - 8)
    4 bytes         'IFRS'          FORM type
    n-4 bytes       ...             The chunks, concatenated

Each chunk has the following format:

    4 bytes         id              Chunk type
    4 bytes         m               Chunk length
    m bytes         ...             Chunk data

If a chunk has an odd length, it *must* be followed by a single padding byte
whose value is zero. (This padding byte is not included in the chunk length
m.) This allows all chunks to be aligned on even byte boundaries.

All numbers are two-byte or four-byte unsigned integers, stored big-endian
(most significant byte first.) Character constants such as 'FORM' are stored
as four ASCII bytes, in order from left to right.

When reading an IFF file, a program should always ignore any chunk it
doesn't understand.

13: Other Resource Arrangements

It may be convenient for an interpreter to be able to access resources in
formats other than a resource file. In particular, when developing a game,
an author will want to load images and sounds from individual files, rather
than having to re-package all the resources whenever any one of them
changes.

Such resource arrangements are platform-specific, and the details are left
to the interpreter. However, one suggestion is to have a single directory
which contains all the resources as files, with one file per resource. (PNG
files for images, and so on. The contents of each file would be exactly the
same as the contents of the equivalent chunk, minus the initial eight bytes
of type/length information.) Files would be named something like "PIC1",
"PIC2"..., "SND1", "SND2"..., and so on. A Z-code file (if present) would be
named "STORY"; a color palette would be named "PALETTE", a resolution chunk
"RESOL", a looping chunk "LOOPING", a release number "RELEASE", and a game
identifier chunk "IDENT". (Naturally, file suffixes would be added in
platforms that require them.) The interpreter would be started up and handed
the entire directory as an argument; or possibly the directory along with a
separate Z-code file.

14: Rationales and Rationalizations

- Why have a common resource collection format?

Infocom chose not to standardize their resource formats; they had a
different picture format for each platform. This was a reasonable choice for
them, since they were writing all the games, all the art, and all the
interpreters. They therefore had the capacity to translate the art into
platform-specific formats for all the platforms they supported.

In the modern age, an IF author does not have access to all the platforms
his game will be played on. It is therefore reasonable to distribute art in
a single format, and leave interpreter writers the job of supporting that
format.

- Why an IFF-based format?

IFF does what we want; it's a known, very simple way to concatenate chunks
of data together.

Also, the common save-file format is IFF-based. This allows interpreters to
use the same code for reading both save files and resource files.

- Why not compress data as well as archiving it? Why just concatenate
everything together as chunks?

Any reasonable sound or image format already incorporates compression.

- Why is there a "number" field in the entries in the resource index chunk?
Why not just assume chunks are numbered consecutively?

Pictures are not necessarily numbered contiguously (Z-Spec 8.8.6.1.) Sounds
are numbered consecutively, but sounds 1 and 2 are bleeps, so the
game-specific sound resources start at 3. (Z-Spec 9.2.) Rather than jigger
the numbering or require place-holder chunks, I decided there should be an
index which maps resource numbers to chunks.

- Why does the "usage" field in the resource index entries contain, for
example, 'Pict' instead of 'PNG '? The proposal requires that all pictures
be PNG files.

Why not? We may go insane someday and support more image formats. In that
case, resources with usage 'Pict' could have other chunk types besides 
'PNG '. Similarly, the usage of sound resources is always 'Snd ', but the
chunk type can be either 'MOD ', 'SONG', or 'FORM AIFF'.

- Why only one image format? Why not allow both PNG and JPEG, or maybe allow
any image format?

The whole point of this exercise is to assure the author that the player can
view his art. If we allow lots of different formats, we can't possibly
insist that every interpreter must display all the formats. This leaves us
just about back where we started. Individual game authors would be
negotiating with individual interpreter authors to support particular
formats, and it would just be icky.

Therefore, we *do* insist that every interpreter be able to display all the
formats listed in this standard. That means one, or at the most two,
formats. See the next two questions.

- Okay, then, why three sound formats?

Because a sampled-sound format (like AIFF) can reproduce anything; and a
note format (like MOD) can reproduce music with much less data than AIFF.
It's effectively a very efficient form of compression which only works for
musical sound. Song files are even more efficient, if you have several
songs, because the note samples are shared. But standard MOD playing and
composition tools work on MODs, so it wouldn't make sense not to allow those
too. Sigh.

- Why PNG for images?

The format is not burdened with patent restrictions; it is free; it's not
lossy; and it can efficiently store many types of images, from 1-bit
(monochrome) images up to 48-bit color images. In contrast, GIF is owned by
twits who restrict its use; JPEG is lossy and not optimal for images other
than photographs. TIFF has been suggested, but it seems to be overly
baroque, and also, hey, you gotta pick something.

- Why not JPEG also?

It would be reasonable to support two image formats -- a non-lossy format
(PNG) for detailed images, and a better-compressed, lossy format (such as
JPEG) for photograph-quality images. They're two different tasks and it
would be more efficient to handle them with two different tools.

It would also be twice as much work for interpreter authors. We may dive
into this pit of rocks in the future; we are not going to do it right away.
PNG *can* be used for any image. If several V6 games are produced and it
becomes clear that a JPEG option would be of practical benefit, we'll add it
to the standard.

- What is the Blorb Policy on Color Depth?

The idea is that each author can decide what kind of color requirements his
game will have. 

The alternative (which we did *not* choose) would be to mandate a fixed set
of color requirements for all V6 games -- for example, an 8-bit color
display set to a color-cube set of colors. This seems like a dumb idea. Any
fixed set of requirements is going to be impossible for some machines and
standard equipment on others, and both these sets will change over time. The
requirements would quickly become obsolete.

Instead, we choose to allow any kind of art in V6 games. If the author
includes only monochrome images, the game will run anywhere. If the author
includes full-color 32-bit images, he is creating a game which wants a
powerful graphics machine to display itself on. That's the author's choice.
If the player's machine only allows 8-bit color, his interpreter will have
to dither or otherwise reduce the color information of the game art. The
player can accept this, buy a more powerful computer, or throw away the
game. There's no way around that. The problem can only be avoided by
outlawing 32-bit color images, which we do not wish to do.

- What's the idea of the palette chunk? 

The palette chunk itself is provided for the benefit of interpreters which
can control their display palettes or color depth. The palette declares the
minimum set of colors (or direct-color depth) which the author wants you to
have in order for the game to "look okay." It may be a good idea to switch
palettes in order to play a particular game; the palette chunk tells the
interpreter this advice.

Now, the interpreter is not *required* to follow this advice. This is for
the player's benefit; if the player has a monochrome machine, or just
doesn't like changing palettes, he is not denied the opportunity to play the
game. He'll just get reduced-quality art. That's his choice. As stated
above, he can accept it, upgrade, or throw the game away.

- What is the Blorb Policy on Pixel Size?

We make a couple of assumptions.

Assumption one: Image pixels are square. Your images should have the correct
aspect ratio when drawn with square pixels -- that is, when the number of
image-pixels-per-inch is the same vertically and horizontally. If your art
program doesn't understand square pixels, get a real art program. There.
That's resolved.

(This means that if an image is 400 pixels wide and 200 pixels high, the
interpreter should draw it on the screen with a physical width twice its
physical height. Anything else will look distorted.)

(It has been noted that this does not exactly apply to Infocom's V6 games;
their art was probably designed for an era of computers that did not have
exactly square pixels (IBM EGA, Apple II machines displaying on television
screens, and other such barbarisms.) However, this does not seem to have
concerned them. Infocom interpreters which are running on modern machines,
with square-pixel displays, display their art with square pixels. We will do
the same.)

Assumption two: It is always okay to draw images at their "actual size" --
one image pixel per screen pixel.

Now you think I'm crazy. It is true that many modern screens can be adjusted
to different pixel sizes; mine allows 55, 72, or 88 pixels per inch.
However, *I declare this to be an illusion.* If a user sets his monitor to
smaller pixels, it's because he wants a given image to be smaller. So he can
fit more of them on screen. He also wants his text to be smaller, and his
windows. That's the way web browsers work, that's the way Adobe Photoshop
works, and that's damn well good enough for the Z-machine.

Perhaps in the future there will be monitors that break this rule -- much
smaller pixels, 300 or 600 pixels per inch, for example. At that time there
will be some consensus on how to display images. (Frankly, I expect it will
be "draw them at 55, 72, or 88 pixels per inch, depending on the user's
previous preference.") Z-machine interpreters can follow that plan when it
emerges. 

Until then, the right size for a non-scaled picture is one image pixel per
screen pixel. If an image is to be scaled by a ratio of 2.0, then the right
size for it is one image pixel per two screen pixels (vertically and
horizontally). And so on.

- Where do Z-pixels come into all this?

The definition of Z-pixels is entirely up to the interpreter. This standard
says nothing on the subject, and does not care.

It is true that the interpreter must tell the game what the window size and
image sizes are, as measured in Z-pixels. That's the interpreter's job. The
interpreter knows how big its window is, in screen pixels; it translates
that into Z-pixels -- using whatever definition it has -- and reports it to
the game. Then, the scaling rules of this spec define what the display sizes
of the images will be, as measured in screen pixels. The interpreter
translates these sizes into Z-pixels -- using the same definition -- and
reports them to the game. All consistent and well-defined.

- What is the Blorb Policy on Interpreters that do Funky Stuff?

The interpreter is Allowed. It's okay to be ugly.

For example, someone may (in a fit of insanity) write a Blorb-compliant
interpreter for the Apple II. The Apple II had non-square pixels. But
(assume) it doesn't have the processing power to scale all its images by a
factor of 1.2 (or whatever) to adjust for this. Well, it's legal to write an
interpreter that draws art at one image pixel per (non-square) screen pixel.
The art will look distorted; the user can like it or play on a different
machine.

For another example, someone may want their entire game display doubled in
size. All the art twice as large (in screen pixels) as this spec says it
should be. An interpreter which has this option is legal. It's the moral
equivalent of mounting a magnifying glass on your monitor -- that certainly
doesn't violate any software standards.

- What about playing Blorb-packaged games on original Infocom interpreters?

It's possible. You'll have to unpack the PNG art and translate it into the
format that Infocom used. Since the Infocom interpreters had a hard-wired
screen size, you can precompute all the scaling factors, and do any
necessary scaling in the translation process. 32-bit color images will have
to be color-reduced; that's the way it goes. But the result should be fully
playable on Infocom's interpreters.

- Have you considered --

Yes.


