Version 1.0; 10-SEP-2018.
With the introduction of new tools for the development of Commodore 64 art on modern systems, there has arisen a need for a universal standard image format for the storing and transferring of Commodore image formats between programs, including original hardware.
This specification proposes just such a format that is simple to understand, simple to parse -- even on original hardware -- and simple to implement.
Unlike modern image formats where the image is composed of pixels, a Commodore image can consist of multiple separate layers of different format, including text-characters (PETSCII), separate colour data and global colour such as the foreground / background colour.
Therefore a format is needed that can store the individual parts of an image, but also leave irrelevant parts out.
The file extension is ".pet"
The PET file format consists of blocks of data split into 254-byte "sectors". This is done to make reading on original hardware very simple, where a disk sector is exactly 254 bytes. A parser can read and process the file a sector at a time and not have to seek backward or forward.
The last sector in a PET file can be truncated, that is have no padding bytes to fill a full sector. All sectors other than last must, of course, be a full 254 bytes. Unused space within a sector should be zero-filled.
The first block of data is the meta-data block and consists of 1 sector (254-bytes). It stores various image meta-data and properties.
The first four bytes of the meta-data block (and therefore, the file itself) form the "magic number" used to identify a PET file. The four bytes are the characters "PET" and a version number character all in PETSCII; that is, the first four bytes of a file (in hexadecimal) will be:
$50 ("P"), $45 ("E"), $54 ("T"), $31 ("1")
This version number of "1" will only be changed should the file format change to an encoding that would be incompatible with the "version 1" specification.
If your program encounters a version number that is not "1" (in PETSCII), it should not parse the file any further!
Immediately following the "PET1" magic-number is the meta-data table. This table consists of a set of names and values, each entry in the meta-data table consists of 8 bytes.
The table ends when you come across a name consisting of four nulls (0, 0, 0, 0). A further four bytes exist (reserved for future use), like any other entry, but these have no defined value. Your parser should skip over these when reading, and write four zeroes when writing a file. I.e., you should not preserve these bytes if your parser does not understand them.
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The first 4 bytes are a name; see the headings on each different name
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The next four bytes depend upon the name, see below
If the meta-data table contains no entries other than the terminator (0, 0, 0, 0), the file is still considered "valid", but you should stop parsing and inform the user that the file "contains no image data".
The meta-data names allowed are as follows:
Allows embedding an author's name.
The 4 name bytes used are "AUTH" ($41, $55, $54, $48). The next one byte gives an offset in bytes from the beginning of the sector to some PETSCII text stored anywhere within the sector.
If multiple such meta-data ID entries exist, consider this to mean more than one author
"TITL" ($54, $49, $54, $45): Title; PETSCII text. A title for the image
"DATE" ($44, $41, $54, $45): Date-time for the image; PETSCII numerals:
...
"DESC" ($44, $45, $52, $43): Description; PETSCII text. A long form description of the image
"EDIT" ($45, $44, $49, $54): Editor; PETSCII text. The editor used to produce the image, e.g. "PETMATE"
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Actually, I like that a lot. But, perhaps one byte in the header to specify the rom character set that's being modified. A stand alone viewer could then copy the correct rom charset into ram, and modify it with the data at the end of the file.
C64 OS, would just need to encode the characters it knows are custom AND which are in use in the screen data for that particular capture.