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Low-rank matrix completion: AltMin, ISTA, FISTA

This example illustrates low-rank matrix completion via alternating projection, ISTA (PGM), and FISTA (FPGM), using the Julia language.

(This approach is related to "projection onto convex sets" (POCS) methods, but the term "POCS" would be a misnomer here because the rank constraint is not a convex set.)

History:

  • 2021-08-23 Julia 1.6.2
  • 2021-12-09 Julia 1.6.4 and use M not Ω
  • 2023-06-04 Julia 1.9.0 in Literate

This page comes from a single Julia file: lrmc-m.jl.

You can access the source code for such Julia documentation using the 'Edit on GitHub' link in the top right. You can view the corresponding notebook in nbviewer here: lrmc-m.ipynb, or open it in binder here: lrmc-m.ipynb.

Setup

Add the Julia packages used in this demo. Change false to true in the following code block if you are using any of the following packages for the first time.

if false
    import Pkg
    Pkg.add([
        "InteractiveUtils"
        "LaTeXStrings"
        "LinearAlgebra"
        "MIRTjim"
        "Plots"
        "Random"
        "Statistics"
    ])
end

Tell Julia to use the following packages. Run Pkg.add() in the preceding code block first, if needed.

using InteractiveUtils: versioninfo
using LinearAlgebra: svd, svdvals, rank, norm, Diagonal
using LaTeXStrings
using MIRTjim: jim, prompt
using Plots: default, gui, plot, savefig, scatter, scatter!, xlabel!, xticks!
using Plots.PlotMeasures: px
using Random: seed!
using Statistics: mean
default(markersize=7, markerstrokecolor=:auto, label = "",
 tickfontsize = 10, legendfontsize = 18, labelfontsize = 16, titlefontsize = 18,
)

The following line is helpful when running this jl-file as a script; this way it will prompt user to hit a key after each image is displayed.

isinteractive() && prompt(:prompt);
jim(:prompt, true)
true

Latent matrix

Make a matrix that has low rank

tmp = [
    zeros(1,20);
    0 1 0 0 0 0 1 0 0 0 1 1 1 1 0 1 1 1 1 0;
    0 1 0 0 0 0 1 0 0 0 0 1 0 0 1 0 0 1 0 0;
    0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 1 0 0;
    0 0 1 1 1 1 0 0 0 0 1 1 0 0 0 0 0 1 1 0;
    zeros(1,20)
]';
rank(tmp)

Xtrue = kron(10 .+ 80*tmp, ones(9,9))
rtrue = rank(Xtrue)
5

plots with consistent size

jim1 = (X ; kwargs...) -> jim(X; size = (600,300),
 leftmargin = 10px, rightmargin = 10px, kwargs...);

and consistent display range

jimc = (X ; kwargs...) -> jim1(X; clim=(0,100), kwargs...);

and with NRMSE label

nrmse = (Xh) -> round(norm(Xh - Xtrue) / norm(Xtrue) * 100, digits=1)
args = (xaxis = false, yaxis = false, colorbar = :none) # book
args = (;) # web
jime = (X; kwargs...) -> jimc(X; xlabel = "NRMSE = $(nrmse(X)) %",
 args..., kwargs...,
)
title = latexstring("\$\\mathbf{\\mathit{X}}\$ : Latent image")
pt = jimc(Xtrue; title, xlabel = " ", args...)
# savefig(pt, "mc_ap_x.pdf")
Example block output

Noisy / incomplete data

seed!(0)
M = rand(Float32, size(Xtrue)) .>= 0.75 # 75% missing
Y = M .* (Xtrue + randn(size(Xtrue)));

title = latexstring("\$\\mathbf{\\mathit{Y}}\$ : Corrupted image matrix\n(missing pixels set to 0)")
py = jime(Y ; title)
# savefig(py, "mc_ap_y.pdf")
Example block output

What is rank(Y) ??

  • A 5-9
  • B 10-49
  • C 50-59
  • D 60-70
  • E 71-200

rank(Y) svdvals(Y)

Show mask, count proportion of missing entries

frac_nonzero = count(M) / length(M)
title = latexstring("\$\\mathbf{\\mathit{M}}\$ : Locations of observed entries")
pm = jim1(M; title, args...,
    xlabel = "sampled fraction = $(round(frac_nonzero * 100, digits=1))%")
# savefig(pm, "mc_ap_m.pdf")
Example block output

Low-rank approximation

A simple low-rank approximation works poorly for missing data.

r = 5
U,s,V = svd(Y)
Xr = U[:,1:r] * Diagonal(s[1:r]) * V[:,1:r]'
title = latexstring("Rank $r approximation of data \$\\mathbf{\\mathit{Y}}\$")
pr = jime(Xr ; title)
# savefig(pr, "mc_ap_lr.pdf")
Example block output

Alternating projection

Alternating projection is an iterative method that alternates between projecting onto the set of rank-5 matrices and onto the set of matrices that match the data.

function projC(X, r::Int)
    U,s,V = svd(X)
    return U[:,1:r] * Diagonal(s[1:r]) * V[:,1:r]' # project onto "𝒞" 𝒞 U+1D49E
end;

function lrmc_alt(Y, r::Int, niter::Int)
    Xr = copy(Y)
    Xr[.!M] .= mean(Y[M]) # fill missing values with mean of other values
    @show nrmse(Xr)
    for iter in 1:niter
        Xr = projC(Xr, r) # project onto "𝒞" 𝒞 U+1D49E
        Xr[M] .= Y[M] # project onto "𝒟" 𝒟 U+1D49F
        if 0 == iter % 40
            @show nrmse(Xr)
        end
    end
    return Xr
end;

niter_alt = 400
r = 5
Xr = lrmc_alt(Y, r, niter_alt)
title = "Alternating Projection at $niter_alt iterations"
pa = jime(Xr ; title)
# savefig(pa, "mc_ap_400.pdf")
Example block output

What is rank(Xr) here ??

  • A 5-9
  • B 10-49
  • C 50-59
  • D 60-70
  • E 71-200

rank(Xr) svdvals(Xr)

Run one more projection step onto the set of rank-r matrices

Xfinal = projC(Xr, r)
pf = jime(Xfinal ; title="Alternating Projection at $niter_alt iterations")
# savefig(pf, "mc_ap_xh.pdf")
Example block output

What is rank(Xfinal) here ??

  • A 5-9
  • B 10-49
  • C 50-59
  • D 60-70
  • E 71-200

rank(Xfinal)

Plot singular values

sr = svdvals(Xr)
rankeff = s -> count(>(0.01*s[1]), s); # effective rank
ps = plot(title="singular values",
 xaxis=(L"k", (1, minimum(size(Y))), [1, rankeff(sr), minimum(size(Y))]),
 yaxis=(L"σ",), labelfontsize = 18,
 leftmargin = 15px, bottommargin = 20px, size = (600,350), widen = true,
)
scatter!(ps, svdvals(Y), color=:red, label="Y (data)", marker=:dtriangle)
scatter!(ps, svdvals(Xtrue), color=:blue, label="Xtrue", marker=:utriangle)
pa = deepcopy(ps)
scatter!(pa, sr, color=:green, label="Alt. Proj. output")

# savefig(pa, "mc_ap_sv.pdf")
Example block output
prompt()

Nuclear norm approach

Now we will try to recover the matrix using low-rank matrix completion with a nuclear-norm regularizer.

The optimization problem we will solve is:

\[\arg\min_{\mathbf{\mathit{X}}} \frac{1}{2} ‖ \mathbf{\mathit{M}} ⊙ (\mathbf{\mathit{X}} - \mathbf{\mathit{Y}}) ‖_{\mathrm{F}}^2 + \beta ‖ \mathbf{\mathit{X}} ‖_* \quad\quad (\text{NN-min})\]

  • $\mathbf{\mathit{Y}}$ is the zero-filled input data matrix
  • $\mathbf{\mathit{M}}$ is the binary sampling mask.

Define cost function for optimization problem

nucnorm = (X) -> sum(svdvals(X)) # nuclear norm
costfun1 = (X,beta) -> 0.5 * norm(M .* (X - Y))^2 + beta * nucnorm(X); # regularized cost

Q. The cost function above is (convex, strictly convex):

  • A: F,F
  • B: F,T
  • C: T,F
  • D: T,T

Define singular value soft thresholding (SVST) function

SVST = (X,beta) -> begin
    U,s,V = svd(X) # see below
    sthresh = max.(s .- beta, 0)
    return U * Diagonal(sthresh) * V'
end;

Q. Which svd is that?

  • A compact
  • B economy
  • C full
  • D none of these

U,s,V = svd(Y) @show size(s), size(U), size(V)

ISTA

The iterative soft-thresholding algorithm (ISTA) is an extension of gradient descent for (often convex) "composite" cost functions that look like $\min_x f(x) + g(x)$ where $f(x)$ is smooth and $g(x)$ is non-smooth.

ISTA is also known as the proximal gradient method (PGM).

ISTA algorithm for solving (NN-min):

  • Initialize $\mathbf{\mathit{X}}_0 = \mathbf{\mathit{Y}}$ (zero-fill missing entries)

  • for k=0,1,2,...

  • $[\mathbf{\mathit{X}}_k]_{i,j} = \begin{cases}[\mathbf{\mathit{X}}_k]_{i,j} & \text{if } (i,j) ∉ Ω \\ [\mathbf{\mathit{Y}}]_{i,j} & \text{if } (i,j) ∈ Ω \end{cases}$ (Put back in known entries)

  • $\mathbf{\mathit{X}}_{k+1} = \text{SVST}(\mathbf{\mathit{X}}_k, \beta)$ (Singular value soft-thresholding)

  • end

ISTA for matrix completion, using functions SVST and costfun1

function lrmc_ista(Y, M, beta::Real, niter::Int)
    X = copy(Y)
    Xold = copy(X)
    cost = zeros(niter+1)
    cost[1] = costfun1(X, beta)
    for k in 1:niter
        @. X[M] = Y[M] # in place
        X = SVST(X, beta)
        cost[k+1] = costfun1(X, beta)
    end
    return X, cost
end;

Apply ISTA

niter = 1000
beta = 0.8 # chosen by trial-and-error here
xh_ista, cost_ista = lrmc_ista(Y, M, beta, niter)
pp = jime(xh_ista ; title="ISTA result at $niter iterations")

# savefig(pp, "mc-nuc-ista.pdf")
Example block output

That result is not good. What went wrong? Let's investigate. First, check if the ISTA solution is actually low-rank.

sp = svdvals(xh_ista)
psi = deepcopy(ps)
scatter!(psi, sp, color=:orange, label=L"\hat{X} \mathrm{(ISTA)}")
xticks!(psi, [1, rtrue, rank(Diagonal(sp)), minimum(size(Y))])
Example block output
prompt()

Now check the cost function. It is decreasing monotonically, but quite slowly.

scatter(cost_ista, color=:orange,
    title="cost vs. iteration",
    xlabel="iteration",
    ylabel="cost function value",
    label="ISTA")
Example block output
prompt()

FISTA

The fast iterative soft-thresholding algorithm (FISTA) is a modification of ISTA that includes Nesterov acceleration for much faster convergence. Also known as the fast proximal gradient method (FPGM).

Reference:

FISTA algorithm for solving (NN-min)

  • initialize matrices $\mathbf Z_0 = \mathbf X_0 = \mathbf Y$

  • for k=0,1,2,...

  • $[\mathbf{Z}_k]_{i,j} = \begin{cases}[\mathbf Z_k]_{i,j} & \text{if}~(i,j) ∉ Ω \\ [\mathbf{Y}]_{i,j} & \text{if}~(i,j) ∈ Ω \end{cases}$ (Put back in known entries)

  • $\mathbf{X}_{k+1} = \text{SVST}(\mathbf{Z}_k, \beta)$

  • $t_{k+1} = \frac{1 + \sqrt{1+4t_k^2}}{2}$ (Nesterov step-size)

  • $\mathbf{Z}_{k+1} = \mathbf{X}_{k+1} + \frac{t_k-1}{t_{k+1}} (\mathbf{X}_{k+1} - \mathbf{X}_k)$ (Momentum update)

  • end

FISTA algorithm for low-rank matrix completion, using SVST and costfun1

function lrmc_fista(Y, M, beta::Real, niter::Int)
    X = copy(Y)
    Z = copy(X)
    Xold = copy(X)
    told = 1
    cost = zeros(niter+1)
    cost[1] = costfun1(X, beta)
    for k in 1:niter
        @. Z[M] = Y[M]
        X = SVST(Z, beta)
        t = (1 + sqrt(1+4*told^2))/2
        Z = X + ((told-1)/t) * (X - Xold)
        Xold = copy(X)
        told = t
        cost[k+1] = costfun1(X, beta) # comment out to speed-up
    end
    return X, cost
end;

Run FISTA

niter = 300
xh_nn_fista, cost_fista = lrmc_fista(Y, M, beta, niter)
p1 = jime(xh_nn_fista ; title="FISTA with nuclear norm at $niter iterations")

# savefig(p1, "lrmc-nn-fs300.pdf")
Example block output

Plot showing that FISTA converges much faster! POGM would be even faster.

plot(title="cost vs. iteration for NN regularizer",
    xlabel="iteration", ylabel="cost function value")
scatter!(cost_ista, color=:orange, label="ISTA")
scatter!(cost_fista, color=:magenta, label="FISTA")
Example block output
prompt()

See if the FISTA result is "low rank"

sf = svdvals(xh_nn_fista)
rfista = rank(Diagonal(sf))
rfista, rankeff(sf)
(30, 7)
psf = deepcopy(ps)
scatter!(psf, sf, color=:magenta, label="Xh (output of FISTA)")
xticks!(psf, [1, rtrue, rfista, minimum(size(Y))])
Example block output
prompt()
  • Optional exercise: think about why $σ_1(Y) < σ_1(\hat{X}) < σ_1(Xtrue)$
  • Optional: try ADMM too

Your work goes below here

The results below are place-holders that will be much improved when implemented properly.

if true # replace these place-holder functions with your work
    shrink_p_1_2(v, reg::Real) = v
    lr_schatten(Y, reg::Real) = Y
    fista_schatten(Y, M, reg::Real, niter::Int) = Y
else # instructor version
    mydir = ENV["hw551test"] # change path
    include(mydir * "shrink_p_1_2.jl") # 1D shrinker for |x|^(1/2), previous HW
    include(mydir * "lr_schatten.jl")
    include(mydir * "fista_schatten.jl")
end;

Apply FISTA for Schatten p=1/2

niter = 150
reg_fs = 120
xh_fs = fista_schatten(Y, M, reg_fs, niter)

p2 = jime(xh_fs; title="FISTA for Schatten p=1/2, $niter iterations")
# savefig("schatten_complete_fs150_sp.pdf")
Example block output

See if the Schatten FISTA result is "low rank"

ss = svdvals(xh_fs)
rank_schatten_fista = rank(Diagonal(ss))
rank_schatten_fista, rankeff(ss)

pss = deepcopy(ps)
scatter!(pss, ss, color=:cyan, label="Xh (FISTA for Schatten)")
xticks!(pss, [1, rank_schatten_fista, minimum(size(Y))])
Example block output
prompt()

error image for nuclear norm

p3 = jimc(xh_nn_fista - Xtrue; title = "FISTA Nuclear Norm: Xh-X", clim=(-80,80))
# savefig("schatten_complete_fs300_nn_err.pdf")
Example block output

error image for schatten p=1/2

p4 = jimc(xh_fs - Xtrue; title = "FISTA Schatten p=1/2 'Norm': Xh-X", clim=(-80,80))
# savefig("schatten_complete_fs150_sp_err.pdf")
Example block output

Reproducibility

This page was generated with the following version of Julia:

using InteractiveUtils: versioninfo
io = IOBuffer(); versioninfo(io); split(String(take!(io)), '\n')
12-element Vector{SubString{String}}:
 "Julia Version 1.10.1"
 "Commit 7790d6f0641 (2024-02-13 20:41 UTC)"
 "Build Info:"
 "  Official https://julialang.org/ release"
 "Platform Info:"
 "  OS: Linux (x86_64-linux-gnu)"
 "  CPU: 4 × AMD EPYC 7763 64-Core Processor"
 "  WORD_SIZE: 64"
 "  LIBM: libopenlibm"
 "  LLVM: libLLVM-15.0.7 (ORCJIT, znver3)"
 "Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)"
 ""

And with the following package versions

import Pkg; Pkg.status()
Status `~/work/book-la-demo/book-la-demo/docs/Project.toml`
  [6e4b80f9] BenchmarkTools v1.5.0
  [aaaa29a8] Clustering v0.15.7
  [35d6a980] ColorSchemes v3.24.0
  [3da002f7] ColorTypes v0.11.4
⌅ [c3611d14] ColorVectorSpace v0.9.10
  [717857b8] DSP v0.7.9
  [72c85766] Demos v0.1.0 `~/work/book-la-demo/book-la-demo`
  [e30172f5] Documenter v1.2.1
  [4f61f5a4] FFTViews v0.3.2
  [7a1cc6ca] FFTW v1.8.0
  [587475ba] Flux v0.14.12
⌅ [a09fc81d] ImageCore v0.9.4
  [71a99df6] ImagePhantoms v0.7.2
  [b964fa9f] LaTeXStrings v1.3.1
  [7031d0ef] LazyGrids v0.5.0
  [599c1a8e] LinearMapsAA v0.11.0
  [98b081ad] Literate v2.16.1
  [7035ae7a] MIRT v0.17.0
  [170b2178] MIRTjim v0.23.0
  [eb30cadb] MLDatasets v0.7.14
  [efe261a4] NFFT v0.13.3
  [6ef6ca0d] NMF v1.0.2
  [15e1cf62] NPZ v0.4.3
  [0b1bfda6] OneHotArrays v0.2.5
  [429524aa] Optim v1.9.2
  [91a5bcdd] Plots v1.40.1
  [f27b6e38] Polynomials v4.0.6
  [2913bbd2] StatsBase v0.34.2
  [d6d074c3] VideoIO v1.0.9
  [b77e0a4c] InteractiveUtils
  [37e2e46d] LinearAlgebra
  [44cfe95a] Pkg v1.10.0
  [9a3f8284] Random
Info Packages marked with ⌅ have new versions available but compatibility constraints restrict them from upgrading. To see why use `status --outdated`

This page was generated using Literate.jl.