found using ICM with and without parameter reesti-
mation, denoted ICM and ICM(P) respectively. The
fourth and fifth methods were MRF labellings found
using α−expansion with and without parameter rees-
timation, denoted α and α(P) respectively. The free
parameter in the MRF models was set to λ=6 (see
Eq.(7)) for all images.
Table 1 reports results when the number of labels
was assumed known (i.e., K was the same as in the
ground-truth). Table 2 reports results for the more
difficult (and more realistic) situation in which K was
unknown. The value of K was estimated based on the
BIC values obtained when EM was used to optimise
the parameters of the GMM. The number of colour
dyes used is often limited by production costs in the
type of textile production represented in the data set.
Therefore, very large values of K can be ruled out a
priori. Specifically, values of K from 1 to 10 were
considered.
Tables 1 and 2 show the errors evaluated us-
ing the method in Section 4 as well as the execu-
tion times. The images, their ground-truth, and seg-
mentations are shown in Tables 3 to 7. In every
case, α−expansion with parameter estimation gave
the lowest error. In many cases ICM with param-
eter estimation performed better than α−expansion
without parameter estimation. This suggests that
parameter reestimation is important for these meth-
ods. GMM gave the highest error. In every case,
α−expansion with parameter reestimation was the
slowest. ICM(P) was slower than ICM, α(P) was
slower than α and GMM was fastest. For each al-
gorithm, computational expense increases with K.
Printing was not always used to create the textiles.
In the textile image in Table 3, there are four differ-
ent colours of filling yarn (i.e., the yarn that runs hor-
izontally). However, the strong texture and uneven
appearance resulted in BIC estimating K as 7. The
graph plots the BIC value against K. Error bars denote
± a standard deviation (σ) estimated over 10 runs for
each value of K. Segmentation results using K = 4
and K
BIC
= 7 are shown. In both cases, α-expansion
with parameter reestimation is clearly superior.
It is interesting to note that although K was usu-
ally overestimated by BIC, the effect of this on the
segmentation error for α−expansion with parameter
reestimation was not always large. For example, it can
be seen that there is no large difference in error with
different K in Table 3. The α−expansion algorithm
can be understood as a competition between different
labels. In every expansion, α takes one value from
{1,2,...K} and makes some of the pixels become α
simutaneously. If the change is accepted, parameters
will be reestimated, a learning process. This helps
to balance the percentage of different labels. The
ground-truth for the textile image in Table 3 contains
four colours, while K
BIC
= 7, but after α−expansion
with re-estimation only 0.4% of pixels are assigned
labels 5, 6, or 7. Similar comments apply to the tex-
tile images in Table 5 and Table 6. Sometimes, larger
K can help to distinguish similar colors. For exam-
ple, in Table 6, K = 8 is not enough to distinguish
the brown and red colors due to the texture effects
and dye degradation, but K
BIC
= 9 could and gave a
smaller error rate.
In the fabric in Table 7, filling yarns were inserted
into the warp yarns to create the floral pattern. Here,
when K = 3, α(P) is superior to the other methods, but
when K
BIC
= 7, although α(P) is better than others, it
loses accuracy.
Tables 4 and 5 show less strongly-textured fab-
rics. Nevertheless, they are problematic. A small
cropped patch of the textile image in Table 5 is shown
magnified in Figure 4(b) where yellow and blue re-
gions overlap and create green. In order to avoid
gaps between different colour regions, two dyes may
be overlapped slightly, thus producing a third colour
and causing difficulty for colour segmentation. Fig-
ure 4 graphically illustrates this problem. Figure 4(c)
shows an annotated ground-truth. However, the α-
expansion method separates the overlapping area into
another colour region as shown in Figure 4(d).
6 CONCLUSIONS
In this paper, we have addressed colour separation
of textiles in order to recover printed designs and
woven patterns from archival image data. An eval-
uation method was proposed and used to compare
pixel labellings obtained using MRFs optimised using
α−expansion and ICM both with and without param-
eter reestimation. These algorithms were initialised
based on Gaussian mixture colour models and BIC
was used to select the number of distinct class labels.
For all the images tested, α−expansion with
parameter reestimation was the most accurate
and the most computationally expensive method.
α−expansion without parameter reestimation gave
similar performance to ICM with parameter reesti-
mation. BIC based on GMM usually overestimated
the number of class labels. However, this did not al-
ways result in α−expansion with parameter reestima-
tion obtaining a less accurate result. In fact, in one
case the accuracy was improved.
The pixel labellings produced suggest ways of
compactly representing image content in terms of
colour and shape. Future work will explore their use
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