MISREGISTRATION ERRORS IN CHANGE

MISREGISTRATION ERRORS IN CHANGE
MISREGISTRATION ERRORS IN CHANGE DETECTION ALGORITHMS
AND HOW TO AVOID THEM
Dirk Farin1 and Peter H. N. de With1,2
1
Univ. of Technol. Eindhoven, PO Box 513
5600 MB Eindhoven, Netherlands
[email protected]
ABSTRACT
Background subtraction is a popular algorithm for video object segmentation. It identifies foreground objects by comparing the input images with a pure background image. In
camera-motion compensated sequences, small errors in the
motion estimation can lead to large image differences along
sharp edges. Consequently, the errors in the image registration finally lead to segmentation errors. This paper proposes a computationally efficient approach to detect image
areas having a high risk of showing misregistration errors.
Furthermore, we describe how existing change detection algorithms can be modified to avoid segmentation errors in
these areas. Experiments show that our algorithm can improve the segmentation quality. The algorithm is memory
efficient and suitable for real-time processing.
1. INTRODUCTION
Video object segmentation is a prerequisite for a number
of applications, such as surveillance, intelligent video databases, or object-oriented video coding. A popular approach
within video object segmentation is the background subtraction algorithm. This algorithm assumes that a picture of
the scene background is available, which does not show any
foreground objects. The background subtraction algorithm
compares the current input image with the background image and it detects foreground objects at places where the
differences between both images are large.
From an algorithmic point of view, we can partition the
applications into two classes, based on the typical type of
video that is observed. The first class contains videos that
are recorded with a static camera. In this case, the background image can be recorded explicitly, or it can be estimated by some temporal averaging process. The second
class covers videos with a (rotationally) moving camera.
Hence, the camera-motion has to be compensated prior to
the background subtraction process.
For videos with a moving camera, the background image is usually generated by estimating the camera-motion
and aligning all input frames into a large background sprite
2
LogicaCMG, PO Box 7089
5605 JB Eindhoven, Netherlands
[email protected]
image [4]. The background image can then be synthesized
with a temporal averaging process. To carry out the background subtraction, the current camera view has to be extracted from this background sprite image and aligned to
the current input image. After the camera-motion compensation, corresponding pixels in the foreground and the background should be co-located at the same position. However,
in practice, small inaccuracies in the motion estimation can
occur that lead to image misregistration. Even though this
is usually less than a pixel distance, it can cause interpolation artifacts along sharp edges. If there is a large difference
in brightness across the edge, a tiny inaccuracy in the motion model or aliasing in the input video can cause a large
difference between the images. The same effect occurs for
setups with static cameras, where the camera moves a little
bit because of vibrations or wind in outdoor environments.
A similar misregistration problem is already known from
remote sensing applications [3, 6] and a few algorithms have
been proposed to reduce them. For example, in [2] an algorithm for multispectral images is proposed. The idea is to
estimate the distribution of the registration noise for each
pixel by synthetically generating a number of misregistered
background images and comparing these images to the original one. However, the approach is designed for multispectral images. Moreover, the high computational complexity
for estimating the distributions makes it inappropriate for
real-time processing of sequences with moving camera.
To our knowledge, current algorithms for video object
segmentation do not consider this misregistration error in
their input and consequently, erroneous foreground objects
are detected in areas with strong edges (for example, see
Fig. 3(b)). This paper presents a new technique to detect
areas that are likely to show misregistration errors, and it
describes how standard change-detection algorithms can be
modified to accomodate for these special areas.
We will further explore the misregistration effect and
introduce the concept of a misregistration risk map in Section 2. The integration of these risk maps into background
subtraction algorithms will be described in Section 3. Results are presented in Section 4 and the paper concludes with
Section 5.
luminance
It
registration error
image difference
IB
position
luminance
(a) smooth edge
It
image difference
registration error
IB
position
(b) sharp edge
Fig. 1. Two edge-profiles, with (a) smooth, (b) sharp edges.
At the sharp edge, a misregistration of two edges induces a
larger luminance error than at the smooth edge.
2. MISREGISTRATION ERRORS
Change detection algorithms are usually based on the assumption that pixels in the input image It correspond exactly with the same pixels in the background image IB .
However, errors in the camera-motion compensation lead
to small displacements of the pixels. These displacements
are usually smaller than one pixel distance, but along strong
edges, they may induce large values in the difference image
(see Fig. 1).
As a first option to reduce this effect, we investigated
to change the image difference measurement. Instead of
using the direct luminance difference d(x, y) = |IB (x, y) −
It (x, y)|, we compensate for the expected misregistration
along edges by dividing by the luminance gradient in the
background image, leading to
|IB (x, y) − It (x, y)|
.
d0 (x, y) =
||∇IB (x, y)||
(1)
Note that this defines the pixel difference as the horizontal
distance that a pixel would have to be shifted to meet a pixel
of equal luminance (under the assumption that the change
of luminance in the neighborhood is linear). Even though
this approach succeedes to reduce the misregistration errors, we observe two problems. In flat image areas (where
∇IB (x, y) ≈ 0), the image difference signal is amplified
excessively. On the other hand, the difference signal is small
if there is high contrast in the background, but no texture in
the foreground object. The latter case reduces the ability to
detect foreground objects in front of textured background
even though the foreground might have a clearly different
color. However, since we can distinguish high-contrast texture areas in the background image from low-contrast areas
in the foreground, this first algorithm is not optimal for the
described case.
For this reason, we propose an algorithm that employs
a boolean image indicating for each pixel whether we expect large differences that result from misregistration. We
denote this boolean image as the risk map. To determine
the risk map, we consider the gradient strength in the background image as well as in the foreground image. This leads
to the following four cases. (A) If the gradient strength is
low in both the background image and the input image, misregistration has little effect to the difference image. (B) If
the gradient is strong in both the background image as well
as the input image, this is possibly because the input image
shows the same content as the background image. Since
both images show a strong edge at the same position, misregistration can lead to large differences and segmentation
errors. (C,D) If only the background image or only the foreground image shows a strong edge, the image content varies
from each other, so that large differences are not the result
of misregistration.
As a consequence, misregistration errors will only have
serious influence at pixels of case (B). Expressed as a formal rule, we detect those pixels if the gradient magnitude
in the background image ||∇IB (x, y)|| exceeds a threshold
τm and the input image gradient ||∇It (x, y)|| also exceeds
this threshold at the same position (x, y). The boolean risk
map RM is constructed accordingly, using
RM (x, y) = (||∇IB (x, y)|| > τm )
(||∇It (x, y)|| > τm ) .
∧
(2)
We apply a simple (1/2 0 -1/2) gradient filter and a threshold of τm ≈ 0.2 for a maximum pixel value of 1.0.
3. SEGMENTATION ALGORITHM
In this section, we show how the misregistration risk map
can be integrated into the change-detection algorithm proposed in [1]. We used this algorithm as part of our automatic
video object segmentation system, which is described in [5].
Here, we will only describe the background subtraction process, where we assume that a suitable camera-motion compensated background sequence has already been computed.
The input data for the background subtraction algorithm
is the current input image and a corresponding view of the
pure background. First, the difference image d(x, y) between both inputs is calculated. Instead of classifying the
pixels independently as foreground and background pixels,
we apply two algorithms that also consider the neighborhood of the pixel, to improve the robustness (see Fig. 2
for an overview). Both algorithms follow the approach described in [1], but are modified to integrate the previously
computed risk map. The first is a χ2 significance test to increase the robustness to camera noise. The result is used
as initialization of a Markov random field based statistical
determine risk map
background
input image
χ2 significance test
MRF optimization
post-processing
Fig. 2. Data-flow in the background subtraction algorithm.
MAP segmentation. After that, a morphological postprocessing step (which is not described here) removes small
and instable clutter regions from the segmentation mask.
3.1. χ2 significance test
The first algorithm assumes that all pixels in a small neighborhood window W (of typically 5 × 5 pixels), are either
changed or unchanged. Furthermore, it assumes that the
camera noise is Gaussian. With these two assumptions, a
χ2 significance test can be carried out on the pixels in a
small neighborhood. In this test, the sum of squared differences ∆ in the neighborhood is compared to a threshold tα .
This threshold is obtained from the cumulative function of a
χ2 distribution Pχ2 ;|W| with |W| degrees of freefom and a
chosen significance level α. More specifically, the threshold
tα is selected such that
Pχ2 ;|W| (∆ > tα ) = α,
(3)
where ∆ is the sum of squared differences, normalized with
the camera noise variance σ,
X
∆=
(d(x, y)/σ)2 .
(4)
(x,y)∈W
Considering several pixels in a neighborhood helps to
increase the robustness to noisy pixels. However, this is not
sufficient to compensate for the misregistration errors (see
Fig. 3(b)). Hence, we modified the algorithm to include
only those pixels in the computation that are not marked as
risky in RM . More specifically, we replace the computation
of ∆ to
(
X
(d(x, y)/σ)2
if RM (x, y) =false,
∆=
(5)
0
if RM (x, y) =true.
(x,y)∈W
The result of this processing step is taken as initialization
for the successive Markov random field optimization.
3.2. Markov field based segmentation
The Markov field based segmentation employs a model about
the probability of segmentation mask shapes. The probability that a pixel is changed increases with the number of
changed pixels in its neighborhood, and vice versa. Assuming again that the luminance differences are Gaussian distributed with variance σ for unchanged and σc for changed
pixels, the decision rule
d(x, y)2 ≷cu 2
σc
σc2 σ 2
ln +(vB (c) − vB (u))B +
σc2 − σ 2
σ
(6)
(vC (c) − vC (u))C
can be derived, where the right hand side determines the
threshold for a specific pixel. The ln σc /σ term together
with the factor in front of the parenthesis can be considered
the base threshold for the segmentation. It is depending only
on the noise variances σ, σc . Additionally, the segmentation
threshold is shifted by considering the labels of the pixels
in the neighborhood. The configuration of the pixels in the
neighborhood is described by the values vB , vC while the
strength of the regularization is controlled with the parameters B,C. See [1] for more details.
For pixels that were classified as risky, we cannot rely
on the difference values d(x, y), such that we classify these
pixels only based on the shape prior. Specifically, we use the
spatial context bias to decide if a pixel is more likely to be
changed or unchanged. This leads to the decision function
0 ≷cu (vB (c) − vB (u))B + (vC (c) − vC (u))C
(7)
if the considered pixel is risky. Otherwise, Eq. (6) is used
without modification.
4. RESULTS
To quantify the quality of the segmentation result, we compared the results with manually generated reference segmentation masks. Since soft shadows or motion blurs complicate the definition of reference masks, we classified the
pixels into three classes: background pixels, foreground pixels, and don’t-care pixels in unclear cases. These don’t-care
pixels were not considered in the evaluation.
Table 1 summarizes the improvements that we obtained
for some example sequences. Generally, it can be observed
that our algorithm can clearly reduce the segmentation errors that are due to image misregistration. At the same
time, our algorithm slightly increases the number of errors
in the foreground object if foreground and background are
both textured. Pictures from two of the example sequences
are shown in Figures 3 and 4. The stefan sequence is a
well-known MPEG-4 test-sequence. Note that the stefan
sequence shows almost no misregistration errors. Hence,
the segmentation result of our algorithm for this sequence is
similar to a segmentation without misregistration reduction.
(a) Input image.
(b) χ2 test without misregistration reduction.
(c) Markov field segmentation without misregistration
reduction.
(d) Markov field segmentation with misregistration reduction.
Fig. 3. Example sequence (surveillance). Misregistration is caused by a camera that is slightly moving in the wind.
(a) Input image.
(b) Risk map (note that the
athlet is not marked as risky).
(c) Without misregistration
reduction.
(d) With misregistration reduction.
Fig. 4. Example sequence (sport) with camera-motion where severe misregistration errors occur along sharp edges.
surveil. (A)
surveil. (B)
sport (A)
sport (B)
stefan (A)
stefan (B)
correct
bkg. (%)
96.5
99.9
87.2
99.9
99.6
99.9
correct
fgr. (%)
86.9
79.3
100.0
98.1
97.3
93.9
wrong
bkg. (#)
3478
42
13141
75
365
28
wrong
fgr. (#)
116
241
0
8
108
251
Table 1. Segmentation quality. (A) without misregistration
reduction, (B) our algorithm. Depicted is the percentage of
correct pixels and the average number of wrong pixels per
frame.
5. CONCLUSIONS
We have described the misregistration effect that results from
interpolation artifacts introduced in the camera-motion compensation. This effect usually leads to an increased number
of false detections in change detection algorithms. A new
algorithm was proposed to explicitly detect areas in which
these misregistration effects are likely to occur. With this
information available, standard change detection algorithms
can be modified to adapt to these areas.
Our detector for misregistration errors considers the contrast and sharpness of edges. Furthermore it combines information about the local texture in the background and the
input image. Nevertheless, the proposed the algorithm is
computationally efficient and suitable for integration into
real-time segmentation systems.
6. REFERENCES
[1] T. Aach and A. Kaup. Statistical model-based change detection in moving video. Signal Processing, 31:165–180, 1993.
[2] L. Bruzzone and R. Cossu. An adaptive approach to reducing registration noise effects in unsupervised change detection. IEEE Transactions on Geoscience and Remote Sensing,
41:2455–2465, 2003.
[3] X. Dai and S. Khorram. The effects of image misregistration
on the accuracy of remotely sensed change detection. IEEE
Transactions on Geoscience and Remote Sensing, 36:1566–
1577, 1998.
[4] D. Farin, P. H. N. de With, and W. Effelsberg. Minimizing
MPEG-4 sprite coding-cost using multi-sprites. In SPIE Proc.
Visual Communications and Image Processing, pages 234–
245, 2004.
[5] D. Farin, P. H. N. de With, and W. Effelsberg. Video-object
segmentation using multi-sprite background subtraction. In
Proc. IEEE International Conference on Multimedia and Expo
(ICME), pages 343–346, 2004.
[6] D. A. Stow. Reducing the effects of misregistration on pixellevel change detection. International Journal on Remote
Sensins, 20(12):2477–2483, Aug. 1999.
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