Here is a description of technology at the core of the Vision Egg.
The computer language used is Python, and graphics are drawn using the
OpenGL standard. Ample documentation for these technologies is
available, both online and in print. This document covers why the
choice was made to rely on these technologies, and why you can trust
them with your experiments. It also provides some insight into how
the Vision Egg works and the philosophy of its construction.
The Python computer programming language is free, easy to learn, easy
to use, powerful, and flexible. Python supports modern programming
techniques such as object oriented programming, threading, and
high-level data types. There are countless modules for virtually any
programming task. Therefore, Python is well suited for complex
programming tasks. There is much Python advocacy on the internet, but
let me just say that Python was chosen for the Vision Egg because
creating something like this project would be a much larger burden
without such a fantastic language.
- Why an interpreted language for real-time stimulus control?
A program written in an interpreted language will run more slowly than
a well-written C program. But there are two reasons why Python works
well for this job. The first reason is that data array manipulation
is performed with high-performance, compiled C code via the Numeric
module of Python. In other words, high-level Python code is only used
to direct computationally intensive tasks.
The second reason why Python works for this task is that computer
monitors can only be refreshed at their maximum vertical frequency,
which typically ranges up to 200 Hz. At 200 Hz, your program and the
graphics card have about 5 milliseconds to prepare the next
frame. Modern computers can perform enormous numbers of calculations
in that time, especially if the hardware is specialized for the task,
such as graphics processing on a graphics card. In other words, it's
fast enough. This is evidenced by the fact that running under Windows
2000, on an Athlon 1800+ system with an nVidia geForce 2 Pro card, no
frames are skipped while updating the monitor at 200 Hz. Although I
do not know of any exhaustive testing, the Vision Egg runs for
hours at a time without skipping a single frame on standard,
consumer-level hardware.
The biggest timing-related concern is unrelated to choice of
programming language. A multitasking operating system might take
control of the CPU from the stimulus generating program for an
unacceptably long period. This is the biggest potential cause of
frame-skipping latencies.
- Comparison of Python with other languages
Matlab is frequently used by vision scientists because it is well
suited for data analysis and numerical modeling. However, it would be
difficult to reproduce the functionality of the Vision Egg directly in
Matlab because of the lack of many language features such as (useful)
object oriented programming, an OpenGL interface, and easy access to
routines written in C.
The C language is used for various bits of the Vision Egg, but it
would be a very difficult tool with which to write an application of
the scale of the Vision Egg. With todays fast computers, execution
speed is at less of a premium than a scientist or programmer's
time. The main role of C within the Vision Egg is as interface code
that allows Python programs to call platform-specific functions.
Performance critical routines could be written in C, but these
optimizations have not been needed in general. It does remain an
option to re-code the drawing performed by a particular stimulus class
or even the entire main loop in C.
If you prefer another language, you are welcome to use the ideas found
within the Vision Egg because it is open-source. Also, it would be
possible to embed Python and the Vision Egg within a C program.
OpenGL is the ubiquitous cross platform way to access hardware
accelerated graphics processing in today's video cards. It is an open
standard 2D and 3D graphics programming interface that recent video
cards support. While OpenGL is defined in C, the PyOpenGL project
makes the OpenGL interface available in Python.
OpenGL offers features not available on traditional 2D graphics cards.
Recent graphics cards are capable of realtime image scaling, sub-pixel
image re-sampling and movement, perspective projections and other
image warping, true color support for framebuffers and DACs greater
than 8 bits (no more color lookup tables!). These are just a few of
the features that OpenGL offers and that the Vision Egg uses.
The Vision Egg comes with many standard stimuli, such as sinusoidal
gratings, moving rectangles, random dots, images, and
checkerboards. Therefore, you may not need to learn OpenGL to take
advantage of it. But if you do learn OpenGL, you can extend the
Vision Egg to do anything your graphics card is capable of. OpenGL is
complex and is therefore challenging to learn, but it is a standard,
so there is an incredible wealth of information on it.
