From dwalker@art.acorn.co.uk Wed Nov 13 11:07:38 1996 Return-Path: Received: from ether310.art.acorn.co.uk by art-1.acorn.co.uk (SMI-8.6/SMI-SVR4) id LAA28579; Wed, 13 Nov 1996 11:07:37 GMT Date: Wed, 13 Nov 1996 11:05:07 +0000 (GMT) From: Dave Walker Subject: Developer News 12/11/96: (fwd) To: jabela Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; CHARSET=ISO-8859-1 X-Organization: Acorn Risc Technologies (a division of Acorn Computer Group plc) X-Mailer: ANT RISCOS Marcel [ver 1.08] -- Dave Walker, Engineering Support Manager Acorn Risc Technologies (a division of the Acorn Computer Group plc), Acorn House, 645 Newmarket Road, Cambridge CB5 8PB ---------- Forwarded message ---------- Date: Tue, 12 NOV 1996 10:51:39 +0000 (GMT) From: Dave Walker To: isvquery@art-1 Subject: Developer News 12/11/96: Hello, There follows a note we have produced to clarify and (hopefully) help people solve a number of outstanding issues involving printing. We are currently looking into doing some significant work on the RISC OS printing system, and it is possible that this work may break or otherwise pessimise the code relating to any printing metrics other than those described below. As these pessimisable metrics have effectively been obsoleted by the release of !Printers 1.52 (issued just over 12 months ago), we are taking this opportunity to inform you how to ensure that your applications are ready for our new printing system when it arrives. Cheers, Dave [snip] Technical Note 11-Nov-96 ------------------------- RISC OS Printing advice for developers -------------------------------------- Purpose ------- This note mainly clarifies printing issues already covered in the PRM, but highlights information from several parts and volumes for convenience. This note also notifies third parties of some minor changes that are being made to advice currently in the PRM. These changes are to allow freedom for more radical improvements in a future version of Printers. This note affects both applications that print and also third party printing extensions. Applications that comply with the advice in this note will offer maximum performance (speed and quality) from the current version of Printers, as well as being more ready for future enhancements. Background ---------- !Printers 1.52 was issued approximately one year ago. This offered significant enhancements to Acorn supported printers, while remaining entirely within any printers API defined in the PRM (mainly, volumes 3 and 5). The current version of !Printers is 1.53 (disc based), or 1.54 (ROM based) for users of RISC OS 3.7. Advice ------ 1) Available printing modes --------------------------- Both applications that print and also third party printer dumper modules should be aware that the lower quality printing modes (and corresponding dumper strip types) may not be supported at all. This has been so since !Printers 1.52, but with some backward compatibility options allowed. In !Printers 1.54 there is no backward compatibility option. The reasons for this restriction are to remove low quality printing, and to allow only the much faster, more sophisticated code introduced in !Printers 1.52 to run. Referring to PRM 5a-576, you should now assume that only the following modes (strip types) are available in general: 1 grey scale 3 multiple pass 24 bit colour (RISC OS 3.1) 5 single pass 24 bit colour (RISC OS 3.5 onwards) This means that applications _must_ be capable of printing colour properly in modes 3 and 5. The most common issue is with mode 3; applications _must not_ assume that colour translation tables can be static for all print rectangles, since mode 3 may be in operation. Typically, the simple approach is to generate colour tables anew for each rectangle. This is not a big change. Firstly, applications should already support these modes. Secondly, since RISC OS 3.6 colour translation tables cannot be relied upon generally between rectangles, because of pre-scanning (see PRM 5a-577). The version of !Paint used with RISC OS 3.1 is not fully compliant here, so users have been advised for some time to drop Sprites into !Draw for printing. 2) Printer definition files and palette files --------------------------------------------- The format of printer definition files (PDFs) is not defined in the PRM. Some of the contents of some PDFs are discussed in PRM 3-697 onwards, insofar as they relate to !PrintEdit. You should note that this covers only some bitmap printer language classes, and that these are dated. !Printers 1.52 onwards already includes some enhanced PDFs. The format of printer palette files is not defined in the PRM. The possibility of editing palette files is mentioned in PRM 5a-579. You should note that this refers to older format palette files, not compatible with !Printers 1.52 onwards. A future version of Printers may enhance PDFs further. It may also remove palette files altogether, in favour of a compact parametric specification. The reason for this clarification is that the current level of PDFs and the need for palette files may needlessly limit the enhancements that a future version of Printers could offer. 3) Wimp protocol for printing ----------------------------- It is _unacceptable_ for any Wimp application to skip the Wimp protocol stage when initiating printing; PRM 3-558 states that a Wimp application must use the protocol. Failure to use the protocol means that the application is incompatible with RISC OS. Using the RISC OS 2 protocol has been deprecated since RISC OS 3; the RISC OS 3 protocol has been recommended (see PRM 3-259). You should now regard the RISC OS 3 protocol as mandatory. In a future version of Printers, behaviour is _undefined_ if an application attempts to use the RISC OS 2 protocol. The reason for this clarification and for the change is that failure to use the RISC OS 3 protocol breaks the background printing and queueing system that has been supported since RISC OS 3. Moreover, future improved methods for background printing, and extra options such as for network related printing simply will not work if the prescribed protocols are bypassed. 4) Extensions to the printing system ------------------------------------ Possible extensions to the current printing system fall into two broad classes. (Some extensions may have components in each class). The first class consists of extensions to the printer drivers themselves; typically new dumper modules. This class has a defined API described in the PRM (PRM 3-553 onwards, Part 9 - Printing). Extensions restricted to the first class will be relatively easy to keep compatible with a future version of Printers. (Some work may be required to take advantage of any API extensions.) The second class consists of extensions to the printer manager. These are referred to as 'back ends' to the Printers application in the PRM (PRM 3-594). This class _does not_ have an API defined in the PRM. The second class has traditionally relied on internal implementation details of the printer manager that have so far remained relatively stable. However, since there is no defined API, use of these details by third party extensions is subject both to the 'as is' nature of a current implementation, and to any changes introduced in a future implementation. Extensions in the second class may be entirely obsoleted by a future version of Printers. This is a clarification of the situation since RISC OS 3.6, rather than a change of policy. PRM 5a-576 states: "In RISC OS 3.6, the front and back ends of the Printers application have been merged. It is important that any software you write does not assume the internal structure of the Printers application, as it may change again in subsequent releases of RISC OS." The structure that may change clearly includes any internal implementation details. The reason for this clarification is that a future version of the Printers manager may be implemented in a radically different way, and implemented in any language. A new, defined API for manager extensions may be issued at a later date.