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Automated Testing Framework
---------------------------

Goal: build a testing framework that covers most of the methods of all internal
and external classes that ship with Purr Data. This should include the "dsp"
method of all signal-based externals, so that we can compare the block output
of each across commits to serve as both a regression and code coverage test.

Benefits: this will make it easier for newcomers to make fixes and improvements
to both the core of Purr Data and its many external libraries. Clear errors
from the CI will help them catch bugs earlier in the development process,
leading to overall cleaner code in merge requests and less disruption to
users from regressions.

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Difficulty level: Moderate. One of the challenges here is that not all classes
have sane defaults for their methods. For the most obvious example, the internal
class [until] will iterate in an infinite loop when the "bang" method is
invoked. Even with external libraries, such poor defaults are entrenched. A
robust testing system must therefore work around these challenges.

Another challenge is that a substantial number of the external libraries have
never been tested at all. Many are not 64-bit clean. With this in mind, there
will probably be an initial high number of crashes with any testing system.
A maintainable and sensible testing system will therefore take an incremental
approach, starting with the most well-tested and popular libraries and
branching out from there to the more obscure ones.

Languages: thanks to some introspection classes availabe in Purr Data, the
testing framework can be fully implemented in Purr Data itself. However, some
familiarity with shell scripting tools and the platforms supported by
Purr Data will be useful (i.e., Windows, OSX, and GNU/Linux).

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Change to Double Precision Floating Format
------------------------------------------

Goal: change Purr Data's numeric data type from single-precision floating point
to double-precision floating point.

Benefits: Purr Data has a single numeric type to represent numeric data.
Changing this type to double-precision has many benefits:
* increased precision in the internal sample format used in the audio engine
* increases the maximum index into an array of audio data without losing
precision. Current single-point floating point indicies severely limit
array indexing, requiring the user to manage both an index and an onset into
the array to retain precision.
* creates a consistent interface between the core audio engine and the HTML5
GUI which itself uses double-precision floating point for represented numbers.
* makes it easier for cases where users want integers, as a double-precision
floating point type can itself contain a 32-bit integer. (Single-precision
floating point cannot.)
* makes it easier for third-party developers to interface with Purr Data. For
example, the upcoming MIDI revision uses 32-bit integers which can simply
be converted to double (while single-precision would lose data and require a
more complicated approach).

Details: this requires some changes to the core, leveraging Katja Vetter's
previous work to find performant replacements for the core DSP classes. It also
requires changes to some of the external libraries-- such as freeverb and
others-- which either have separate APIs based on numeric precision or rely on
the numeric type being single-precision.

This work will benefit immensely from the automated testing framework listed
above. That framework will immediately reveal many crashers and discrepancies
in output of the "dsp" methods of external classes. Developers can then use
the output of those tests to get a better sense of the scope of the work and
prioritize which libraries to refactor first.
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Difficulty: Hard. This is mitigated somewhat by the prior work of Katja Vetter
and others who have done double-precision implementations for most of the
core audio engine. Still, this change touches many different parts of the code
and will no doubt result in some insidious crashes and runtime errors that
will require a well-rounded knowledge of the C programming language, floating
point type standards, and Purr Data's internals including the core DSP
API and algorithms.

Languages: C, some C++ (for some of the external libraries coded in C++),
plus basic shell scripting familiarity (both `find` and `grep` will come in
handy here).

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API for HTML5 Web Apps
----------------------

Goal: create an API and simple user interface for loading a web app
that is linked to a Pd patch.

Details: Currently Pd patches are essentially just an HTML page with
an inline SVG which displays an editable diagram. It should be possible
for the user to instead open an arbitrary HTML document that contains
a web app which communicates using a javascript API with a Pd patch.

Implementation suggestion: since the current pdgui.js module already has
an interface for sending messages to the Pd backend, we should probably
leverage that to communicate from the web app to Pd. Then it's only
necessary to add a feature that allows a Pd Patch to send messages back
to the web app.

The user should then be able to load an HTML file using the existing
File->Open dialog. Then the web app will need something like the following
interface:

* ability to send messages to Pd (already exists)
* ability to tell Pd to load a particular patch in the same directory as
the web app (or perhaps using Pd's search path).
* ability to close a Pd patch
* ability to specify whether the patch we're loading should default to
  being visible or hidden
* ability to set callbacks for the web app to receive the following events:
  * the Pd patch we wanted to open has loaded
  * the Pd patch we loaded has closed
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Languages: Javascript. Some basic knowledge of C will be helpful if we need
to add a method in the Purr Data engine. However, such methods will probably
be quite simple and won't require implementing any complex algorithms in the
C language.

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K12 Mode
--------

Goal: Port the K12 Mode that Pd-l2ork introduced.

Details: Currently we ship a lot of the K12 abstractions with Purr Data.
However, I never ported the special mode that starts the program up in a
"K12" mode that replaces the window/application menubar with a more
user-friendly, clickable object menu.

That mode makes it easier for beginners to get started. In porting it, we
can take advantage of the new HTML5 GUI:
* The menu itself can leverage HTML/CSS to be more responsive, autohide, etc.
* The entire K12 Mode could itself just be a web-app separate from normal
  Pd app operation.

To figure out which is best, it's probably helpful to know more about the
history of K12 mode, and whether the ability to also open and interact with
normal Pd objects turned out to be an important feature.
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Ivica Bukvic has a lot of experience using this mode when teaching
kids, so we can look to him for some more insight into the right path to
take.
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Either way, this feature vastly improves the usability of Purr Data for new
users, giving them a way to quickly generate sounds and experiment with the
interface.
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Difficulty: Hard. A lot of the initial implementation of K12 mode was made
in the core engine in C. This will require at least reading a lot of the
source code and making sense of the changes made for K12 mode, plus possibly
refactoring some of that code so that it can be handled in the GUI instead.
On the other hand, there is a working version of the old K12 mode which can
be consulted as a reference implementation.

Languages: Javascript, C, CSS, HTML.

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Profile and Optimize Purr Data for Realtime Safety
--------------------------------------------------

Goal: Find the "pain points" in Purr Data's core message dispatcher and
audio engine, and optimize the code where possible to improve the realtime
scheduling of DSP.

Details: The core DSP algorithms of Purr Data tend to avoid system calls,
unnecessary branching, and other calls which would make the performance of
the audio process unreliable. However, there are many areas of Purr Data
which are not optimized for realtime scheduling.

The first order of business would be to profile Purr Data in several areas
to see where problems may be. Some likely culprits are the following:

* parse time for incoming messages from GUI to the audio/messaging engine
* overhead of sending messages to the GUI, especially for visual arrays and
  drawing instructions for data structures
* overhead of sending streams of "motion" messages from GUI to Pd to track
  mouse position
* overhead of walking the linked list of objects in order to find the object
  under the mouse/mouseclick
* overhead of tracking mouse position over a visual array
* overhead of calculating bboxes in the audio process as opposed to GUI
* overhead of calculating text positioning in audio process as opposed to GUI
* overhead of counting UTF-8 code points in audio process to calculate
  line breaks
* overhead of handling incoming/outgoing socket traffic in the same thread
  as the audio scheduler
* probably other areas

Once we get a sense of the pain points, we can tackle the problem of how
to optimize in a maintainable manner. Some references for this:

* [Guilio Moro's work](https://github.com/giuliomoro/purrdata/commit/9dc3223ece79be5f60a6a629450b52a79b9e050c) on using a separate thread for the socket connections from GUI to the core audio engine (plus all the other socket connections
like netsend, netreceive, etc.)
* Guilio Moro's work on a threaded microsleep for the event loop.
* [Guilio Moro's work](https://github.com/giuliomoro/purrdata/commits/simpler-motion) to simplify GUI communication by handling more of the mouse motion/click
  logic in the GUI. This results in fewer messages from GUI to audio engine,
  but still requires a linked-list walk in the audio engine to find the
  relevant object.
* Matt Barber's link to a new cosine wave generator algorithm that may be
  more performant than the current implementation. (Not so important for
  current performance, but this may become more relevant once we switch to
  double-precision for block samples.)
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* Possibility to vectorize DSP algos using SIMD. Also more crude experiments
  by just hand-unrolling one or two classes when N=64 (i.e., the most common
  block size) and measuring the performance impact (if any).
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Difficulty: Moderate to hard. The initial profiling will take some time but
isn't particularly challenging. Making changes to the core audio engine,
however, will require some knowledge of Linux system interfaces and some of
Purr Data's internals. Properly assessing and testing any threading techniques
in C is also frought with peril and will require extreme care in order to
keep the code maintainable and avoid insidious bugs.

Languages: basic to advanced shell scripting, C, plus familiarity with
profiling tools like gprof and others.

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ASCII art to Purr Data diagram conversion
-----------------------------------------

Goal: make a GUI interface where the user can either type or paste in
ASCII art for a Purr Data diagram and have it converted to a floating selection
in the current Purr Data diagram.

Details: Often Purr Data users rely on ASCII art in forums, mailing lists
and other documentation to give examples of Purr Data programs. This is
convenient because it's quicker than taking a screenshot of the Purr Data
GUI and takes up less space, too.

The ability for the user to type ASCII art into an object box and get a new
selection of a Purr Data object chain would be useful. This would let users
paste ASCII art from the mailing list directly into the interface. It would
also make it easier to create Purr Data diagrams using only the keyboard.

Bonus Goal: make it possible to convert a subset of Purr Data diagrams into
ASCII art.

Bonus Goal Details: Unfortunately Purr Data allows arbitrary positioning
of objects on an integer-based x/y axis. Thus, not all Purr Data diagrams
can be represented as ASCII art.

The bonus goal would add a feature to check if the currently selected Purr
Data diagram is able to be represented using only ASCII art. This could be
either a) a button that does an analysis, or b) realtime feedback to let the
user know when they have positioned an object or connection in such a way
that makes ASCII art impossible.

This is a non-trivial problem. For example, one could add a button to Purr
Data that would constrain object positioning to a predefined grid. Then one
could disallow objects to overlap with each other. Still, the connections
among the various objects (i.e., the arcs which connect the nodes) could
overlap in such a way that makes unambiguous ASCII art impossible.

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Difficulty: Easy to hard. This idea runs the gamut-- a simple GUI-side
parser for simple ASCII art is a quick exercise. Handling some of the more
complicated art with multiple and/or crossed connections is moderate. Scanning
the mailing lists for examples and covering the majority of them is hard.

Additionally, converting an existing Purr Data graphical diagram back to
ASCII art is a difficult problem. This will require reading up on the current
state of the art and comparing various approaches from other fields like
OCR and some of the current work converting Photoshop mockups to HTML pages.
However, Purr Data diagrams tend to be much simpler than arbitrary Photoshop
art, so the problem may be more tractable here than in other fields.

Languages: Javascript if converting ASCII art to Purr Data diagram, C if
attempting to convert from graphical diagram *back* to ASCII art.

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Navigation of "Wireless" Objects
--------------------------------

Goal: make it possible for the user to navigate among all extant wireless
objects that are bound to a particular symbol.

Details: Purr Data diagrams typically consist of objects-- English words in
boxes-- connected by Bezier curves. In a complex diagram, however, the
Bezier curves can end up obscuring the flow of the data in the diagram. For
these instances, "wireless" objects like `[send]` and `[receive]` may be
used to send data from one object to another without making an explicit
connection.

However, such "nonlocal" connections can quickly make diagrams difficult to
maintain. It would be helpful if the user could query a particular "wireless"
object to find out how many other objects it communicates with.

Internally, all related "wireless" objects are bound to the same immutable
symbol. This makes it easy to find all related objects for a given symbol.

Probably the easiest UI for this would be a button to query all related
connections. Once Purr Data's engine finishes the search, it can send back
the ids of all the objects it found. Finally, the gui can just print a list
of hyperlinks to the console. When the user clicks any hyperlink, the
corresponding object can be highlighted (or the relevant diagram brought up
with the object in it highlighted).

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Difficulty: Easy to moderate. Purr Data's bindelem and pd_bind API are a little
tricky if one hasn't used them before. However, once that interface is
understood it is all that is needed to return a complete list of all
nonlocal objects bound to a particular symbol.

The remainder of the problem is easy and can be done in Javascript. However,
this problem "upgrades" gracefully. Once the initial UI is done, it can be
tested on users and iteratively improved from there.

Languages: Javascript, C.

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Terminal REPL
-------------

Goal: make a little REPL interface with which the user can interact with
Purr Data programs and program state.

Details: Purr Data is being used in situations where the hardware is an embedded
device. While the current GUI runs on most common hardware including the RPI,
there are situations where it would be more convenient to simply interact using
a text interface (locally or over ssh).

The user can already communicate with Purr Data's audio engine over a socket
connection. So the "read" and "evaluate" part already exists. However, Purr
Data does not print a response nor loop in this situation.

Additionally, the UX of sending raw messages to Purr Data's interpreter is
quite lacking. The syntax for creating new environments and objects was not
meant to be used directly. Objects are referenced by index number, and the
diagrams themselves must be referenced using hex identifiers.

It would be very beneficial to create a REPL UI that is more user-friendly
and well-documented/specified. This way Purr Data users can always interact
with and create programs easily on any embedded device, even if there is
no direct display. (This would also be very handy for debugging purposes.)
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Difficulty: Moderate. An initial REPL can be created with the current Purr
Data API, but it won't be particularly user-friendly. To achieve that requires
more work and an understanding of Purr Data's message dispatching system.

Languages: C, some shell scripting.