Copyright (c) Kazuki Yasumatsu, 1995. All rights reserved.

I am an abstract class to provide for encoding and/or decoding an image on a stream.

Instance Variables:
stream <ReadStream | WriteStream> stream for image storages

Class Variables:
ImageNotStoredSignal <Signal> image not stored error signal
MagicNumberErrorSignal <Signal> magic number error signal

Subclasses must implement the following messages:
accessing
nextImage
nextPutImage:
testing
canUnderstand (added tao 10/26/97)

	private
	changePadOfBits:width:height:depth:from:to: hasMagicNumber: on: unpackBits:depthTo8From:with:height:pad:

	stream access
	atEnd close contents cr lf next next: nextLong nextLongPut: nextPut: nextPutAll: nextWord nextWordPut: peekFor: position position: size skip: space tab

	testing
	understandsImageFormat

	instance creation
	on:

	accessing	top
	nextImage Dencoding an image on stream and answer the image.
	nextPutImage: Encoding anImage on stream.

	private	top
	changePadOfBits:width:height:depth:from:to: Change padding size of bits.
	hasMagicNumber:
	on:
	unpackBits:depthTo8From:with:height:pad: Unpack bits of depth 1, 2, or 4 image to it of depth 8 image.

	stream access	top
	atEnd
	close close if you can
	contents
	cr
	lf PPM and PBM are used LF as CR.
	next
	next:
	nextLong Read a 32-bit quantity from the input stream.
	nextLongPut: Write out a 32-bit integer as 32 bits.
	nextPut:
	nextPutAll:
	nextWord Read a 16-bit quantity from the input stream.
	nextWordPut: Write out a 16-bit integer as 16 bits.
	peekFor:
	position
	position:
	size Primitive. Answer the number of indexable variables in the receiver. This value is the same as the largest legal subscript. Essential. See Object documentation whatIsAPrimitive.
	skip:
	space
	tab

	testing	top
	understandsImageFormat Test to see if the image stream format is understood by this decoder. This should be implemented in each subclass of ImageReadWriter so that a proper decoder can be selected without ImageReadWriter having to know about all possible image file types.

	image reading/writing	top
	formFromFileNamed: Answer a ColorForm stored on the file with the given name.
	formFromServerFile: Answer a ColorForm stored on the file with the given name. Meant to be called from during the getting of updates from the server. That assures that (Utilities serverUrls) returns the right group of servers.
	formFromStream: Answer a ColorForm stored on the given stream. closes the stream
	putForm:onFileNamed: Store the given form on a file of the given name.
	putForm:onStream: Store the given form on a file of the given name.

	instance creation	top
	on: Answer an instance of the receiver for encoding and/or decoding images on the given.