Skip to content Skip to navigation Skip to collection information

Connexions

You are here: Home » Content » Automatic Generation of Prime Length FFT Programs » Appendix: A Matlab Program for Generating Prime Length FFT Programs

Navigation

Lenses

What is a lens?

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

This content is ...

Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
  • Rice Digital Scholarship

    This collection is included in aLens by: Digital Scholarship at Rice University

    Click the "Rice Digital Scholarship" link to see all content affiliated with them.

  • Featured Content display tagshide tags

    This collection is included inLens: Connexions Featured Content
    By: Connexions

    Click the "Featured Content" link to see all content affiliated with them.

    Click the tag icon tag icon to display tags associated with this content.

Also in these lenses

  • UniqU content

    This collection is included inLens: UniqU's lens
    By: UniqU, LLC

    Click the "UniqU content" link to see all content selected in this lens.

  • Lens for Engineering

    This module and collection are included inLens: Lens for Engineering
    By: Sidney Burrus

    Click the "Lens for Engineering" link to see all content selected in this lens.

Recently Viewed

This feature requires Javascript to be enabled.

Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.
 

Appendix: A Matlab Program for Generating Prime Length FFT Programs

Module by: Ivan Selesnick, C. Sidney Burrus. E-mail the authors


 function [u,ip,op,ADDS,MULTS] = ff(p,e);
% [u,ip,op,ADDS,MULTS] = ff(p,e);
% u  : multiplicative constants
% ip : input permutation
% op : output permutation

K = length(p);
N = prod(p.^e);
P = N + 1;
[pr, ipr] = primitive_root(P);
Red_Adds = 2 * N * (K - sum(1./(p.^e)) );
ADDS = 2 * Red_Adds;

FS = sprintf('fft%d.m',P);
fid = fopen(FS,'w');

fprintf(fid,'function y = fft%d(x,u,ip,op)\n',P);
fprintf(fid,'%% y = fft%d(x,u,ip,op)\n',P);
fprintf(fid,'%% y  : the %d point DFT of x \n',P);
fprintf(fid,'%% u  : a vector of precomputed multiplicative constants\n');
fprintf(fid,'%% ip : input permutation\n');
fprintf(fid,'%% op : ouput permutation\n');

Pstr = sprintf('[%d',p(1));
for k = 2:K, Pstr = [Pstr, sprintf(',%d',p(k))]; end
Pstr = [Pstr,']'];
Estr = sprintf('[%d',e(1));
for k = 2:K, Estr = [Estr, sprintf(',%d',e(k))]; end
Estr = [Estr,']'];
PEstr = sprintf('[%d',p(1)^e(1));
for k = 2:K, PEstr = [PEstr, sprintf(',%d',p(k)^e(k))]; end
PEstr = [PEstr,']'];

fprintf(fid,'\n');
S = sprintf('y = zeros(%d,1);\n',P);
fprintf(fid,S);
S1 = sprintf('x = x(ip);');
S2 = sprintf('%% input permutation\n');
fprintf(fid,'%-50s%s',S1,S2);
S1 = sprintf(['x(2:%d) = KRED(',Pstr,',',Estr,',%d,x(2:%d));'],P,K,P);
S2 = sprintf('%% reduction operations\n');
fprintf(fid,'%-50s%s',S1,S2);

e_table = [0:e(1)]';
a = e(1)+1;
for i = 2:K
   e_table = [kron(ones(e(i)+1,1),e_table), kron([0:e(i)]',ones(a,1))];
   a = a * (e(i)+1);
end
R = prod(e+1);


% ------------------------ MULTIPLICATIVE CONSTANTS ------------------------
k = rp(P,ipr,0:N);
I = sqrt(-1);
W = exp(-I*2*pi*k/P);
h = W(2:P);
h = h(N:-1:1);
h = pfp(p.^e,K,h);
h = itKRED(p,e,K,h);
u = h(1);

S1 = sprintf('y(1) = x(1)+x(2);');
S2 = sprintf('%% DC term calculation\n');
fprintf(fid,'%-50s%s',S1,S2);

DC_ADDS = 2;
ADDS = ADDS + DC_ADDS;

SLINE = '--------------------------------------------------------------------------------';
SB = ' block : 1 ';
SC = SLINE;
BL = 21;
SC(BL:BL-1+length(SB)) = SB;
fprintf(fid,'%% %s\n',SC);
S1 = sprintf('y(2) = x(2)*u(1);');
fprintf(fid,'%-40s\n',S1);
a = 1;
MULTS = 1;
for i = 2:R
   v = e_table(i,:);
   f = find(v>0);
   q = p(f);
   t = v(f);
   L = prod(q-1)*prod(q.^(t-1));

   B = prod(q.^t);
   bs = sprintf('%d',q(1)^t(1));
   for k = 2:length(q), bs = [bs, sprintf(' * %d',q(k)^t(k))]; end
   if length(q) > 1
	SB = sprintf(' block : %d = %s ',B,bs);
	SC = SLINE;
	SC(BL:BL-1+length(SB)) = SB;
	fprintf(fid,'%% %s\n',SC);
   else
	SB = sprintf(' block : %d ',B);
        SC = SLINE;
        SC(BL:BL-1+length(SB)) = SB;
        fprintf(fid,'%% %s\n',SC);
   end
   if prod(q.^t) == 2
      S1 = sprintf('y(%d) = x(%d)*u(%d);',a+2,a+2,MULTS+1);
      fprintf(fid,'%-40s\n',S1);
      Mk = 1;
   else
      d = []; r = []; c = []; Q = []; Qt = [];
      for j = 1:length(q)
         [dk,rk,ck,Qk,Qtk] = A_data(q(j)^t(j));
         if dk > 1
            d = [d dk]; r = [r rk]; c = [c ck]; Q = [Q Qk]; Qt = [Qt Qtk];
         end
      end
      [g,C1] = cgc(Q,r,c,length(Q));
      ADDS = ADDS + C1;
      Mk = prod(r);
      BEG = int2str(a+2); FIN = int2str(a+1+L);
      XX = ['x(',BEG,':',FIN,')']; YY = 'v';
      kpi(d,g,r,c,length(Q),YY,XX,fid);
      S1 = ['v = v.*u(',int2str(MULTS+1),':',int2str(MULTS+Mk),');'];
      fprintf(fid,'%-40s\n',S1);
      [g,C2] = cgc(Qt,c,r,length(Q));
      ADDS = ADDS + C2;
      XX = 'v'; YY = ['y(',BEG,':',FIN,')'];
      kpit(d,g,c,r,length(Q),YY,XX,fid);
   end

   c = [];
   r = [];
   lq = length(q);
   for j = 1:lq
      [fk,rk,ck] = C_data(q(j),t(j));
      r = [r rk]; c = [c ck];
   end
   f = (q-1).*(q.^(t-1));
   temp = Kcrot(q,t,lq,h(a+1:a+L));
   temp = KFt(f,r,c,temp);
   u = [u; temp(:)];
   a = a + L;
   MULTS = MULTS + Mk;
end

u(1) = u(1)-1;
fprintf(fid,'%% %s\n',SLINE);
S1 = sprintf('y(2) = y(1)+y(2);');
S2 = sprintf('%% DC term calculation\n');
fprintf(fid,'%-50s%s',S1,S2);
S1 = sprintf(['y(2:%d) = tKRED(',Pstr,',',Estr,',%d,y(2:%d));'],P,K,P);
S2 = sprintf('%% transpose reduction operations\n');
fprintf(fid,'%-50s%s',S1,S2);
S1 = sprintf('y = y(op);');
S2 = sprintf('%% output permutation\n');
fprintf(fid,'%-50s%s',S1,S2);
fprintf(fid,'\n');

MULTS = 2 * MULTS;
ADDS = 2* ADDS;
fprintf(fid,'%% For complex data - \n');
fprintf(fid,'%% Total Number of Real Multiplications : %d\n',MULTS);
fprintf(fid,'%% Total Number of Real Additions: %d\n\n',ADDS);
fclose(fid);

%%%%%%%%%%%%%%%%%%%% COMPUTE INPUT AND OUTPUT PERMUTATIONS %%%%%%%%%%%%%%%%%%%%%%%%%%%%

id = 1:P;   % identity permutation
ip = rp(P,pr,id);
ip(2:P) = pfp(p.^e,K,ip(2:P));

op = id;
op(2:P) = pfpt(p.^e,K,op(2:P));
op(2:P) = op(P:-1:2);
op = rpt(P,ipr,op);

%%%%%%%%%%%%%%%%% PUT MULTIPLICATIVE CONSTANTS AND PERMUTATIONS IN A FILE %%%%%%%%%%%%%%

CFS = sprintf('cap%d.m',P);
fid = fopen(CFS,'w');
fprintf(fid,'\n%% The multiplicative constants for the %d point FFT\n\n',P);
fprintf(fid,'I = sqrt(-1);\n');
fprintf(fid,'u = [\n');
for k = 1:MULTS/2
   if abs(real(u(k))) < 0.000001
      fprintf(fid,'%25.15f*I\n',imag(u(k)));
   elseif abs(imag(u(k))) < 0.00001
      fprintf(fid,'%25.15f\n',real(u(k)));
   else
      fprintf(fid,'%25.15f + %25.15f*I\n',real(u(k)),imag(u(k)));
   end
end
fprintf(fid,'];\n\n');
fprintf(fid,'\n%% The input permutation for the %d point FFT\n\n',P);
fprintf(fid,'ip = [\n');
for k = 1:P
	fprintf(fid,'   %d\n',ip(k));
end
fprintf(fid,'];\n\n');
fprintf(fid,'\n%% The output permutation for the %d point FFT\n\n',P);
fprintf(fid,'op = [\n');
for k = 1:P
        fprintf(fid,'   %d\n',op(k));
end
fprintf(fid,'];\n\n');
fclose(fid);

The following programs print the program statements that carry out the operation IDkIIDkI and IDktIIDktI. They are modeled after kpi in the text.

function kpi(d,g,r,c,n,Y,X,fid)
% kpi(d,g,r,c,n,Y,X,fid);
% Kronecker Product : A(d(1)) kron ... kron A(d(n))
% g : permutation of 1,...,n 
% r : [r(1),...,r(n)]
% c : [c(1),..,c(n)]
% r(i) : rows of A(d(i))
% c(i) : columns of A(d(i))
% n : number of terms
for i = 1:n
   a = 1;
   for k = 1:(g(i)-1)
      if i > find(g==k) 
         a = a * r(k);
      else
         a = a * c(k);
      end
   end
   b = 1;
   for k = (g(i)+1):n
      if i > find(g==k)
         b = b * r(k);
      else
         b = b * c(k);
      end
   end
   % Y = (I(a) kron A(d(g(i))) kron I(b)) * X;
   if i == 1
      S1 = sprintf([Y,' = ID%dI(%d,%d,',X,');    '],d(g(i)),a,b);
      S2 = sprintf(['%% ',Y,' = (I(%d) kron D%d kron I(%d)) * ',X],a,d(g(i)),b);
      fprintf(fid,'%-35s%s\n',S1,S2);
   elseif d(g(i)) ~= 1
      S1 = sprintf([Y,' = ID%dI(%d,%d,',Y,');    '],d(g(i)),a,b);
      S2 = sprintf(['%% ',Y,' = (I(%d) kron D%d kron I(%d)) * ',Y],a,d(g(i)),b);
      fprintf(fid,'%-35s%s\n',S1,S2);
   end
end
function kpit(d,g,r,c,n,Y,X,fid)
% kpit(g,r,c,n,Y,X,fid);
% (transpose)
% Kronecker Product : A(d(1))' kron ... kron A(d(n))'
% g : permutation of 1,...,n 
% r : [r(1),...,r(n)]
% c : [c(1),..,c(n)]
% r(i) : rows of A(d(i))'
% c(i) : columns of A(d(i))'
% n : number of terms

for i = 1:n
   a = 1;
   for k = 1:(g(i)-1)
      if i > find(g==k)
         a = a * r(k);
      else
         a = a * c(k);
      end
   end
   b = 1;
   for k = (g(i)+1):n
      if i > find(g==k)
         b = b * r(k);
      else
         b = b * c(k);
      end
   end
   % x = (I(a) kron A(d(g(i)))'' kron I(b)) * x;
   if i == n
      S1 = sprintf([Y,' = ID%dtI(%d,%d,',X,');    '],d(g(i)),a,b);
      S2 = sprintf(['%% ',Y,' = (I(%d) kron D%d'' kron I(%d)) * ',X],a,d(g(i)),b);
      fprintf(fid,'%-35s%s\n',S1,S2);
   elseif d(g(i)) ~= 1
      S1 = sprintf([X,' = ID%dtI(%d,%d,',X,');    '],d(g(i)),a,b);
      S2 = sprintf(['%% ',X,' = (I(%d) kron D%d'' kron I(%d)) * ',X],a,d(g(i)),b);
      fprintf(fid,'%-35s%s\n',S1,S2);
   end
end

Programs for Computing Multiplicative Constants

The following programs carry out the operation of Fd1FdKFd1FdK where FF is the reconstruction matrix in a linear convolution algorithm. See the appendix, `Bilinear Forms for Linear Convolution.'

function u = KFt(f,r,c,u)
% u = (F^t kron ... kron F^t)*u
% (transpose)
% f = [f(1),...,f(K)]
% r : r(i) = rows of F(i)
% c : c(i) = columns of F(i)
% u : length(u) = prod(c);
K = length(f);
for i = 1:K
   m = prod(c(1:i-1));
   n = prod(r(i+1:K));
   u = IFtI(f(i),r(i),c(i),m,n,u);
end
function y = IFtI(s,r,c,m,n,x);
% y = (I(m) kron F(s)^t kron I(n))*x
% (transpose)
% r : rows of F(s)
% c : columns of F(s)
v = 0:n:n*(c-1);
u = 0:n:n*(r-1);
for i = 0:m-1
   for j = 0:n-1
      y(v+i*c*n+j+1) = Ftop(s,x(u+i*r*n+j+1));
   end
end
function y = Ftop(k,x)
if k == 1, y = x;
elseif k == 2, y = F2t(x);
elseif k == 3, y = F3t(x);
elseif k == 4, y = F4t(x);
elseif k == 6, y = F6t(x);
elseif k == 8, y = F8t(x);
elseif k == 18, y = F18t(x);
end

The following programs carry out the operation of Gp1e1GpKeKGp1e1GpKeK were GG is given by Equation 13 and Equation 14 from Bilinear Forms for Circular Convolution.

function x = Kcrot(p,e,K,x)
% Kronecker product of Cyclotomic Reduction Operations.
% x = (G(p(1)^e(1)) kron ... kron G(p(K)^(K)))^t*x
% (transpose)
% p : p = [p(1),...,p(K)];
% e : e = [e(1),...,e(K)];
a = (p-1).*((p).^(e-1));
r = a;		% r(i) = number of rows of G(i)
c = 2*a-1;	% c(i) = number of columns of G(i)
m = 1;
n = prod(r);
for i = 1:K
   n = n / r(i);
   x = IcrotI(p(i),e(i),m,n,x);
   m = m * c(i);
end
function y = IcrotI(p,e,m,n,x)
%  y = (eye(m) kron G(p^e)^t kron eye(n))*x
%  (transpose)
a = (p-1)*(p^(e-1));
c = a;
r = 2*a-1;
y = zeros(r*m*n,1);
v = 0:n:(r-1)*n;
u = 0:n:(c-1)*n;
for i = 0:m-1
   for j = 0:n-1
      y(v+i*r*n+j+1) = crot(p,e,x(u+i*c*n+j+1));
   end
end
function y = crot(p,e,x)
% y = crot(p,x)
% cyclotomic reduction matrix (transpose)
% length(x) == 2*n-1
% length(y) == n
% where n = (p-1)*(p^(e-1))
n = (p-1)*(p^(e-1));
y = zeros(2*n-1,1); 
if p == 2
  n = p^(e-1);
  y(1:n) = x;
  y(n+1:2*n-1) = -x(1:n-1);
else
  y(1:n) = x;
  L = p^(e-1);
  y(n+1:n+L) = -x(1:L);
  a = L;
  for k = 2:p-1
     y(n+1:n+L) = y(n+1:n+L) - x(a+1:a+L);
     a = a + L;
  end
  b = 2*n-1 - p*(p^(e-1));
  y(p*L+1:p*L+b) = x(1:b);
end

The following programs tell the programs for code generation relevant information about the bilinear forms for cyclotomic convolution. Specifically, they indicates the linear convolution out of which these cyclotomic convolution are composed, and the dimensions of the corresponding matrices. See the appendix Bilinear Forms for Linear Convolution.

function [d,r,c,Q,Qt] = A_data(n)
% A : A matrix in bilinear form for cyclotomic convolution
% d : linear convolution modules used
% r : rows
% c : columns
% Q : Q(i) = cost associated with D(d(i))
% Qt : Qt(i) = cost associated with D(d(i))'
if n == 2, d = [1];
elseif n == 4, d = [2];
elseif n == 8, d = [2 2];
elseif n == 16, d = [2 2 2];
elseif n == 3, d = [2];
elseif n == 9, d = [2 3];
elseif n == 27, d = [2 3 3];
elseif n == 5, d = [2 2];
elseif n == 7, d = [2 3];
end
r = []; c = []; Q = []; Qt = [];
for k = 1:length(d)
   [rk, ck, Qk, Qtk] = D_data(d(k));
   r = [r rk]; c = [c ck]; Q = [Q Qk]; Qt = [Qt Qtk];
end
function [r,c,Q,Qt] = D_data(d);
% D : D matrix in bilinear form for linear convolution
% r : rows
% c : columns
% Q : cost associated with D(d)
% Qt : cost associated with D(d)'
if d == 1, r = 1; c = 1; Q = 0; Qt = 0;
elseif d == 2, r = 3; c = 2; Q = 1; Qt = 2;
elseif d == 3, r = 5; c = 3; Q = 7; Qt = 9;
end
function [f,r,c] = C_data(p,e)
% f : length of linear convolution
% r : rows
% c : columns
f = prod((p-1).*(p.^(e-1)));
% (Euler Totient Function)
r = 2*f-1;
c = F_data(f);
function c = F_data(n)
% c : columns of F matrix
if n == 1, c = 1;
elseif n == 2, c = 3;
elseif n == 4, c = 9;
elseif n == 8, c = 27;
elseif n == 3, c = 5;
elseif n == 6, c = 15;
elseif n == 18, c = 75;
end

Programs for Inverse Transpose Reduction Operations

function x = itKRED(P,E,K,x)
% x = itKRED(P,E,K,x);
% (inverse transpose)
% P : P = [P(1),...,P(K)];
% E : E = [E(K),...,E(K)];
for i = 1:K
   a = prod(P(1:i-1).^E(1:i-1));
   c = prod(P(i+1:K).^E(i+1:K));
   p = P(i);
   e = E(i);
   for j = e-1:-1:0
      x(1:a*c*(p^(j+1))) = itRED(p,a,c*(p^j),x(1:a*c*(p^(j+1))));
   end
end
function y = itRED(p,a,c,x)
% y = itRED(p,a,c,x);
% (inverse transpose)
y = zeros(a*c*p,1);
for i = 0:c:(a-1)*c
   for j = 0:c-1
      A = x(i*p+j+1);
      for k = 0:c:c*(p-2)
         A = A + x(i*p+j+k+c+1);
      end
      y(i+j+1) = A;
      for k = 0:c:c*(p-2)
         y(i*(p-1)+j+k+a*c+1) = p*x(i*p+j+k+1) - A;
      end
   end
end
y = y/p;

Programs for Permutations

The permutation of Equation 18 from Preliminaries is implemented by pfp . It calls the function pfp2I . The transpose is implemented by pfpt and it calls pfpt2I .

function x = pfp(n,K,x)
% x = P(n(1),...,n(K)) * x
% n = [n(1),...,n(K)];
% length(x) = prod(n(1),...,n(K))
a = prod(n);
s = 1;
for i = K:-1:2
  a = a / n(i);
  x = pfp2I(a,n(i),s,x);
  s = s * n(i);
end
function y = pfp2I(a,b,s,x)
% y = kron(P(a,b),I(s)) * x;
% length(x) = a*b*s
n = a * b;
y = zeros(n*s,1);
k1 = 0;
k2 = 0;
for k = 0:n-1
  i1 = s * (k1 + b * k2);
  i2 = s * k;
  for i = 1:s
    y(i1 + i) = x(i2 + i);
  end
  k1 = k1 + 1;
  k2 = k2 + 1;
  if k1 >= b
    k1 = k1 - b;
  end
  if k2 >= a
    k2 = k2 - a;
  end
end
function x = pfpt(n,K,x)
% x = P(n(1),...,n(K))' * x
% (tanspose)
% n = [n(1),...,n(K)];
% length(x) = prod(n(1),...,n(K))
% a = prod(n);
a = n(1);
s = prod(n(2:K));
for i = 2:K
  s = s / n(i);
  x = pfpt2I(a,n(i),s,x);
  a = a * n(i);
end
function y = pfpt2I(a,b,s,x)
% y = P(a,b)' kron I(s) * x;
% (transpose)
% length(x) = a*b*s
n = a * b;
y = zeros(n*s,1);
k1 = 0;
k2 = 0;
for k = 0:n-1
  i1 = s * (k1 + b * k2);
  i2 = s * k;
  for i = 1:s
    y(i2 + i) = x(i1 + i);
  end
  k1 = k1 + 1;
  k2 = k2 + 1;
  if k1 >= b
    k1 = k1 - b;
  end
  if k2 >= a
    k2 = k2 - a;
  end
end

The following Matlab programs implement Rader's permutation and its transpose. They require the primitive root to be passed to them as an argument.

function y = rp(p,r,x)
% Rader's Permutation
% p : prime
% r : a primitive root of p
% x : length(x) == p
a = 1;
y = zeros(p,1);
y(1) = x(1);
for k = 2:p
   y(k) = x(a+1);
   a = rem(a*r,p);
end
function y = rpt(p,r,x)
% Rader's Permutation
% (transpose)
% p : prime
% r : a primitive root of p
% x : length(x) == p
a = 1;
y = zeros(p,1);
y(1) = x(1);
for k = 2:p
   y(a+1) = x(k);
   a = rem(a*r,p);
end
function [R, R_inv] = primitive_root(N)

%  function [R, R_inv] = primitive_root(N)
%  Ivan Selesnick
%  N is assumed to be prime.  This function returns R,
%  the smallest	primitive root of N, and R_inv, the 
%  inverse of R modulo N.

R = 'Not Found';
m = 0:(N-2);
for x = 1:(N-1)
   if ( 1:(N-1) == sort(rem2(x,m,N)) )
      R = x;
      break
   end
end
R_inv = 'Not Found';
for x = 1:N
   if rem(x*R,N) == 1
      R_inv = x;
      break
   end
end

References

    Collection Navigation

    Content actions

    Download:

    Collection as:

    PDF | EPUB (?)

    What is an EPUB file?

    EPUB is an electronic book format that can be read on a variety of mobile devices.

    Downloading to a reading device

    For detailed instructions on how to download this content's EPUB to your specific device, click the "(?)" link.

    | More downloads ...

    Module as:

    PDF | EPUB (?)

    What is an EPUB file?

    EPUB is an electronic book format that can be read on a variety of mobile devices.

    Downloading to a reading device

    For detailed instructions on how to download this content's EPUB to your specific device, click the "(?)" link.

    | More downloads ...

    Add:

    Collection to:

    My Favorites (?)

    'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

    | A lens I own (?)

    Definition of a lens

    Lenses

    A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

    What is in a lens?

    Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

    Who can create a lens?

    Any individual member, a community, or a respected organization.

    What are tags? tag icon

    Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

    | External bookmarks

    Module to:

    My Favorites (?)

    'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

    | A lens I own (?)

    Definition of a lens

    Lenses

    A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

    What is in a lens?

    Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

    Who can create a lens?

    Any individual member, a community, or a respected organization.

    What are tags? tag icon

    Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

    | External bookmarks