Shadow Removal in Front Projection Environments

Shadow Removal in Front Projection Environments
Shadow Removal in Front Projection Environments Using Object Tracking
Samuel Audet and Jeremy R. Cooperstock
Centre for Intelligent Machines, McGill University
3480 University Street, Montréal, Québec, Canada
When an occluding object, such as a person, stands between a projector and a display surface, a shadow results.
We can compensate by positioning multiple projectors so
they produce identical and overlapping images and by using a system to locate shadows. Existing systems work by
detecting either the shadows or the occluders. Shadow detection methods cannot remove shadows before they appear
and are sensitive to video projection, while current occluder
detection methods require near infrared cameras and illumination. Instead, we propose using a camera-based object
tracker to locate the occluder and an algorithm to model the
shadows. The algorithm can adapt to other tracking technologies as well. Despite imprecision in the calibration and
tracking process, we found that our system performs effective shadow removal with sufficiently low processing delay
for interactive applications with video projection.
1. Introduction
Front projection installations use less space than equivalent rear projection configurations and cost less than monitors to cover large display surfaces. However, when a person moves in between a projector and a display surface, a
shadow occurs, negatively affecting the experience in terms
of human-computer interaction [14].
In many situations, we can manage shadows by appropriately positioning projectors and constraining the allowed
locations at which people can stand. When this is not possible, such as in small rooms, we can use techniques where
multiple projectors are positioned far apart and configured
so they produce the same image at the same place on the
display surface [13]. This is also known as Passive Virtual
Rear Projection. The image thus remains visible even if
a person occludes one projector. This configuration can be
achieved by transforming the images before their projection
so they match with the reference frame [11, 18]. For a flat
display surface, the transformation may be a simple homography. Color correction and multi-focal projection are also
(a) Before corrections.
(b) After corrections.
Figure 1. Ideal front projection where distortion, colors and shadows are all corrected.
desirable, although outside the scope of our work.
With the previous approach, although the image remains
visible, the resulting differences in display intensity remain
perceptible. To compensate, more advanced methods compute and determine the display region in which a shadow
occurs. Another projector can then fill the region with equal
intensity and identical color to that of the occluded projector, as in the ideal case depicted in Figure 1. This is also
known as Active Virtual Rear Projection. However, as described below, current methods suffer from a number of limitations.
1.1. Related Work
The first methods that were developed directly detect
shadows on the display surface [3, 6, 8, 13] and thus can
only remove shadows after they appear. Visual echo may
also result when the algorithm no longer observes a shadow
and incorrectly assumes the region to be unoccluded. This
method works by comparing the image displayed by a projector to the image as seen by a camera, after compensating
for geometric and radiometric color differences between the
projector and the camera. Therefore, this operation is subject to numerous errors in the case of video projection, since
to compare correctly we need to know the precise delay incurred in the projectors, in the cameras, and in processing.
An additional constraint is that the cameras need an unoccluded view of the entire display surface, or the system cannot function properly.
Other methods use a near infrared camera mounted
alongside each projector [4, 17] to detect the occlusion
rather than the shadow. Because these cameras do not
see projector light, a simple background subtraction technique can be used to generate a pixel-mask of the occlusions in front of each camera-projector pair. Since
background subtraction requires good contrast between the
background (display surface) and the foreground (people),
infrared floodlights are used to illuminate the display surface, while not illuminating the people. However, this
method, on its own, cannot predict where a shadow will appear next as people move within the environment. To deal
with this limitation, Flagg et al. [4] perform much of their
computation on Graphics Processing Units (GPUs), which
greatly improves processing speed and the results. The
main drawback of this approach is the requirement of hardware specifically for shadow removal purposes, which may
include infrared floodlights, and for each projector, a near
infrared camera and additional processing power (GPUs).
Distortion and
Centralized Processing
Continuous Flow
Distributed Processing
Onetime Flow
Figure 2. Illustration of the flow of data in our system.
2. Calibration
1.2. Object Tracking for Shadow Removal
Instead, we propose object tracking as the basis of our
approach to shadow removal. Many interactive applications
already require such tracking, for example, to ensure that
images are rendered from the proper perspective [12] or to
adjust 3D audio according to user movement [20]. This
leverages technology that may already be in place, since our
method can use data from any object tracker. Also, tracking
can take advantage of temporal information to predict the
motion of people as occluders.
Our approach requires a calibration method for both
the cameras and projectors, an adequately reliable object
tracker, and an algorithm that combines this information to
perform shadow removal. While calibration and tracking
are often imprecise, we show that good results can be obtained from such a system.
A high-level diagram of our system architecture appears
in Figure 2. Our work focuses on the Calibration, Object
Tracking, Shadow Removal, and Distortion and Perspective Correction modules. We did not address issues of color
correction and multi-focal projection. In the following sections, we elaborate on our approach. Section 2 first discusses the camera calibration procedure we employed, then
describes how we adapted it to projector calibration. Next,
Section 3 describes how we adapted a camera-based object
tracker for our purposes. The main challenge we faced was
to achieve reasonably reliable tracking, despite the projection of dynamic video in the background, which can easily be misinterpreted as moving people. Section 4 explains
how we integrated these components to model shadows and
achieve shadow removal. Section 5 summarizes our experimental results and Section 6 concludes with a discussion of
limitations and future work.
We first needed to find the intrinsic and extrinsic parameters of both the cameras and the projectors. These parameters are required to obtain the location and size of people
in 3D and to model their shadows occurring on the display
2.1. Intrinsic Parameters
To find the intrinsic parameters of our perspective devices (cameras and projectors), we used Zhang’s calibration
method [21] as implemented by Bouguet [2]. This method
was designed for camera calibration, so we used it directly
to calibrate our cameras. For the projectors, we developed a
method based on work done by Raskar and Beardsley [10],
and by Ashdown et al. [1]. Their methods involves locating
projected points on a physical board using the homography
between the projector and a camera viewing the board. In
the second paper, the red and blue channels of a color camera were used respectively as filters for a calibration board
composed of white and cyan squares, and for a projected
pattern with blue and black squares. This way, one can use
the same surface for both a projected pattern and a physical
pattern, maximizing the use of the sensor of a color camera,
so we chose this method. We tested cyan, magenta, yellow,
red, green and blue colors under complementary projector
light, and we also came to the conclusion that cyan was the
best color to use. During calibration of a projector, since
we did not have mobile projectors as Ashdown et al. [1],
we held our cyan calibration board, a printed sheet attached
to a foam board, at different angles in front of the projector as depicted in Figure 3. We found that our projectors
were too powerful and they emitted too much blue light in
comparison to the ambient red light. Rather than simply reducing the intensity of the projector, we took advantage of
Figure 3. Holding the cyan calibration board in front of a camera
and a projector displaying the yellow pattern.
the opportunity. We used the projector to light up the calibration board with red and green light as well, resulting in a
projected pattern with yellow and white squares. This way,
we were able to guarantee the ratio of blue pixel intensity
to red pixel intensity without relying on the environment or
fine-tuning camera parameters. Typical images are shown
in Figure 4.
We also took care of other important factors that can influence the precision of the resulting calibration. Previous
research [16] indicates that to obtain near optimal results
with the method of Zhang [21], the calibration board should
have more than 150 corners and the set of images should
contain more than 1500 corners. For these reasons we used
at least 15 images for the calibration of each device, and the
calibration board used for the cameras had 18 × 14 squares
of 15 mm (221 corners), the cyan calibration board had
19 × 14 squares of 50 mm (234 corners), and the projected
pattern had 16 × 12 squares (165 corners). Also, we held
the calibration boards at angles near 45◦ as recommended by
Zhang [21]. Last, when calibrating a projector, we installed
the camera on top of the projector so that the projected pattern image would use the largest possible area of the camera
sensor even when holding the board at different angles.
2.2. Extrinsic Parameters
To obtain the extrinsic parameters of the cameras and
projectors, we first found the homographies induced by
the flat display surface for each camera-projector pair by
projecting a known calibration pattern onto the surface.
For two calibrated devices, an algorithm developed by
Triggs [19] can then decompose the homography into the
rotation and translation separating the two devices. It also
provides the equation of the inducing plane. The algorithm
actually returns two sets of values, only one of which corresponds to the physical reality. In our case, the display
surface remained the same for at least two projectors, so
we could easily resolve the ambiguity and retain the set of
values for which the plane equations were closest to one
Because of unavoidable calibration errors in the intrinsic
parameters, the plane equations were not actually equal. To
model the shadows on the display surface, a unique plane
equation is required to properly backproject from two or
more devices. Also the homographies between projectors
were very precise (less than one camera pixel of error),
and we did not want to lose this precision. To satisfy both
goals, we devised the following procedure. First, we decided to use a plane equation pc found with the homography between two cameras, since it was probably more accurate given that the calibration errors for the projectors were
larger. Then, the idea is to find a corrective homography H
to remove the discrepancies between the projection of 3D
points X and known corner locations xp , which should be
equal on the image plane of the projector:
xp = HPp X
X = Bc xc ,
Bc =
[pc tc I4x4 − tc pc ]P+
T −1
Pc (Pc Pc )
where X are the 3D location of the points on the display
surface pc according to their projection xc in the camera
and its backprojection matrix Bc where tc is the camera
center and P+
c is the pseudoinverse of the projection matrix Pc of the camera. Hartley and Zisserman [5] provide
more detailed explanations on this subject. H is the corrective homography we are looking for so that once added to
the projection matrix Pp of the projector, X project to xp
as closely as possible with least squares. We ignored the
radial and tangential distortion coefficients of the projector
as found during calibration. This approximation should be
valid as the projectors did not exhibit noticeable distortion.
Also, this simple procedure as a whole does not attempt
to minimize geometric errors induced inside the projection
matrix, but assuming relatively precise calibration results,
the correction, and consequently the errors, should be minimal.
Finally, to obtain equation of the floor plane required by
the tracking module as detailes below, we manually located
the edge vector in the camera image where the floor and the
display surface met. We also estimated the length of the
vector in a physical unit. Assuming the floor and the surface were at right angles, we found the normal of the floor
by computing the cross-product of the normal of the surface and of the edge vector. We also rescaled all calibration
parameters according to the physical length of the vector.
(a) RGB image.
(b) Red channel.
(c) Blue channel.
Figure 4. Sample images we used for projector calibration. (We adjusted the level of the red channel for better contrast.)
3. Tracking
In theory, we could use any tracking algorithm that provides 3D information. However, we wanted to test our system with an algorithm that did not require the user to wear
sensors, tags or markers, and that could also work in an environment filled with video projections. We devised a simple method based on previous research using computer vision [7, 15]. We did not focus our efforts into developping
a new tracking method, and although we could have used
more complex methods for better 3D tracking, we were motivated in using an approach we could easily implement and
that would be fast enough for interactive purposes.
3.1. Disparity Contours
First, we used the calibration data of the room (floor and
display surface) to build a background disparity map. This
map can be seen as a function x2 = M (x1 ) that maps points
x1 from the image plane of one camera into points x2 on the
image plane of the other camera. As detailed by Ivanov et
al. [7], with a pair of frames I1 and I2 and their disparity
map, one can compute a difference image
D(x1 ) = |I1 (x1 ) − I2 (M (x1 ))|
for all points x1 . In theory, x1 and M (x1 ) should backproject to the same physical point when no object is present
in front of the cameras, thus producing a completely black
difference image D regardless of changes in lighting conditions. However, when the disparity map does not correspond to the geometry of what is actually present in front of
closely positioned cameras, this procedure produces contour images, referred to as disparity contours after Sun [15],
similar to the results of an edge detector. Although in theory it is possible to extract depth information from those
contours, the task has proven challenging [15], and we instead used the information for 2D tracking only.
We decided to apply to the resulting difference image a
3 × 3 square mask erosion, to deal with ±0.5 pixel registration errors, followed by a background subtraction algorithm
using a Mixture of Gaussian [9], which worked best at minimizing the importance of the various causes of errors. We
then routed the processed images to the blob tracker. On
the output of the tracker, we used a Kalman filter to predict
movement. The filter runs two prediction steps, accounting for the delays in the camera/tracker module and in the
post-processing/projector module, assuming they are close.
In this manner, we obtained an object tracker insensitive to
dynamic video projection on the display surface as showed
in Figure 5.
3.2. 3D Tracking
The tracker only provides 2D information, but we used
the following method to recover 3D information by assuming that the objects of interest would stand vertically on the
floor, which is usually the case with people. With equation 2, we can backproject a point xc corresponding to the
feet of a person in the image plane of the camera into a point
X on the floor whose normal is known. We can then approximate the height of the person by considering a plane parallel to the floor on top of the person ptop = [ a b c e ],
which consequently has the same normal as the plane of
the floor pfloor = [ a b c d ], where the normal is
n = [ a b c ]T . The distance we are interested in is
thus the height h = d − e. Looking back at equation 2, the
backprojection matrix Bc has rank 3, so it can solve for 3
unknowns. Our unknowns are the height h and the scale
of the resulting X. The system is thus overdetermined.
We could have used least squares to minimize errors with
xc = [ u v 1 ], but under the assumption that the up
vector of the camera is in the general direction of the nor-
(a) Camera image.
(b) Disparity contours and tracking.
Figure 5. Sample camera image and the corresponding disparity
contour image with tracking information.
mal, simply dropping the u component and using only the
v component should produce acceptable results, so this is
what we did. In this manner, we decided to model people as
flat rectangles parallel to the display surface. For the current
implementation of the algorithm, this simplifying assumption appeared sufficient.
4. Shadow Removal
Our system runs the tracker and generates masks for all
projectors as a centralized process. On the other hand, the
post-processing of the masks just before display can be done
in parallel and be distributed.
4.1. Centralized Processing
Shadow removal works by creating a rectangular mask
image for each of the n projectors in a centralized manner.
The area of the mask image represents the rectangular portion of the display surface where we desire images to be
displayed. For the sake of clarity, we will use [0, 1] as the
range of pixel values for discussion in this section, but our
implementation appropriately scales the results for use with
8-bit grayscale images. After processing, a pixel with value
of 0 means that no light should be emitted for this pixel, and
1, the full brightness of the projector should be used for this
First, the algorithm sets all the pixels of the mask images
to 0. Then using the calibration information it sets to 1 the
pixels inside the quadrilateral region that is coverable by the
projector. At this point, the images contain only information
about which area of the display surface any particular projector can cover. Using tracking and calibration information, it then localizes shadows on the display surface. We
can model shadows similarly to how the operation is done
in computer graphics. For each projector, we can backproject the vertices of the rectangle onto the display surface,
and draw with 0 valued pixels in mask images the resulting
shadow. Finally it normalizes each pixel so that the sum of
all pixels in the same position from all masks M does not
exceed 1, as described by the following equation:
M̂j (u, v) =
PnMj (u,v)
k=1 Mk (u,v)
4.2. Distributed Processing
The masks are then dispatched over the network and used
to modulate the brightness of the pixels of the projectors.
First, to soften the transition between projectors with different color emission properties, we added a smoothing stage.
We adapted the box blur algorithm to dynamically select the
direction of the blurring and in this manner to try to clamp
the pixel values in two kinds of regions of the masks: the
shadow regions and regions near the edges of coverable areas. Figure 6 shows the difference before and after blurring.
Next, the linear representation of intensity was not physically reproduced by our projectors. Since video systems
usually follow a power law of 2.2, we performed gamma
correction, i.e.: M̂0 (u, v) = M̂(u, v) 2.2 for all pixels (u, v).
Finally, the distributed program also corrected for linear
(perspective) and non-linear (radial and tangential) distortion by approximating the desired correction function using
OpenGL textures [3, 18]. Tardif et al. [18] used a grid of
12065 squares, so we divided our texture in a similar manner, more precisely into 101 × 101 vertices.
5. Results
We tested our system using front projection on an interior wall surface. We used two Sanyo PLC-EF30 projectors
and two Point Grey Research Flea2 cameras equipped with
4.0-12.0 mm varifocal lenses adjusted at their widest angle.
These cameras automatically synchronize with each other.
The experiments involved three distinct steps; calibration,
tracking, and shadow removal, each of which is described
in detail below.
5.1. Calibration
Based on our modified version of Bouguet’s calibration
software [2], the reprojection errors (three times the standard deviation in pixels) achieved after calibration of the
cameras and projectors are shown in Table 1. We performed
the reprojection using the native resolution of the device as
defined during calibration. The resolution of the cameras
was 1024×768, and that of the projectors was 1280×1024.
if k=1 Mk (u, v) = 0
for j = 1..n where Mj is the mask image before normalization, and M̂j after, for all pixels (u, v). However, when
an occluder stands too close to the display surface, a region
of the display surface will have all its pixels set to 0 for all
projectors. To regain any small portion that is not actually
occluded, the algorithm resets to 1 all pixels of the first projector in this area.
(a) Before blur.
(b) After blur.
Figure 6. Sample result of our modified box blur for smooth transition between two or more projectors.
During projector calibration, for consistency, we used images and calibration results from Camera 1 only.
Camera 1
Camera 2
Projector 1
Projector 2
Reprojection error
(a) Without shadow removal.
(b) With shadow removal.
Figure 7. Sample images of the room with one person standing,
with and without shadow removal.
Table 1. Reprojection error (three times the standard deviation in
pixels) after calibration.
5.2. Tracking
As for the extrinsic parameters, Table 2 details the position and orientation in space as found by our method. The
normal of the floor was (0, 0, 1), while the normal of the
wall was (0, -1, 0), and the up vector of the perspective device reference system was (0, 0, 1) while its viewing or
projecting direction was (0, 1, 0), all axes following the
right-hand rule convention. After applying corrective homographies, the errors between corresponding points from
cameras or projectors all dropped to less than one pixel, as
expected, according to the initial homographies. Interestingly enough, the corrective homographies did not significantly alter the rotations and translations of the projection
matrices, preferring to skew the internal parameters instead.
Although the homographies are close to the identity matrix,
the operation minimizes the algebraic error, not the geometric error. Without proper normalization, it is not surprising
to see such results. Nevertheless, our implementation performed well even with these errors.
Camera 1
Camera 2
Projector 1
Projector 2
Position (cm)
−22.5 −425 215
−6.28 −429 215
226 −451 227
−94.4 −418 202
Orientation (degrees)
Roll Pitch Yaw
−2.65 −28.3 −8.00
−2.18 −29.2 −5.61
−1.74 −5.54 11.7
−2.97 −4.68 −24.4
Table 2. External parameters of the perspective devices in the
We verified the precision of the projector calibration by
having a person stand at every corners of the the tiling in
the room (61 × 61 cm) and by giving the algorithm the exact position on the floor. To account for the non-flatness
of people, we used values for the width and height of the
person that were respectively 110% and 103% of their real
values. The maximum error we found on the floor was 54.3
cm, and the maximum error in z, 12.1 cm, while on average
the errors were 28.0 cm and 2.0 cm respectively, all measurements precise within 2.0 cm.
The focus of our work is not to compare different tracking algorithms, and we simply adopted the blob tracker
from the OpenCV Video Surveillance Module. Although
its performance was not tested thoroughly, it was obvious
from our observations that it could not track with a precision greater than 10 centimeters. Reliability was also problematic, especially when a person occluded another person.
5.3. Shadow Removal
Typical results are shown in Figures 8 and 9, for
a static image projection and a dynamic video projection, respectively.
The full video is available at
. Although the system currently achieves reasonable results
in the case of static images, some additional refinement
is necessary for it to perform well with video projection.
In the case of static images, we could compensate for
calibration and tracker imprecision by dilating the masks
appropriately. Visible artefacts, such as the ones in
Figures 8(a) and 8(b), are due mostly to fast movements
not correctly predicted by the Kalman filter. However,
in the case of video projection, the tracker frequently
fails, resulting in large visible shadows such as seen in
Figure 9(d).
An additional problem is that the system generates artefacts during occluder movement, such as in Figure 9(b).
This is due, in part because it does not properly synchronize
the mask display. Currently, each machine receives a mask,
processes it, and sends it for display with no regards to
whether other machines are ready for display. Obviously, a
rudimentary synchronization technique would improve the
Since one of our objectives was to implement a system that could perform shadow removal in real time, we
tested the performance in terms of processing delay. We
benchmarked our current largely unoptimized codebase on
an Intel Pentium 4 2.60 GHz. Applying the disparity map
on 512 × 384 color images and tracking, using the resulting difference image resized down to 256 × 192, took
(a) frame 100
(b) frame 150
(c) frame 200
(d) frame 250
(e) frame 300
(f) frame 350
(g) frame 400
(f) frame 1150
(g) frame 1200
Figure 8. Frames from demo video with static image projection.
(a) frame 150
(b) frame 250
(c) frame 350
(d) frame 900
(e) frame 1100
Figure 9. Frames from demo video with dynamic video projection.
39.2 ± 3.4 (SD) ms. The generation of a 640 × 512 mask
took 9.38 ± 0.94 (SD) ms, while mask post-processing (including a 50 pixel blurring) and correction took 79.1 ± 2.5
(SD) ms, for a grand total of 127.7 ± 6.8 (SD) ms. This
does take into account delays in capture, display or network
6. Discussion
Compared to other methods for shadow removal, our
object tracking method has several pros and cons. First,
shadow detection [3, 6, 8, 13] requires that at least one camera be placed strategically to have an unoccluded view of
the entire scene. In practice, this can sometimes be challenging. The infrared occlusion detection method [4, 17]
requires one camera alongside each projector, in addition
to the installation of near infrared floodlights at the display
surface. In contrast, our method only requires that two cameras be placed appropriately for tracking people. Second,
shadow detection methods cannot remove shadows before
they appear. Although the simple approach of the infrared
occlusion detection method, which preemptively dilates the
occluded region to account for possible movement in any
direction, appears effective at solving this problem in most
cases, our method goes one step further and uses prediction information from a Kalman filter. Third, since both
our method and infrared occlusion detection methods do not
make use of projected images, it does not matter, in principle, whether we project static images or dynamic video.
On the other hand, shadow detection becomes dramatically
more complex in the case of video projection. Although
Jaynes et al. [8] achieved nine frames per second, it is uncertain whether this can be easily increased to at least 24
frames per second. Flagg et al. [4] found that the combined
latency of their camera and projector was in the order of
50 ms. For this reason, with commonly available hardware,
the algorithm as developped by Jaynes et al. [8] cannot scale
well beyond 15 frames per second.
Our method has drawbacks as well. The calibration required for both the shadow detection and infrared occlusion detections method can be relatively easily automated,
whereas our approach currently requires a rather tedious
calibration phase. Furthermore, tracking using cameras requires a sufficiently illuminated environment to make the
occluders visible. Also, the simple tracking we used does
not model limbs, and as such, effective shadow removal
cannot be attained if the users wave their hands in front
of the display. Moreover, tracking fails when a person occludes another person in the camera view. In this regard,
shadow detection and infrared occlusion detection methods
are superior. Fortunately, our approach is not bound to any
particular tracker, so these limitations may be overcome by
using better or simply more appropriate tracking methods.
In our implementation, we identified various sources of
calibration and tracking errors. We found that large calibration errors may result because of improperly modeled
non-linear distortion in the periphery of the camera image,
uneven floor or wall, and the corrective homographies used
for the projectors, which currently do not attempt to minimize geometric errors. We will investigate how to manage
these errors, but considering all the issues related to calibration alone, we will also attempt to redesign the method to
not require an elaborate calibration phase. In the case of the
tracker, although it performs well for static images, it does
not give acceptable results with video. Right now, it uses
foreground information based solely on geometric properties of the cameras. However, we noted that, in the area of
the display surface, even though the disparity map was precise, using it produced too much noise, probably cause by
other intrinsic differences between the two cameras. We do
not believe near infrared to be required, unless a dark environment is desired. A photometric color calibration should
give us as good or even better performance with color cameras.
The work presented here provides an initial proof of con-
cept that shadow removal can be performed on today’s hardware using conventional object tracking rather than requiring the more elaborate configuration of the near infrared occlusion detection approach. While there remains considerable effort ahead to refine and optimize our method, current
results clearly demonstrate its feasibility. We hope that this
will lead to better projector-camera systems now that we
have shown that we can leverage object tracking technology
already in place in many front projection environments, and
achieve effective shadow removal.
This work was supported by a scholarship from the Natural Sciences and Engineering Research Council of Canada
(NSERC). Picture from Mars, courtesy of NASA/JPLCaltech.
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