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diff --git a/doc/misc/pdp_forth.html b/doc/misc/pdp_forth.html new file mode 100644 index 0000000..0f39fd2 --- /dev/null +++ b/doc/misc/pdp_forth.html @@ -0,0 +1,94 @@ +<!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> |