optical_flow_mg — Compute the optical flow between two images.
optical_flow_mg(ImageT1, ImageT2 : VectorField : Algorithm, SmoothingSigma, IntegrationSigma, FlowSmoothness, GradientConstancy, MGParamName, MGParamValue : )
optical_flow_mg computes the optical flow between two
images. The optical flow represents information about the movement
between two consecutive images of a monocular image sequence. The
movement in the images can be caused by objects that move in the
world or by a movement of the camera (or both) between the
acquisition of the two images. The projection of these 3D movements
into the 2D image plane is called the optical flow.
The two consecutive images of the image sequence are passed in
ImageT1 and ImageT2. The computed optical flow is
returned in VectorField. The vectors in the vector field
VectorField represent the movement in the image plane
between ImageT1 and ImageT2. The point in
ImageT2 that corresponds to the point (r,c) in
ImageT1 is given by (r',c') = (r+u(r,c),c+v(r,c)), where
u(r,c) and v(r,c) denote the value of the row and column
components of the vector field image VectorField at the
point (r,c).
The parameter Algorithm allows the selection of three
different algorithms for computing the optical flow. All three
algorithms are implemented by using multigrid solvers to ensure an
efficient solution of the underlying partial differential equations.
For Algorithm = 'fdrig', the method proposed by
Brox, Bruhn, Papenberg, and Weickert is used. This approach is
flow-driven, robust, isotropic,
and uses a gradient constancy term.
For Algorithm = 'ddraw', a robust variant of the
method proposed by Nagel and Enkelmann is used. This approach is
data-driven, robust,
anisotropic, and uses warping (in contrast to the
original approach).
For Algorithm = 'clg' the combined
local-global method proposed by Bruhn, Weickert,
Feddern, Kohlberger, and Schnörr is used.
In all three algorithms, the input images can first be smoothed by a
Gaussian filter with a standard deviation of SmoothingSigma
(see derivate_gauss).
All three approaches are variational approaches that compute the
optical flow as the minimizer of a suitable energy functional. In
general, the energy functionals have the following form:
where w=(u,v,1) is the optical flow vector
field to be determined (with a time step of 1 in the third
coordinate). The image sequence is regarded as a continuous
function f(x), where
x=(r,c,t) and (r,c) denotes the position
and t the time. Furthermore,
denotes the data term,
while denotes the
smoothness term, and is a regularization
parameter that determines the smoothness of the solution. The
regularization parameter is passed in
FlowSmoothness. While the data term encodes assumptions
about the constancy of the object features in consecutive images,
e.g., the constancy of the gray values or the constancy of the first
spatial derivative of the gray values, the smoothness term encodes
assumptions about the (piecewise) smoothness of the solution, i.e.,
the smoothness of the vector field to be determined.
The FDRIG algorithm is based on the minimization of an energy functional that contains the following assumptions:
Constancy of the gray values: It is assumed that corresponding pixels in consecutive images of an image sequence have the same gray value, i.e., that f(r+u,c+v,t+1) = f(r,c,t). This can be written more compactly as f(x+w) = f(x) using vector notation.
Constancy of the spatial gray value derivatives: It is assumed that corresponding pixels in consecutive images of an image sequence additionally have have the same spatial gray value derivatives, i.e, that also holds, where . This can be written more compactly as . In contrast to the gray value constancy, the gradient constancy has the advantage that it is invariant to additive global illumination changes.
Large displacements: It is assumed that large displacements, i.e., displacements larger than one pixel, occur. Under this assumption, it makes sense to consciously abstain from using the linearization of the constancy assumptions in the model that is typically proposed in the literature.
Statistical robustness in the data term: To reduce the influence of outliers, i.e., points that violate the constancy assumptions, they are penalized in a statistically robust manner, i.e., the customary non-robust square penalization is replaced by a linear penalization via , where is a fixed regularization constant.
Preservation of discontinuities in the flow field I: The solution is assumed to be piecewise smooth. While the actual smoothness is achieved by penalizing the first derivatives of the flow , the use of a statistically robust (linear) penalty function with provides the desired preservation of edges in the movement in the flow field to be determined. This type of smoothness term is called flow-driven and isotropic.
Taking into account all of the above assumptions, the energy functional of the FDRIG algorithm can be written as
Here, is the regularization parameter passed in
FlowSmoothness, while is the gradient
constancy weight passed in GradientConstancy. These two
parameters, which constitute the model parameters of the FDRIG
algorithm, are described in more detail below.
The DDRAW algorithm is based on the minimization of an energy functional that contains the following assumptions:
Constancy of the gray values: It is assumed that corresponding pixels in consecutive images of an image sequence have the same gray value, i.e., that f(x+w) = f(x).
Large displacements: It is assumed that large displacements, i.e., displacements larger than one pixel, occur. Under this assumption, it makes sense to consciously abstain from using the linearization of the constancy assumptions in the model that is typically proposed in the literature.
Statistical robustness in the data term: To reduce the influence of outliers, i.e., points that violate the constancy assumptions, they are penalized in a statistically robust manner, i.e., the customary non-robust square penalization is replaced by a linear penalization via , where is a fixed regularization constant.
Preservation of discontinuities in the flow field II: The solution is assumed to be piecewise smooth. In contrast to the FDRIG algorithm, which allows discontinuities everywhere, the DDRAW algorithm only allows discontinuities at the edges in the original image. Here, the local smoothness is controlled in such a way that the flow field is sharp across image edges, while it is smooth along the image edges. This type of smoothness term is called data-driven and anisotropic.
All assumptions of the DDRAW algorithm can be combined into the following energy functional:
where is a normalized projection matrix orthogonal to , for which
holds. This matrix ensures that the smoothness of the flow field is only assumed along the image edges. In contrast, no assumption is made with respect to the smoothness across the image edges, resulting in the fact that discontinuities in the solution may occur across the image edges. In this respect, serves as a regularization parameter that prevents the projection matrix from becoming singular. In contrast to the FDRIG algorithm, there is only one model parameter for the DDRAW algorithm: the regularization parameter . As mentioned above, is described in more detail below.
As for the two approaches described above, the CLG algorithm uses certain assumptions:
Constancy of the gray values: It is assumed that corresponding pixels in consecutive images of an image sequence have the same gray value, i.e., that f(x+w) = f(x).
Small displacements: In contrast to the two approaches above, it is assumed that only small displacements can occur, i.e., displacements in the order of a few pixels. This facilitates a linearization of the constancy assumptions in the model, and leads to the approximation , i.e., should hold. Here, denotes the gradient in the spatial as well as the temporal domain.
Local constancy of the solution: Furthermore, it is assumed
that the flow field to be computed is locally constant. This
facilitates the integration of the image data in the data term over
the respective neighborhood of each pixel. This, in turn, increases
the robustness of the algorithm against noise. Mathematically, this
can be achieved by reformulating the quadratic data term as
. By performing a local
Gaussian-weighted integration over a neighborhood specified by the
(passed in IntegrationSigma), the following
data term is obtained: . Here, denotes a convolution of the 3x3 matrix
with a Gaussian filter with a standard
deviation of (see derivate_gauss).
General smoothness of the flow field: Finally, the solution is assumed to be smooth everywhere in the image. This particular type of smoothness term is called homogeneous.
All of the above assumptions can be combined into the following
energy functional:
The corresponding model parameters are the regularization parameter
as well as the integration scale
(passed in IntegrationSigma), which determines the size of
the neighborhood over which to integrate the data term. These two
parameters are described in more detail below.
To compute the optical flow vector field for two consecutive images of an image sequence with the FDRIG, DDRAW, or CLG algorithm, the solution that best fulfills the assumptions of the respective algorithm must be determined. From a mathematical point of view, this means that a minimization of the above energy functionals should be performed. For the FDRIG and DDRAW algorithms, so called coarse-to-fine warping strategies play an important role in this minimization, because they enable the calculation of large displacements. Thus, they are a suitable means to handle the omission of the linearization of the constancy assumptions numerically in these two approaches.
To calculate large displacements, coarse-to-fine warping strategies use two concepts that are closely interlocked: The successive refinement of the problem (coarse-to-fine) and the successive compensation of the current image pair by already computed displacements (warping). Algorithmically, such coarse-to-fine warping strategies can be described as follows:
1. First, both images of the current image pair are zoomed down to a very coarse resolution level.
2. Then, the optical flow vector field is computed on this coarse resolution.
3. The vector field is required on the next resolution level: It is applied there to the second image of the image sequence, i.e., the problem on the finer resolution level is compensated by the already computed optical flow field. This step is also known as warping.
4. The modified problem (difference problem) is now solved on the finer resolution level, i.e., the optical flow vector field is computed there.
5. The steps 3-4 are repeated until the finest resolution level is reached.
6. The final result is computed by adding up the vector fields from all resolution levels.
This incremental computation of the optical flow vector field has the following advantage: While the coarse-to-fine strategy ensures that the displacements on the finest resolution level are very small, the warping strategy ensures that the displacements remain small for the incremental displacements (optical flow vector fields of the difference problems). Since small displacements can be computed much more accurately than larger displacements, the accuracy of the results typically increases significantly by using such a coarse-to-fine warping strategy. However, instead of having to solve a single correspondence problem, an entire hierarchy of these problems must now be solved. For the CLG algorithm, such a coarse-to-fine warping strategy is unnecessary since the model already assumes small displacements.
The maximum number of resolution levels (warping levels), the resolution ratio between two consecutive resolution levels, as well as the finest resolution level can be specified for the FDRIG as well as the DDRAW algorithm. Details can be found below.
The minimization of functionals is mathematically very closely related to the minimization of functions: Like the fact that the zero crossing of the first derivative is a necessary condition for the minimum of a function, the fulfillment of the so called Euler-Lagrange equations is a necessary condition for the minimizing function of a functional (the minimizing function corresponds to the desired optical flow vector field in this case). The Euler-Lagrange equations are partial differential equations. By discretizing these Euler-Lagrange equations using finite differences, large sparse nonlinear equation systems result for the FDRIG and DDRAW algorithms. Because coarse-to-fine warping strategies are used, such an equation system must be solved for each resolution level, i.e., for each warping level. For the CLG algorithm, a single sparse linear equation system must be solved.
To ensure that the above nonlinear equation systems can be solved efficiently, the FDRIG and DDRAW use bidirectional multigrid methods. From a numerical point of view, these strategies are among the fastest methods for solving large linear and nonlinear equation systems. In contrast to conventional non-hierarchical iterative methods, e.g., the different linear and nonlinear Gauss-Seidel variants, the multigrid methods have the advantage that corrections to the solution can be determined efficiently on coarser resolution levels. This, in turn, leads to a significantly faster convergence. The basic idea of multigrid methods additionally consists of hierarchically computing these correction steps, i.e., the computation of the error on a coarser resolution level itself uses the same strategy and efficiently computes its error (i.e., the error of the error) by correction steps on an even coarser resolution level. Depending on whether one or two error correction steps are performed per cycle, a so called V or W cycle is obtained. The corresponding strategies for stepping through the resolution hierarchy are as follows for two to four resolution levels: Here, iterations on the original problem are denoted by large markers, while small markers denote iterations on error correction problems.
Algorithmically, a correction cycle can be described as follows:
1. In the first step, several (few) iterations using an interactive linear or nonlinear basic solver are performed (e.g., a variant of the Gauss-Seidel solver). This step is called pre-relaxation step.
2. In the second step, the current error is computed to correct the current solution (the solution after step 1). For efficiency reasons, the error is calculated on a coarser resolution level. This step, which can be performed iteratively several times, is called coarse grid correction step.
3. In a final step, again several (few) iterations using the interactive linear or nonlinear basic solver of step 1 are performed. This step is called post-relaxation step.
In addition, the solution can be initialized in a hierarchical manner. Starting from a very coarse variant of the original (non)linear equation system, the solution is successively refined. To do so, interpolated solutions of coarser variants of the equation system are used as the initialization of the next finer variant. On each resolution level itself, the V or W cycles described above are used to efficiently solve the (non)linear equation system on that resolution level. The corresponding multigrid methods are called full multigrid methods in the literature. The full multigrid algorithm can be visualized as follows: This example represents a full multigrid algorithm that uses two W correction cycles per resolution level of the hierarchical initialization. The interpolation steps of the solution from one resolution level to the next are denoted by i and the two W correction cycles by and . Iterations on the original problem are denoted by large markers, while small markers denote iterations on error correction problems.
In the multigrid implementation of the FDRIG, DDRAW, and CLG algorithm, the following parameters can be set: whether a hierarchical initialization is performed; the number of coarse grid correction steps; the maximum number of correction levels (resolution levels); the number of pre-relaxation steps; the number of post-relaxation steps. These parameters are described in more detail below.
The basic solver for the FDRIG algorithm is a point-coupled fixed-point variant of the linear Gauss-Seidel algorithm. The basic solver for the DDRAW algorithm is an alternating line-coupled fixed-point variant of the same type. The number of fixed-point steps can be specified for both algorithms with a further parameter. The basic solver for the CLG algorithm is a point-coupled linear Gauss-Seidel algorithm. The transfer of the data between the different resolution levels is performed by area-based interpolation and area-based averaging, respectively.
After the algorithms have been described, the effects of the individual parameters are discussed in the following.
The input images, along with their domains (regions of interest) are
passed in ImageT1 and ImageT2. The computation of
the optical flow vector field VectorField is performed on
the smallest surrounding rectangle of the intersection of the
domains of ImageT1 and ImageT2. The domain of
VectorField is the intersection of the two domains. Hence,
by specifying reduced domains for ImageT1 and
ImageT2, the processing can be focused and runtime can
potentially be saved. It should be noted, however, that all methods
compute a global solution of the optical flow. In particular, it
follows that the solution on a reduced domain need not (and cannot)
be identical to the resolution on the full domain restricted to the
reduced domain.
SmoothingSigma specifies the standard deviation of the
Gaussian kernel that is used to smooth both input images. The
larger the value of SmoothingSigma, the larger the low-pass
effect of the Gaussian kernel, i.e., the smoother the preprocessed
image. Usually, SmoothingSigma = 0.8 is a suitable
choice. However, other values in the interval [0,2] are also
possible. Larger standard deviations should only be considered if
the input images are very noisy. It should be noted that larger
values of SmoothingSigma lead to slightly longer execution
times.
IntegrationSigma specifies the standard deviation
of the Gaussian kernel that is
used for the local integration of the neighborhood information of
the data term. This parameter is used only in the CLG algorithm and
has no effect on the other two algorithms. Usually,
IntegrationSigma = 1.0 is a suitable choice. However,
other values in the interval [0,3] are also possible. Larger
standard deviations should only be considered if the input images
are very noisy. It should be noted that larger values of
IntegrationSigma lead to slightly longer execution times.
FlowSmoothness specifies the weight of
the smoothness term with respect to the data term. The larger the
value of FlowSmoothness, the smoother the computed optical
flow field. It should be noted that choosing
FlowSmoothness too small can lead to unusable results, even
though statistically robust penalty functions are used, in
particular if the warping strategy needs to predict too much
information outside of the image. For byte images with a gray value
range of [0,255], values of FlowSmoothness around 20 for
the flow-driven FDRIG algorithm and around 1000 for the data-driven
DDRAW algorithm and the homogeneous CLG algorithm typically yield
good results.
GradientConstancy specifies the weight
of the gradient constancy with respect to the gray value constancy.
This parameter is used only in the FDRIG algorithm. For the other
two algorithms, it does not influence the results. For byte images
with a gray value range of [0,255], a value of
GradientConstancy = 5 is typically a good choice, since
then both constancy assumptions are used to the same extent. For
large changes in illumination, however, significantly larger values
of GradientConstancy may be necessary to achieve good
results. It should be noted that for large values of the gradient
constancy weight the smoothness parameter FlowSmoothness
must also be chosen larger.
The parameters of the multigrid solver and for the coarse-to-fine
warping strategy can be specified with the generic parameters
MGParamName and MGParamValue. Usually, it
suffices to use one of the four default parameter sets via
MGParamName = 'default_parameters' and
MGParamValue = 'very_accurate',
'accurate', 'fast_accurate', or 'fast'.
The default parameter sets are described below. If the parameters
should be specified individually, MGParamName and
MGParamValue must be set to tuples of the same length. The
values corresponding to the parameters specified in
MGParamName must be specified at the corresponding position
in MGParamValue.
MGParamName = 'warp_zoom_factor' can be used to
specify the resolution ratio between two consecutive warping levels
in the coarse-to-fine warping hierarchy.
'warp_zoom_factor' must be selected from the open interval
(0,1). For performance reasons, 'warp_zoom_factor' is
typically set to 0.5, i.e., the number of pixels is halved in each
direction for each coarser warping level. This leads to an increase
of 33% in the calculations that need to be performed with respect
to an algorithm that does not use warping. Values for
'warp_zoom_factor' close to 1 can lead to slightly better
results. However, they require a disproportionately larger
computation time, e.g., 426% for 'warp_zoom_factor' =
0.9.
MGParamName = 'warp_levels' can be used to
restrict the warping hierarchy to a maximum number of levels. For
'warp_levels' = 0, the largest possible number of levels
is used. If the image size does not allow to use the specified
number of levels (taking the resolution ratio
'warp_zoom_factor' into account), the largest possible
number of levels is used. Usually, 'warp_levels' should be
set to 0.
MGParamName = 'warp_last_level' can be used to
specify the number of warping levels for which the flow increment
should no longer be computed. Usually,
'warp_last_level' is set to 1 or 2, i.e., a flow increment
is computed for each warping level, or the finest warping level is
skipped in the computation. Since in the latter case the
computation is performed on an image of half the resolution of the
original image, the gained computation time can be used to compute a
more accurate solution, e.g., by using a full multigrid algorithm
with additional iterations. The more accurate solution is then
interpolated to the full resolution.
The three parameters that specify the coarse-to-fine warping strategy are only used in the FDRIG and DDRAW algorithms. They are ignored for the CLG algorithm.
MGParamName = 'mg_solver' can be used to specify
the general multigrid strategy for solving the (non)linear equation
system (in each warping level). For 'mg_solver' =
'multigrid', a normal multigrid algorithm (without
coarse-to-fine initialization) is used, while for
'mg_solver' = 'full_multigrid' a full multigrid
algorithm (with coarse-to-fine initialization) is used.
Since a resolution reduction of 0.5 is used between two consecutive
levels of the coarse-to-fine initialization (in contrast to the
resolution reduction in the warping strategy, this value is
hard-coded into the algorithm), the use of a full multigrid
algorithm results in an increase of the computation time by
approximately 33% with respect to the normal multigrid algorithm.
Using 'mg_solver' to 'full_multigrid' typically
yields numerically more accurate results than 'mg_solver'
= 'multigrid'.
MGParamName = 'mg_cycle_type' can be used to
specify whether a V or W correction cycle is used per multigrid
level. Since a resolution reduction of 0.5 is used between two
consecutive levels of the respective correction cycle, using a W
cycle instead of a V cycle increases the computation time by
approximately 50%. Using 'mg_cycle_type' = 'w'
typically yields numerically more accurate results than
'mg_cycle_type' = 'v'.
MGParamName = 'mg_levels' can be used to
restrict the multigrid hierarchy for the coarse-to-fine
initialization as well as for the actual V or W correction cycles.
For 'mg_levels' = 0, the largest possible number of
levels is used. If the image size does not allow to use the
specified number of levels, the largest possible number of levels is
used. Usually, 'mg_levels' should be set to 0.
MGParamName = 'mg_cycles' can be used to specify
the total number of V or W correction cycles that are being
performed. If a full multigrid algorithm is used,
'mg_cycles' refers to each level of the
coarse-to-fine initialization. Usually, one or two cycles are
sufficient to yield a sufficiently accurate solution of the equation
system. Typically, the larger 'mg_cycles', the more
accurate the numerical results. This parameter enters almost
linearly into the computation time, i.e., doubling the number of
cycles leads approximately to twice the computation time.
MGParamName = 'mg_pre_relax' can be used to
specify the number of iterations that are performed on each level of
the V or W correction cycles using the iterative basic solver
before the actual error correction is performed. Usually,
one or two pre-relaxation steps are sufficient. Typically, the
larger 'mg_pre_relax', the more accurate the numerical
results.
MGParamName = 'mg_post_relax' can be used to
specify the number of iterations that are performed on each level of
the V or W correction cycles using the iterative basic solver
after the actual error correction is performed. Usually, one
or two post-relaxation steps are sufficient. Typically, the larger
'mg_post_relax', the more accurate the numerical results.
Like when increasing the number of correction cycles, increasing the number of pre- and post-relaxation steps increases the computation time asymptotically linearly. However, no additional restriction and prolongation operations (zooming down and up of the error correction images) are performed. Consequently, a moderate increase in the number of relaxation steps only leads to a slight increase in the computation times.
MGParamName = 'mg_inner_iter' can be used to
specify the number of iterations to solve the linear equation
systems in each fixed-point iteration of the nonlinear basic solver.
Usually, one iteration is sufficient to achieve a sufficient
convergence speed of the multigrid algorithm. The increase in
computation time is slightly smaller than for the increase in the
relaxation steps. This parameter only influences the FDRIG and
DDRAW algorithms since for the CLG algorithm no nonlinear equation
system needs to be solved.
MGParamName = 'parallel_solver' can be used to
enable a parallel processing of the FDRIG algorithm.
This changes the results of the operator slightly, but can allow
for a faster processing.
The value can be set to 'true' or 'false' (default).
As described above, usually it is sufficient to use one of the
default parameter sets for the parameters described above by using
MGParamName = 'default_parameters' and
MGParamValue = 'very_accurate',
'accurate', 'fast_accurate', or 'fast'.
If necessary, individual parameters can be modified after the
default parameter set has been chosen by specifying a subset of the
above parameters and corresponding values after
'default_parameters' in MGParamName and
MGParamValue (e.g., MGParamName =
['default_parameters','warp_zoom_factor'] and
MGParamValue = ['accurate',0.6]).
The default parameter sets use the following values for the above parameters:
'default_parameters' = 'very_accurate': 'warp_zoom_factor' = 0.5, 'warp_levels' = 0, 'warp_last_level' = 1, 'mg_solver' = 'full_multigrid', 'mg_cycle_type' = 'w', 'mg_levels' = 0, 'mg_cycles' = 1, 'mg_pre_relax' = 2, 'mg_post_relax' = 2, 'mg_inner_iter' = 1.
'default_parameters' = 'accurate': 'warp_zoom_factor' = 0.5, 'warp_levels' = 0, 'warp_last_level' = 1, 'mg_solver' = 'multigrid', 'mg_cycle_type' = 'v', 'mg_levels' = 0, 'mg_cycles' = 1, 'mg_pre_relax' = 1, 'mg_post_relax' = 1, 'mg_inner_iter' = 1.
'default_parameters' = 'fast_accurate': 'warp_zoom_factor' = 0.5, 'warp_levels' = 0, 'warp_last_level' = 2, 'mg_solver' = 'full_multigrid', 'mg_cycle_type' = 'w', 'mg_levels' = 0, 'mg_cycles' = 1, 'mg_pre_relax' = 2, 'mg_post_relax' = 2, 'mg_inner_iter' = 1.
'default_parameters' = 'fast': 'warp_zoom_factor' = 0.5, 'warp_levels' = 0, 'warp_last_level' = 2, 'mg_solver' = 'multigrid', 'mg_cycle_type' = 'v', 'mg_levels' = 0, 'mg_cycles' = 1, 'mg_pre_relax' = 1, 'mg_post_relax' = 1, 'mg_inner_iter' = 1.
It should be noted that for the CLG algorithm the two modes 'fast_accurate' and 'fast' are identical to the modes 'very_accurate' and 'accurate' since the CLG algorithm does not use a coarse-to-fine warping strategy.
ImageT1 (input_object) singlechannelimage(-array) → object (byte / uint2 / real)
Input image 1.
ImageT2 (input_object) singlechannelimage(-array) → object (byte / uint2 / real)
Input image 2.
VectorField (output_object) singlechannelimage(-array) → object (vector_field)
Optical flow.
Algorithm (input_control) string → (string)
Algorithm for computing the optical flow.
Default: 'fdrig'
List of values: 'clg', 'ddraw', 'fdrig'
SmoothingSigma (input_control) real → (real)
Standard deviation for initial Gaussian smoothing.
Default: 0.8
Suggested values: 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0
Restriction:
SmoothingSigma >= 0.0
IntegrationSigma (input_control) real → (real)
Standard deviation of the integration filter.
Default: 1.0
Suggested values: 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0
Restriction:
IntegrationSigma >= 0.0
FlowSmoothness (input_control) real → (real)
Weight of the smoothing term relative to the data term.
Default: 20.0
Suggested values: 10.0, 20.0, 30.0, 50.0, 70.0, 100.0, 200.0, 300.0, 500.0, 700.0, 1000.0, 1500.0, 2000.0
Restriction:
FlowSmoothness >= 0.0
GradientConstancy (input_control) real → (real)
Weight of the gradient constancy relative to the gray value constancy.
Default: 5.0
Suggested values: 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 15.0, 20.0, 30.0, 40.0, 50.0, 70.0, 100.0
Restriction:
GradientConstancy >= 0.0
MGParamName (input_control) attribute.name(-array) → (string)
Parameter name(s) for the multigrid algorithm.
Default: 'default_parameters'
List of values: 'default_parameters', 'mg_cycle_type', 'mg_cycles', 'mg_inner_iter', 'mg_levels', 'mg_post_relax', 'mg_pre_relax', 'mg_solver', 'parallel_solver', 'warp_last_level', 'warp_levels', 'warp_zoom_factor'
MGParamValue (input_control) attribute.value(-array) → (string / real / integer)
Parameter value(s) for the multigrid algorithm.
Default: 'accurate'
Suggested values: 'very_accurate', 'accurate', 'fast_accurate', 'fast', 'multigrid', 'full_multigrid', 'v', 'w', 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 'true', 'false'
grab_image (ImageT1, AcqHandle)
while (true)
grab_image (ImageT2, AcqHandle)
optical_flow_mg (ImageT1, ImageT2, VectorField, 'fdrig', 0.8, 1, 10, \
5, 'default_parameters', 'accurate')
threshold (VectorField, Region, 1, 10000)
copy_obj (ImageT2, ImageT1, 1, 1)
endwhile
If the parameter values are correct, the operator
optical_flow_mg returns the value 2 (
H_MSG_TRUE)
. If the input is
empty (no input images are available) the behavior can be set via
set_system('no_object_result',<Result>). If necessary, an
exception is raised.
threshold,
vector_field_length
T. Brox, A. Bruhn, N. Papenberg, and J. Weickert: High accuracy
optical flow estimation based on a theory for warping. In T. Pajdla
and J. Matas, editors, Computer Vision - ECCV 2004, volume 3024 of
Lecture Notes in Computer Science, pages 25--36. Springer, Berlin,
2004.
A. Bruhn, J. Weickert, C. Feddern, T. Kohlberger, and C. Schnörr:
Variational optical flow computation in real-time. IEEE
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