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-<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
-<html>
- <head>
- <title>PDP Forth Scripting</title>
- </head>
-
- <body>
- <h1>PDP Forth Scripting</h1>
- <h2>Introduction</h2>
-
- <p> This document describes the rationale behind the pdp forth scripting
- language, aka "pdp's rpn calculator".
-
-
- <p>The point is to be able to decouple the packet processors from the pd
- object model, in order to be able to port the functional part of pdp to other
- systems. Therefore pdp needs its own processor model.
-
- <p>A requirement for this is that it is both flexible and simple. pd's object
- model is very flexible, but it is hard to port to other architextures, because
- of the tight bindings to the pd kernel. What is needed is a way to abstract the
- functionality of a packet processor in such a way that a pd class can be
- generated from the pdp processor model.
-
- <p>There are a lot of ways to solve this problem. One is to extend a packet
- class to support methods. This seems like a very clean solution, however
- when a processor has more than one packet as internal state, this poses
- a problem. It would require to define an operation like biquad (which has 1 input
- packet and 2 extra state packets) as a method of a packet collection. To
- do this properly requires a much more advanced object model, or a separate
- object model for processors.
-
-
- <p>In short: it is not always clear if a packet processor can be seen as a metod
- 'of' or an operation 'on' a single packet, so extending a packet with methods
- would require a separate packet processor class anyway. Therefore we do not
- define packet operations as methods of packets. (no object oriented solution)
-
- <p>Another approach is to write operators in a pure functional way: a processor
- accepts a list of arguments and returns a new list of arguments. To do this
- cleanly it would require the calling style to be pass by value: i.e. the arguments
- passed will not be modified themselves. Since most of the image processors in
- pdp all use in place processing for performance reasons, this would require
- a lot of workarounds or a big kludge mixing const and non const references
- (like it would be done in C++).
-
- <p>Since one of the goals is to automate
- the generation of wrappers of pdp processors (i.e. pd classes, python classes,
- scheme functions, ...) the interface can best be kept as simple as possible.
- (no pure functional solution)
-
- <p>The most natural solution, given the underlying code base, seems to be to embrace
- an in place processing approach. What comes to mind is to use a data stack to solve
- the communication problem. I.e. the forth language model. A packet operation is then
- a forth word that manipulates a data stack. A data stack is nothing more than a list
- of atoms containing the basic data building blocks: floats, ints, symbols and packets.
-
- <p>An additional advantage is that dataflow and forth mix very well. This would enable
- the possibility to create new words by chaining old ones, without the disadvantage
- that pd abstractions using pdp objects have: passive packets are contained in internal processor
- registers longer than necessary, leading to inefficient memory usage.
-
- <p>Several practical problems need to be solved to implement this. The first being
- description of the stack effect. Each primitive word has a mandatory description of
- the number of stack elements it consumes and produces.
-
- <P>The forth words will support
- polymorphy: each word has a type template to describe the type of atoms it can operate
- on. The type is determined by some word on the stack, or a symbol indicating the type for
- generators.
-
- <p>To solve the additional problem of mapping forth words to processors, the concept
- of a forth process is introduced. A forth process has a stack (representing it's
- state), an init method that constructs a stack, a process method that operates
- on the stack and some description of how to map inputs to stack and stack to output.
-
- <p>There is one last class of objects that don't fit the forth description
- very well: it is the input/output classes. These will probably stay as special
- cases, since they really need an internal state represented as an object,
- incorporating them into the system would require them to be defined as an
- object (quicktime packet, v4l packet, xv packet, ...). More later.
-
- <h2>Implementation</h2>
- Have a look at <code>pdp_forth.h</code>
-
-
- <hr>
- <address><a href="mailto:no@spam">Tom Schouten</a></address>
-<!-- Created: Sun Jun 8 17:42:50 CEST 2003 -->
-<!-- hhmts start -->
-Last modified: Mon Jun 9 13:41:09 CEST 2003
-<!-- hhmts end -->
- </body>
-</html>