# Clustering data for compression with PCA

If I have datapoints in a high dimensional space and want to find a (linear) subspace onto which a data-set projects well, I can use PCA and then discard less important dimensions of the new basis to get compressed datapoints. However, often the data can be projected onto lower dimensional spaces with much smaller error if one first separates them into a couple of classes and then performs PCA for each class individually. What kind of algorithm can find such clusters? Just clustering based on distance in the high dimensional space won't be very useful:

If I'd just cluster first based on distance in the high-dimensional space, I would arrive at the bad clustering. There are 5 clusters and the green and red clusters don't project very well onto a 2D subspace.

As a human looking at the data, I see however that if I separate the data as indicated, red and blue will project very well onto a plane each and green will project very well onto a line, so I can run PCA for each group individually and store the red data points with 2 values each and the gree ones with 1 value each (plus a 2bit index on each datapoint to label which group it belongs to) and get a very low error upon uncompressing.

How can I automate this clustering based on how well it will project onto as low-imensional subspaces as possible?

Something like minimize E = SumOverClusters(SumOverPoints(SquaredDist(projected_point, original_point)) * (number_dims_projected / number_dims_original)) + C * number_of_clusters

What technique is well suited to do that?

(edit: while the example shows a 3d space, I'm more interested in doing that in about 64dimensional spaces)

• In your example it seems it suffices to just choose a larger number of clusters. Are you familiar with the EM algorithm?
– D.W.
Jul 31, 2020 at 0:03
• Yes of course the error would be zero if each point was in it‘s own cluster so it should also be a goal to have as few clusters as possible (that‘s why i have C*number_of_clusters added to the value to be minimized). Since this is for compression, C is something related to the amount of data needed to describe the basis transformation of each cluster plus the datasize of the labels for each datapoint (the more clusters the more bits are required for the labels) Jul 31, 2020 at 1:19
• Have you considered ICA instead of PCA? Is LSH hashing an option? Does projection to lower space require big computational effort? If not, making some random projections is viable option?
– Evil
Jul 31, 2020 at 13:16

(Not really an answer, more like a suggestion)

Since it's almost surely NP-hard, we need some heuristics. The idea is, for each point, to build the cluster it's contained in:

def clustering(P):  # P is the set of points
for each p in P:
build a somewhat-optimal cluster C containing p
remove C from P


Now, for each point we will build a cluster, one dimension at a time. We will build vectors $$(v_1, \ldots, v_k)$$, until the objective doesn't improve:

def build_cluster(p, P, t):  # t is a projection distance threshold
S = ()  # current span
for k = 1, 2, ...:  # cluster dimension
sample m vectors v_1, ..., v_m
for each v_i:
compute the number of points in P within a distance t to p + span(S, v_i)
select the best such v_i
if adding v_i to S is not beneficial:
break
S = union(S, v_i)


Now, there is a lot of engineering which can be done, except for defining stop conditions. E.g. we can sample vectors, and then iteratively improve them (minimizing the loss, I.e. the distance to the span from vertices within distance t) (and we can also decrease t in the process of improvement). Also, maybe we should do something better than sampling random vectors since we are unlikely to hit the right one. We may try to compute some candidates based on our data.

Hope it helps.

• What do you mean by ‚remove C from P‘? Jul 31, 2020 at 14:10
• You generate clusters one by one. When you've generated one cluster, its vertices no longer participate.
– user114966
Jul 31, 2020 at 14:58
• But you say do this for each point. So i should not generate clusters for points that i already added to some other cluster? Then how do I cluster? Jul 31, 2020 at 15:19