GNU CGraph ( )
GNU CGraph
A Tool for Learning about Convolution
GNU CGraph version 2.0, 12 October 2011
Adrienne Gaye Thompson ( [email protected]
)
This is a manual for GNU CGraph version 2.0, a tool for learning about convolution.
Author. GNU CGraph version 2.0. Derived from BSc. dissertation "Interactive Computer
Package Demonstrating: Sampling Convolution and the FFT", University of Aberdeen,
Scotland (1983). For the code from the dissertation, visit <http://codeartnow.com/lawproject>.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no FrontCover Texts, and with no BackCover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
for ALL the VICTIMS of APARTHEID STRUGGLING to be FREE and to REGNIER
You’re sending me discrete signals from across the room,
I respond on impulse, reflecting on the sampling of events
That were a dichotomy from the day you left your mother’s womb;
Multiplied in frequency, integrated in time, a weighted confluence
Of sliding, shifting trains of thought, alternative messages under transformation;
Counterpoint, duality, involution, contradistinction without confusion;
Independence in summation. Silence  this is convoluted conversation.
1
Table of Contents
1 Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1
About . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2
Required Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Invoking CGraph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 The Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1
The Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2
Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3
Defaults & Error Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4
Frequency Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 A CGraph Session . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Reporting Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A Sketch of Convolution Theory . . .
A.1
Introductory Ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2
The Mathematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2.1
Deriving the Convolution Sum . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2.2
The Convolution Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3
Linear and Circular Convolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3.1
Linear Convolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3.2
Circular Convolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix B References . . . . . . . . . . . . . . . . . . . . . . . .
Appendix C GNU Free Documentation License
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
i
Chapter 1: Foreword
1 Foreword
From the shadow cast by light to the echo in a cave convolution, like the ubiquitous Fibonacci series, is a mathematical description of naturally ocurring physical phenomena in any linear, timeinvariant system capable of responding to an input signal. Today, convolution  the combination of two signals to produce a third  has wide ranging applications.
Edge detection in computer vision, algorithms for robot motion, signal and image processing, crystallography, statistics and probability theory, differential equations, linear algebra, numerical analysis, and even recent innovations in music production  all utilise techniques involving convolution.
GNU CGraph (for Convolution Graph) is a tool for visualizing the convolution of two signals. The package is a reproduction of the Fortran 77 program in my BSc. Electrical
Engineering (Honours) dissertation "Interactive Computer Program Demonstrating: Sampling Convolution and the FFT", University of Aberdeen, Scotland, 1983. In this version
I have included pulses, scaling of the signals, and errorhandling  features that were not part of my original Thesis.
Whether student engineer or scientist, aspiring specialeffects animator or roboticist, GNU
CGraph will help you find the adventure in the mathematics of convolution.
– Adrienne Gaye Thompson
3
Chapter 2: Overview
2 Overview
2.1 About
GNU CGraph computes the linear convolution of two signals in the time domain then compares their circular convolution by demonstrating the convolution theorem  convolution of two signals in the time domain corresponds to multiplication in the frequency domain.
Each signal is modelled by a register of discrete values simulating samples of a signal, and the discrete Fourier transform (DFT) computed by means of the Fast Fourier Transform
(FFT). See
Appendix A [Appendix A], page 27
, for an explanation of the convolution theorem.
GNU CGraph is interactive, prompting the user to enter single character or numerical values from the keyboard, thereby dispensing with the learning curve for coding formulae.
The user chooses 2 from a menu of 8 signal types, and up to 5 parameters to define the waveforms. The signals chosen may be periodic, aperiodic, or pulses. CGraph then plots
3 graphs:
1. The time domain representation of both signals;
2. Their Fourier transforms;
3. A comparison of their linear and circular convolution.
See
Chapter 5 [A CGraph Session], page 13
, for a typical CGraph session.
GNU CGraph will be useful to students of signal theory in the study of convolution and spectral analysis.
This version (2.0) uses a simple FFT written by Arthur Wouk and converted to Fortran 90 by Alan Miller (see
Appendix B [Appendix B], page 35 ).
2.2 Required Software
GNU CGraph is written in contemporary Fortran. The package runs on GNU/Linux, was developed with GFortan and G95, and uses Gnuplot 4.2 as well as Image Magick 6.6.
Experienced users wishing to use other compilers may supply the necessary commandline options to configure during installation. See the file ‘INSTALL’ for basic installation instructions.
5
Chapter 3: Invoking CGraph
3 Invoking CGraph
To run GNU CGraph, open up a terminal in X and type cgraph. CGraph supports the following options:
‘dedicate’
‘d’ Print the dedication and exit.
‘help’
‘h’ Print a summary of the command line options and exit.
‘nosplash’
‘n’ Invoke GNU CGraph with no splash screen.
‘version’
‘v’ Print the version number and licensing information of GNU CGraph, then exit.
7
Chapter 4: The Signals
4 The Signals
4.1 The Menu
CGraph presents the following menu of signals from which the user chooses 2:
SIGNAL
Sine
Cosine
Triangle
Square
Sawtooth
Exponential
Ramp
Step
CODE
A
B
C
D
E
F
G
H
Signals ‘A’ to ‘E’ are periodic, while ‘F’, ‘G’, and ‘H’ are aperiodic. Pulses may also be chosen; these are a half period in duration 1/2f , where f is the frequency of the corresponding cyclical waveform.
4.2 Parameters
The user enters up to 5 parameters to generate the signals, their FFTs and their convolution:
1. The number of samples ‘N’
2. The code for the signal ‘A’ to ‘H’
3. The wave/pulse parameter ‘w’ or ‘p’
4. The frequency ‘f’
5. The scaling coefficient ‘sc’
Both signals are constructed from the same number of samples ‘N’. If the user chooses a periodic signal, then he/she is prompted to select either the cyclical waveform or a derived pulse, i.e., ‘w’ or ‘p’. For each periodic signal chosen the user is prompted to enter its frequency ‘f’ and a scale factor ‘sc’.
Pulses are monophasic and are defined on half the period of the modulus of the corresponding periodic waveform.
4.3 Defaults & Error Handling
If the required parameter is a number and the user has erroneously entered character data,
CGraph generates an error message and gives the user another try to enter a number.
Otherwise, for input outside the expected ranges CGraph assumes default values.
Number of samples ‘N’
‘N’ must lie in the range [64, 1024]. Values entered outside of this range will default to 512. ‘N’ is defined to be a power of 2. If the user enters a value that is not a power of 2 CGraph will choose the nearest power of 2.
9
10 GNU CGraph 2.0
Signal code ‘A’ to ‘H’
For input outside the range ‘A’ to ‘H’, the default codes are ‘C’ for the first signal, and ‘D’ for the second.
Wave/Pulse parameter ‘w’ or ‘p’
The default waveform is a pulse.
Frequency ‘f’
CGraph assumes a default frequency of 1Hz for values of ‘f’ entered outside the range [0.5, N/4].
Scaling coefficient ‘sc’
The scaling coefficient ‘sc’ may be positive or negative. The maximum absolute value of ‘sc’ for signals a, b, f, and h is ‘N’,while that for signals c,d,e, and g is 1.
All signals will be scaled to unity for input values of ‘sc’ outside the permitted range.
With the default scaling coefficient of 1, signals a, b, f, and h are unit functions; signals d (square) an e (sawtooth) have a maximum amplitude of half the period
(1/(2f)) while that of c (triangle) is onequarter the period (1/(4f)).
4.4 Frequency Selection
We can express the period P of a periodic signal as
P = N/(number of cycles)
= T /(N/n) where T is the duration ofthe signal register in seconds, N is the number of samples in the register (window length), and n is the number of samples in 1 period.
The frequency of the signal f is the reciprocal of the period, so f = N/(nT ) samples/seconds or Hz
C=Graph assumes that the duration of the signal register is 1 second, so f = N/n) Hz
The sampling rate f s is given by f s
= N/T Hz
The interval h between successive samples being the reciprocal h = 1/f s seconds
Chapter 4: The Signals 11
If the window length and frequency chosen are 512 and 20 Hz (approximately the lower limit of the human audible range) then the number of samples n in each period would therefore be n = N/f
= 512/20
= 25.6
CGraph requires that n be a multiple of 4. For each periodic signal, the frequency entered by the user is accordingly adjusted so that n approximates to the nearest multiple of 4. So the frequency of the signal used by CGraph would become f = N/n
= 512/26
= 19.7 Hz
Chapter 5: A CGraph Session 13
5 A CGraph Session
In this session, we run CGraph twice to compare the convolution of 2 signals of equal length:
1. When the signals are finite sequences and the remaining interval is zero padded to the
‘N’ with at least the same number of zeros as samples in each signal;
2. When both signals extend across the full register of ‘N’ samples.
We use a sawtooth pulse for the first signal, and a rectangular pulse half the amplitude of the sawtooth for the second signal. We also demonstrate the use of default values for unexpected input. The keychord ALTTAB is used to toggle the terminal and the Gnuplot window.
In X, type cgraph. The splash screen will appear for a few seconds. Pressing ESC will kill the display, but you may invoke CGraph without the splash screen with the nosplash option. When the splash screen disappears, the following text will appear:
GNU CGraph version 2.0
Dedicated to Eliezer Regnier and all victims of apartheid.’
Gaye Thompson, Sole Author. GNU CGraph version 2.0. Derived from
BSc. dissertation "Interactive Computer Package Demonstrating: Sampling
Convolution and the FFT", University of Aberdeen, Scotland (1983). For the code from the dissertation, visit
<http://codeartnow.com/lawproject>.
GNU CGraph is free software licensed under the terms of the GNU General
Public License (the GPL) version 3 or later. You are welcome to distribute it under certain conditions.
GNU CGraph is distributed WITHOUT ANY WARRANTY without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GPL section 15 regarding disclaimer of warranty.
Press <g> then <ENTER> to display the GPL, or just press <ENTER> to continue.
CGraph:>> RET
THIS IS GNU CGraph  a tool for visualizing convolution.
Compare the linear convolution of two signals with their circular convolution.
14 GNU CGraph 2.0
Signal
======
Sine
Cosine
Triangle
Square
Sawtooth E
UnitExponential F
UnitRamp
UnitStep
C
D
G
H
Code
====
A
B
Generate 2 signals from the above menu with up to 5 parameters:
1. Number of samples ‘N’
2. Signal code ‘A’ to ‘H’
3. Whether the signal, ifperiodic, is a wave ‘w’ or a pulse ‘p’
4. The frequency ‘f’
5. The scaling coefficient ‘sc’
Choose a value for "N" between 64 and 1024.
CGraph:>> 51
The number of samples "N" is: 512
Signal
======
Sine
Cosine
Triangle
Square
Sawtooth
UnitExponential F
UnitRamp G
UnitStep H
C
D
E
A
B
Code
====
Enter code for first signal
CGraph:>> e
Is the signal periodic or is it a pulse?
Type "w" for periodic wave, or "p" for pulse
CGraph:>> p
Select the frequency "f" of this signal.
CGraph:>> 1.0
Chapter 5: A CGraph Session
The frequency of this signal is 1.00 Hz.
Do you wish to scale this signal?
Enter a value for the scaling coefficient "sc".
A coefficient of 1 will give the unit function.
CGraph:>> 1
Signal
======
Sine
Cosine
Triangle
Square
Sawtooth
UnitExponential F
D
E
UnitRamp
UnitStep
G
H
B
C
Code
====
A
Enter code for second signal
CGraph:>> s
Is the signal periodic or is it a pulse?
Type "w" for periodic wave, or "p" for pulse
CGraph:>> p
Select the frequency "f" of this signal.
CGraph:>> 1
The frequency of this signal is 1.00 Hz.
Do you wish to scale this signal?
Enter a value for the scaling coefficient "sc".
A coefficient of 1 will give the unit function.
CGraph:>> .5
You selected a square signal by default.
15
16 GNU CGraph 2.0
Press <Enter> to see the signals in the time domain:>> RET
ALTTAB
Hit <Enter> to continue:>> RET
View the frequencydomain representation of the signals.
Chapter 5: A CGraph Session
Press <Enter> to see their FFTs:>> RET
17
ALTTAB
Hit <Enter> to continue:>> RET
18 GNU CGraph 2.0
Press <Enter> to compare linear and circular convolution:>> RET
ALTTAB
Hit <Enter> to continue:>> RET
Exiting GNU CGraph ...
Bye.
[email protected]:~$ cgraph nosplash
GNU CGraph version 2.0
Dedicated to Eliezer Regnier and all victims of apartheid.’
Gaye Thompson, Sole Author. GNU CGraph version 2.0. Derived from
BSc. dissertation "Interactive Computer Package Demonstrating: Sampling
Convolution and the FFT", University of Aberdeen, Scotland (1983). For
Chapter 5: A CGraph Session 19 the code from the dissertation, visit
<http://codeartnow.com/lawproject>.
GNU CGraph is free software licensed under the terms of the GNU General
Public License (the GPL) version 3 or later. You are welcome to distribute it under certain conditions.
GNU CGraph is distributed WITHOUT ANY WARRANTY without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GPL section 15 regarding disclaimer of warranty.
Press <g> then <ENTER> to display the GPL, or just press <ENTER> to continue.
CGraph:>> RET
THIS IS GNU CGraph  a tool for visualizing convolution.
Compare the linear convolution of two signals with their circular convolution.
Signal
======
Sine
Cosine
Triangle
Square
Sawtooth
UnitExponential F
UnitRamp G
UnitStep H
A
B
C
D
E
Code
====
Generate 2 signals from the above menu with up to 5 parameters:
1. Number of samples ‘N’
2. Signal code ‘A’ to ‘H’
3. Whether the signal, ifperiodic, is a wave ‘w’ or a pulse ‘p’
4. The frequency ‘f’
5. The scaling coefficient ‘sc’
Choose a value for "N" between 64 and 1024.
CGraph:>> 521
The number of samples "N" is: 512.
20
Signal
======
Sine
Cosine
Triangle
Square
Sawtooth E
UnitExponential F
UnitRamp
UnitStep
C
D
G
H
Code
====
A
B
Enter code for first signal
CGraph:>> g
Do you wish to scale this signal?
Enter a value for the scaling coefficient "sc".
A coefficient of 1 will give the unit function.
CGraph:>> 1
Signal
======
Sine
Cosine
Triangle
Square
Sawtooth
UnitExponential F
UnitRamp G
UnitStep H
D
E
B
C
Code
====
A
Enter code for second signal
CGraph:>> h
Do you wish to scale this signal?
Enter a value for the scaling coefficient "sc".
A coefficient of 1 will give the unit function.
CGraph:>> q56
That was not a number. Try again!
CGraph:>> 256
GNU CGraph 2.0
Chapter 5: A CGraph Session
Press <Enter> to see the signals in the time domain:>> RET
21
ALTTAB
Hit <Enter> to continue:>> RET
View the frequencydomain representation of the signals.
22
Press <Enter> to see their FFTs:>> RET
GNU CGraph 2.0
ALTTAB
Hit <Enter> to continue:>> RET
Chapter 5: A CGraph Session
Press <Enter> to compare linear and circular convolution:>> RET
23
ALTTAB
Hit <Enter> to continue:>> RET
Exiting GNU CGraph ...
Bye.
On exit, the directory cgraphs will be created. Directory cgraphs will have 2 subdirectories containing the graphs, Gnuplot command files, and the data used for plotting generated by the last run. The subdirectories and files are:
• cgraphs/graphs: signals.png, transforms.png, convolutions.png
• cgraphs/coms: signals.cg, transforms.cg, convolutions.cg time.dat trans.dat
One can then print the graphs and display them either by using a graphics editor like Image
Magick, or by executing the command files with Gnuplot.
Chapter 6: Reporting Bugs 25
6 Reporting Bugs
To report bugs or suggest enhancements for GNU CGraph, please send electronic mail to [email protected]
.
For bug reports, please include enough information for the maintainers to reproduce the problem. Generally speaking, that means:
• The version numbers of GNU CGraph (which you can find by running
‘cgraph version’) and any other program(s) or manual(s) involved.
• Hardware and operating system names and versions.
• The contents of any input files necessary to reproduce the bug.
• The expected behavior and/or output.
• A description of the problem and samples of any erroneous output.
• Options you gave to configure other than specifying installation directories.
• Anything else that you think would be helpful.
When in doubt whether something is needed or not, include it. It’s better to include too much than to leave out something important.
Patches are welcome. Please follow the existing coding style.
Appendix A: Sketch of Convolution Theory 27
Appendix A Sketch of Convolution Theory
A.1 Introductory Ideas
GNU CGraph compares the linear and circular convolution of two signals. Subroutine convo computes the linear convolution directly in the time domain, while the FFT is exploited to compute circular convolution through the convolution theorem, which is defined below (see
Section A.2.2 [The Convolution Theorem], page 28 ).
Convolution is an operation by which two functions combine to produce a third that represents a kind of moving average. This is a naturally ocurring phenomenon that presents itself whenever there is a linear system obeying the principles of superposition and shift/time invariance. Accordingly, the mathematics of convolution has found application to much of science and engineering in areas ranging from statistics to computer vision.
The output of any linear shift invariant system may be described as the convolution of the input with the impulse response of the system. In computer vision, for example, where the system being considered is a 2dimensional image, the output of the system may be blurred as a result of the relative motion of the camera and the object. This blurred image can be modelled by convolution of the static image with the 2dimensional impulse response.
The 2dimensional impulse response is called a pointspread function (PSF). Each pixel in the image produces a copy of the PSF, scaled according to the strength of the pixel and spatially shifted. Superposition of these copies form the resultant output signal, the system being linear and shift invariant. The output blurred image is then a convolution that is, in fact, a linear combination of the PSFs. The design of a filter for image restoration must then rely on inverse convolution.
A thorough treatment of the mathematics of convolution is beyond the scope of this manual.
See
Appendix B [Appendix B], page 35 , for some references on the subject, and related
engineering theory.
A.2 The Mathematics
Consideration of the 1dimensional case simplifies the arithmetic. To prove the convolution theorem, we first derive an expression for the convolution of 2 signals, then apply the Fourier transform to this expansion.
A.2.1 Deriving the Convolution Sum
A discretetime signal may be modelled as a series of piecewise rectangular pulses. The summation of all such rectangular pulses approximates the signal f : f (n) = P m f (m) rect(n − m) where n − m denotes the rectangle whose base on the n axis is centred at sample n = m.
In the limit, the series of rectangular pulses approaches a continuous signal as the pulse width tends to zero and each pulse becomes an impulse signal. Each impulse signal can then be represented as a scaled and shifted unit impulse simulating one sample of the discrete signal.
28 GNU CGraph 2.0
f (n) = P m f (m) δ(n − m)
Applying a system transform M that maps the input signal f to the output signal g, g(n) = M [f (n)] g(n) = M [f (n) = P m f (m) δ(n − m)]
=
P m f (m) M [δ(n − m)]
Since the system transforms a delta function to the system impulse response h g(n) =
P m f (m) h(n − m) (1)
The above expression called the convolution sum, denoted by f (∗)h, defines the output g(n) of the system.
A.2.2 The Convolution Theorem
GNU CGraph demonstrates the convolution theorem. The convolution of 2 signals in the time domain is equal to the inverse Fourier transform of the product of their transforms in the frequency domain.
Just as a signal can be represented by a linear combination of scaled and shifted impulses, we can also describe the signal as a linear combination of sinusoidal basis functions. The Fourier transform exploits this representation to deconstruct the signal into frequency components, each corresponding to a basis sinusoid.
Using Euler’s identity e jθ
= cos(θ) + j sin(θ)
The sinusoidal sum representing the discrete time signal may be written in the form f (n) = 1/N
N −1
P
F (k) e jω k n k=0 where ω k
= 2kπ/N , and the F (k) are Fourier transform coefficients indicating the strength of the kth spectral sample of frequency ω k
(how much of the each basis sinusoid is present in the signal).
1
Accordingly, the F (k) may be computed from the signal
F (k) =
N −1
P f (n) e
−jω k n n=0
This is the Fourier transform description of the signal as a function of frequency.
From eqn (1), the convolution of an input signal f with the system impulse response h to give an output g is defined as:
1
For a periodic signal, these coefficients are the δ functions of the Fourier transform.
Appendix A: Sketch of Convolution Theory 29 g(n) = f (∗)h = P m f (m) h(n − m)
Let the Fourier transform of g(n) be denoted by
Γ[g(n)] = g(n) e
−jωn
, then
Γ[f (∗)g] =
P n
[f (∗)g] e
−jωn
Γ[f (∗)h] =
P n
P m f (m) h(n − m)] e
−jωn
=
P m f (m)
P n h(n − m) e
−jωn
Changing the variable to p = n − m
Γ[g(n)] = P m f (m) P p h(p) e
−jω(m+p)
=
P m f (m)e
−jmω
P p h(p)e
−jωp
Taking the inverse Fourier transform, f (∗)g = Γ
−1
[ Γ[f ] Γ[h]
This is the convolution theorem.
A.3 Linear and Circular Convolution
The simulation of actual or linear convolution requires a sequence of multiplications and additions that are computationally too slow for high speed operations such as deblurring filters for precision robotic vision control systems. The FFT, an algorithm for efficiently computing the DFT, dramatically overcomes the computational load by successively decomposing the multiplication of two sequences into subsequences of half the length thereby reducing the number of artithmetic operations by roughly N/logN .
The cost of this additional computational power is the treatment of the convolving signals as periodic with N samples per period. The resulting convolution is termed circular convolution. It can be shown that circular convolution and linear convolution are equivalent if
N ≥ L + P − 1 where L, M are the unpadded lengths of the sequences being convolved.
We illustrate the difference between linear and circular convolution using abbreviated sequences for the pulses demonstrated in
Chapter 5 [A CGraph Session], page 13
A.3.1 Linear Convolution
As noted above (see
Section A.1 [Introductory Ideas], page 27
), in linear convolution, each sample of f contributes a scaled and shifted copy of the h. This is accomplished by the multiplication of the particular sample m of f by each sample of h.
This sequential multiplication can be visualized as a physical reflection of h(m)) about the vertical axis to obtain h(−m) followed by discrete shifts of 1 sample interval (δn = 1) along the time axis with no overlap of the signals at the beginning and end of the translation.
As the impulse response moves along the time axis, the point by point multiplication of
30 GNU CGraph 2.0
coincident samples is summed. The sum at each point in the translation is the value of the convolution sum g(n) at that point, and the length of the convolution is L + M .
For the sequences f (m) = [1, 1, 1], and h(m) = [0, 1, 2]
The series of operations for linear convolution f (∗)h = [0 1 3 3 2], are:
0.
1 1 1
2 1 0

0+0+0+0+0 = 0 g(0) = f(m)h(0m)
1.
g(1) = f(m)h(1m)
2.
1 1 1
2 1 0

0+1+0+0 = 1
1 1 1
2 1 0

2+1+0 = 3 g(2) = f(m)h(2m) =
3.
g(3) = f(m)h(3m) =
1 1 1
2 1 0

0+2+1+0 = 3
Appendix A: Sketch of Convolution Theory 31
4.
g(4) = f(m)h(4m) =
1 1 1
2 1 0

0+0+2+0+0 = 2
A.3.2 Circular Convolution
We can imagine circular convolution in terms of the relative rotation of two concentric cylinders whose circumferences are of length N . A copy of the N samples comprising f is wrapped anticlockwise round one cylinder, while a copy of h is wrapped clockwise round the other cylinder, reflecting h . Rotating the second cylinder anticlockwise by 1 sample interval each time, multiplying the coincident samples and summing will give corresponding values of the convolved signal g.
f (m) = [1, 1, 1], and h(m) = [0, 1, 2]
The series of operations for circular convolution f (∗)h = [0 1 3 3 2], are:
0.
g(0) = f(m)h(0m)
1.
g(1) = f(m)h(1m)
1 1 1
0 2 1

0+2+1 = 3
1 1 1
1 0 2

1+0+2 = 3
32 GNU CGraph 2.0
2.
g(2) = f(m)h(2m)
1 1 1
2 1 0

2+1+0 = 3
By zeropadding each sequence of length L = 3 to length N so that N ≥ L + L − 1 (see
Section A.3 [Linear and Circular Convolution], page 29 ), we obtain the sequences
f (m) = [1, 1, 1, 0, 0] h(m) = [0, 1, 2, 0, 0]
Circular convolution then achieves the same as result as linear convolution: f (∗)h = [0 1 3 3 2]
The operations are:
0.
g(0) = f(m)h(0m)
1 1 1 0 0
0 0 0 2 1

0+0+0+2+1 = 0
1.
g(1) = f(m)h(1m)
2.
g(2) = f(m)h(2m)
1 1 1 0 0
1 0 0 0 2

1+0+0+0+2 = 1
1 1 1 0 0
2 1 0 0 0

2+1+0+0+0 = 3
Appendix A: Sketch of Convolution Theory
3.
g(3) = f(m)h(3m)
4.
g(4) = f(m)h(4m)
1 1 1 0 0
0 2 1 0 0

0+2+1+0+0 = 3
1 1 1 0 0
0 0 2 1 0

0+0+2+1+0 = 2
33
Appendix B: References 35
Appendix B References
The sources below were consulted in the preparation of GNU CGraph and/or the 1983 dissertation [1] from which GNU CGraph is derived.
1. Thompson, Adrienne G. ”Interactive Computer Package Demonstrating: Sampling
Convolution and the FFT”, BSc. Engineering Honours thesis, University of Aberdeen
(Scotland), 1983, see http://codeartnow.com/lawproject
.
2. Horn, Berthold, K. P. Robot Vision. MIT Press, Cambridge, Massachusetts, 1986.
3. McGillem, Clare D., and Cooper, George R. Continuous and Discrete Signal and System Analysis. Holt, Rinehart and Winston, Inc., 1990.
4. Oppenheim, Alan V., and Schafer, Ronald W. Digital Signal Processing. PrenticeHall,
Englewood Cliffs, N.J., 1975.
5. Smith, Steven W. The Scientist and Engineer’s Guide to Digital Signal Processing http://www.dspguide.com
6. Stremler, Ferrel G. Introduction to Communication Systems. AddisonWesley Publishing Co. Inc, 1977.
7. Wouk, Arthur.
fftsimple.f90.
Ed.
Alan Miller.
2003.
Retrieved from http://jblevins.org/mirror/amiller/fft_simple.f90
8. Yuen, C. K. and Fraser, D. Digital Spectral Analysis. CSIRO/Pitman, East Melbourne,
Australia, 1979.
9. Ziemer, Rodger E., Tranter, William H., Fannin, D. Ronald. Signals Systems Continuous and Discrete. 4th ed. Prentice Hall, Upper Saddle River, NJ 07458 1998.
Appendix C: GNU Free Documentation License 37
Appendix C GNU Free Documentation License
Version 1.3, 3 November 2008 http://fsf.org/
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public
License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a worldwide, royaltyfree license, unlimited in duration, to use that work under the conditions stated herein.
The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
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The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released
38 GNU CGraph 2.0
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The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
2. VERBATIM COPYING
Appendix C: GNU Free Documentation License 39
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies.
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If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires
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4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
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40 GNU CGraph 2.0
be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
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G. Preserve in that license notice the full lists of Invariant Sections and required Cover
Texts given in the Document’s license notice.
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I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the
“History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
M. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
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O. Preserve any Warranty Disclaimers.
If the Modified Version includes new frontmatter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their
Appendix C: GNU Free Documentation License 41 titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
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5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical
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In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
6. COLLECTIONS OF DOCUMENTS
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You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
42 GNU CGraph 2.0
7. AGGREGATION WITH INDEPENDENT WORKS
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Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
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Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
Appendix C: GNU Free Documentation License 43
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free
Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.
See http://www.gnu.org/copyleft/
.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free
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An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under
CCBYSA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
44 GNU CGraph 2.0
ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name .
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation; with no Invariant Sections, no FrontCover Texts, and no BackCover
Texts.
A copy of the license is included in the section entitled ‘‘GNU
Free Documentation License’’.
If you have Invariant Sections, FrontCover Texts and BackCover Texts, replace the
“with. . . Texts.” line with this: with the Invariant Sections being list their titles , with the FrontCover Texts being list , and with the BackCover Texts being list .
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If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU
General Public License, to permit their use in free software.
Index 45
Index

dedicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
nosplash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
v . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A aperiodic signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G graphs, printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I impulse response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
impulse signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
inverse Fourier transform . . . . . . . . . . . . . . . . . . . . . .
L linear convolution . . . . . . . . . . . . . . . . . . . . . . . . . .
B bugs, reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
N number of samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C checklist for bug reports . . . . . . . . . . . . . . . . . . . . . . .
circular convolution . . . . . . . . . . . . . . . . . . . . . . . .
,
computer vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
convolution . . . . . . . . . . . . . . . . . . . . . . . . . . .
,
,
convolution sum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
convolution theorem . . . . . . . . . . . . . . . . . . . . . . .
,
convolution, circular . . . . . . . . . . . . . . . . . . . . . . . . . . .
convolution, linear . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
P parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
patches, contributing . . . . . . . . . . . . . . . . . . . . . . . . . .
periodic signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
periodic wavform, selecting . . . . . . . . . . . . . . . . . . . . .
point spread function . . . . . . . . . . . . . . . . . . . . . . . . . .
printing graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pulse signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pulses, selecting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D default values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
delta function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
,
discrete Fourier transform, see DFT . . . . . . . . . . . . .
Dissertation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
,
E erroneous data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Euler’s identity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
example run . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
S samples, number of . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
sampling rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
scaling the signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
signals, types of waveforms . . . . . . . . . . . . . . . . . . . . . .
splash screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
subroutine convo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
system impulse response . . . . . . . . . . . . . . . . . . . . . . .
F
Fast Fourier Transform, see FFT . . . . . . . . . . . . . . . .
FFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
,
,
Fourier coefficient. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fourier transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
frequency domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
frequency, selecting . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
T
Thesis, see Dissertation . . . . . . . . . . . . . . . . . . . . .
time domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V vision, computer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vision, robotic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
W waveform parameters . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project