find_planar_uncalib_deformable_model — Find the best matches of a planar projective invariant deformable model
in an image.
find_planar_uncalib_deformable_model(Image : : ModelID, AngleStart, AngleExtent, ScaleRMin, ScaleRMax, ScaleCMin, ScaleCMax, MinScore, NumMatches, MaxOverlap, NumLevels, Greediness, GenParamName, GenParamValue : HomMat2D, Score)
The operator find_planar_uncalib_deformable_model finds the best
NumMatches instances of the perspectively distorted deformable
model ModelID in the input image Image. The
model must have been created previously by calling
create_planar_uncalib_deformable_model or
read_deformable_model.
The domain of the image Image determines the search space
for the reference point of the model, i.e., for the center of
gravity of the domain (region) of the image that was used to create
the deformable model with create_planar_uncalib_deformable_model.
A different origin set with set_deformable_model_origin is not taken
into account. The model is searched within those points of the domain of
the image, in which the model lies completely within the image.
This means that the model will not be found if it extends beyond the
borders of the image, even if it would achieve a score greater than
MinScore (see below).
Note that, if for a certain pyramid level the model touches the image
border, it might not be found
even if it lies completely within the original image.
As a rule of thumb, the model might not be found if its distance to
an image border falls below .
This behavior can be changed with
set_system('border_shape_models','true'), which will cause
models that extend beyond the image border to be found if they
achieve a score greater than MinScore. Here, points lying
outside the image are regarded as being occluded, i.e., they lower
the score. It should be noted that the runtime of the search will
increase in this mode.
Note further, that in rare cases,
which occur typically only for artificial images, the model might not be
found also if for certain pyramid levels the model touches the border
of the reduced domain. Then, it may help to enlarge the reduced
domain by using,
e.g., dilation_circle.
The parameters AngleStart, AngleExtent,
ScaleRMin, ScaleRMax, ScaleCMin and
ScaleCMax are used to specify a basic range of up to an anisotropic
transformation that is exhaustively searched on the top level of the image
pyramid. The parameters AngleStart and AngleExtent
determine the range of possible rotations in which the model is exhaustively
searched. ScaleRMin, ScaleRMax,
ScaleCMin, and ScaleCMax determine the range of
possible anisotropic scales that are exhaustively searched in the image.
A scale of 1 in both scale factors corresponds to the
original size of the model.
The operator find_planar_uncalib_deformable_model may find
objects outside this range, e.g., when the object is perspectively
distorted. Hence, the range parameters are a kind of suggestion for the
search algorithm. Starting from this, certain models in a wider range of
transformations can be detected, depending on the used pyramid levels, but
also on the model/image content. It is important to note that, e.g., small
scale changes can be tolerated without the need to specify a scale range,
leading to faster execution times.
Often, it is not necessary to use an anisotropic scaling to find the object
on the top level of the pyramid. In these cases, ScaleCMin and
ScaleCMax should be set to 1.
The search is then performed with isotropic scaling only, which is much faster.
If the object should be detected despite severe perspective distortions
anisotropic scaling is required. Here, ScaleRMin and
ScaleRMax specify the anisotropic scaling in row,
ScaleCMin and ScaleCMax in column direction.
Note that the transformations are treated internally such that the scalings are applied first, followed by the rotation. Therefore, the model should usually be aligned such that it appears horizontally or vertically in the model image.
Additionally, the operator find_planar_uncalib_deformable_model
processes the parameters 'angle_step', 'scale_r_step' and
'scale_c_step' which can be set with the operator
create_planar_uncalib_deformable_model or, as described below,
with the generic parameters GenParamName and GenParamValue.
In most cases, the values that can be determined automatically by
create_planar_uncalib_deformable_model lead to good results.
The parameter 'angle_step' determines the step
size within the selected range of angles. 'angle_step' should be
chosen based on the size of the object. Smaller models do not have many
different discrete rotations in the image, and hence 'angle_step'
should be chosen larger for smaller models. If AngleExtent
is not an integer multiple of 'angle_step', 'angle_step' is
modified accordingly. The parameters 'scale_r_step' and 'scale_c_step' determine the
step size within the selected range of scales. Like 'angle_step',
'scale_r_step' and 'scale_c_step' should be chosen based on the
size of the object.
If the respective range of scales is not an integer multiple of
'scale_r_step' and 'scale_c_step', 'scale_r_step' and
'scale_c_step' are modified accordingly.
The parameter MinScore determines what score a potential
match must at least have to be regarded as an instance of the model
in the image. The larger MinScore is chosen, the faster
the search is. If the model can be expected never to be occluded in
the images, MinScore may be set as high as 0.8 or even 0.9.
The maximum number of instances to be found can be determined with
NumMatches. If more than NumMatches instances
with a score greater than MinScore are found in the image,
only the best NumMatches instances are returned. If fewer
than NumMatches are found, only that number is returned,
i.e., the parameter MinScore takes precedence over
NumMatches. If all model instances exceeding
MinScore in the image should be found, NumMatches
must be set to 0.
In rare cases, NumMatches must be set to a
higher value than the required number of matches. This is
the case if, for instance, a small MinScore is set.
When tracking the matches through the image pyramid, on each level,
some less promising matches are rejected based on NumMatches. Thus,
it is possible that some matches are rejected that would have had a higher
score on the lowest pyramid level. Due to this, for example, the found
match for NumMatches set to 1 might be
different from the match with the highest score returned when setting
NumMatches to 0 or > 1.
If multiple objects with a similar score are expected, but only the one with
the highest score should be returned, it might be preferable to raise
NumMatches, and then select the match with the highest score.
If the model exhibits symmetries it may happen that multiple
instances with similar positions but different rotations are found
in the image. The parameter MaxOverlap determines by what
fraction (i.e., a number between 0 and 1) two instances may at most
overlap in order to consider them as different instances, and hence
to be returned separately. If two instances overlap each other by
more than MaxOverlap only the best instance is returned.
The calculation of the overlap is based on the smallest enclosing
rectangle of arbitrary orientation (see smallest_rectangle2)
of the found instances. If MaxOverlap=0, the found
instances may not overlap at all, while for MaxOverlap=1
all instances are returned.
With the generic parameters GenParamName and GenParamValue it is
possible to adjust parameters that typically do not have to be set by the
user. By default the pose is extracted with high subpixel accuracy
('least_squares_very_high') through a least-squares
adjustment, i.e., by minimizing the distances of the model points to
their corresponding image points. However, if no high accuracy is required
by an application, the subpixel precise extraction can be reduced or
switched off as it increases the processing time. Here,
'subpixel' must be passed in GenParamName and 'none',
'least_squares', 'least_squares_high' for
GenParamValue. A further use of GenParamName and
GenParamValue is to override the discretization steps of the search
space 'angle_step', 'scale_r_step' and 'scale_c_step' that have been defined
when the model was created in create_planar_uncalib_deformable_model.
As described in create_planar_uncalib_deformable_model the
deformable matching algorithm searches exhaustively a basic set of
parameters that are specified with AngleStart,AngleExtent,
ScaleRMin,ScaleRMax,ScaleCMin and
ScaleCMax. However, to allow a detection even when the object is
imaged under perspective distortion, an additional transformation is
estimated. This additional transformation transforms the model from the
original search range to a bigger perspectively distorted one.
By allowing perspective distortions, the risk of false positives is also
increased. One possible use of the parameter GenParamName is to help
discarding false positives that occur, if for instance a small score was
specified in MinScore and the image contains significant clutter
with similar shape as the model.
To restrict arbitrary perspective matches from occurring, the values
'angle_change_restriction' and 'aniso_scale_change_restriction'
can be used in GenParamName.
With 'angle_change_restriction' the maximal tolerated angular distortion
can be restricted. As default value is set, which
allows arbitrary distortion. By setting 'angle_change_restriction'
to 0, no distortion is allowed at all. The set value should be within the
interval [0, ].
This parameter tests, if the angle of 90 degree at the corners of the
axis-aligned rectangle around the model points is changed by more than the
corresponding GenParamValue for the found instance of the model.
Note that this parameter helps to restrict both affine (a shear mapping) and
perspective parts of the transformation.
As an example, with 'angle_change_restriction' a square-like model can
be prevented to match with a parallelogram or an arbitrary trapezium.
With the parameter 'aniso_scale_change_restriction' the ratio of anisotropic scaling can be restricted (the smaller scale factor divided by the bigger scale factor). The value of this parameter ranges from the default value 0.0, where arbitrary distortion is allowed, to 1.0, where no distortion is allowed. One typical use for this parameter is to restrict for instance a square-like model to deform to a rectangular model.
The number of pyramid levels used during the search is determined
with NumLevels. If necessary, the number of levels is
clipped to the range given when the deformable model was created with
create_planar_uncalib_deformable_model. If NumLevels is set
to 0, the number of pyramid levels specified in
create_planar_uncalib_deformable_model is used.
The parameter Greediness determines how “greedily” the
search should be carried out. If Greediness=0, a safe
search heuristic is used, which finds the model if it is
visible in the image and the other parameters are set appropriately.
However, the search will be relatively time consuming in this case. If
Greediness=1, an unsafe search heuristic is used, which may cause
the model not to be found in rare cases, even though it is visible in the
image. For Greediness=1, the maximum search speed is achieved.
In almost all cases, the deformable model will be found for
Greediness=0.9.
The projective transformation (homographies) that encode the position of the
found instances of the model are returned in HomMat2D. In case that
multiple objects are found, the different homographies are concatenated. A
single homography can easily be extracted by tuple_select_range(HomMat2D,Index*9,(Index+1)*9-1,
SelectedHomMat2D). The different detection results are sorted in decreasing
order of Score.
The row and column coordinates are
the coordinates of the origin of the deformable model in the search image,
which can be found by calling
projective_trans_pixel(HomMat2D,0,0,Row,Column). By default, the
origin is the center of gravity of the domain (region) of the image that was
used to create the deformable model with
create_planar_uncalib_deformable_model. A different origin
can be set with set_deformable_model_origin.
For visualization purposes, the model contours that are extracted by
get_deformable_model_contours can be projected to the
found location given HomMat2D with
projective_trans_contour_xld.
Additionally, the score of each found instance is returned in Score.
The score is a number between 0 and 1, which is an approximate measure of how
much of the model is visible in the image. If, for example, half of the
model is occluded, the score cannot exceed 0.5.
Image (input_object) (multichannel-)image → object (byte / uint2)
Input image in which the model should be found.
ModelID (input_control) deformable_model → (handle)
Handle of the model.
AngleStart (input_control) angle.rad → (real)
Smallest rotation of the model.
Default value: -0.39
Suggested values: -3.14, -1.57, -0.79, -0.39, -0.20, 0.0
AngleExtent (input_control) angle.rad → (real)
Extent of the rotation angles.
Default value: 0.78
Suggested values: 6.29, 3.14, 1.57, 0.79, 0.39, 0.0
Restriction: AngleExtent >= 0
ScaleRMin (input_control) number → (real)
Minimum scale of the model in row direction.
Default value: 1.0
Suggested values: 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
Restriction: ScaleRMin > 0
ScaleRMax (input_control) number → (real)
Maximum scale of the model in row direction.
Default value: 1.0
Suggested values: 1.0, 1.1, 1.2, 1.3, 1.4, 1.5
Restriction: ScaleRMax >= ScaleRMin
ScaleCMin (input_control) number → (real)
Minimum scale of the model in column direction.
Default value: 1.0
Suggested values: 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
Restriction: ScaleCMin > 0
ScaleCMax (input_control) number → (real)
Maximum scale of the model in column direction.
Default value: 1.0
Suggested values: 1.0, 1.1, 1.2, 1.3, 1.4, 1.5
Restriction: ScaleCMax >= ScaleCMin
MinScore (input_control) real → (real)
Minimum score of the instances of the model to be found.
Default value: 0.5
Suggested values: 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
Typical range of values: 0
≤
MinScore
≤
1
Minimum increment: 0.01
Recommended increment: 0.05
NumMatches (input_control) integer → (integer)
Number of instances of the model to be found (or 0 for all matches).
Default value: 1
Suggested values: 0, 1, 2, 3, 4, 5, 10, 20
MaxOverlap (input_control) real → (real)
Maximum overlap of the instances of the model to be found.
Default value: 1.0
Suggested values: 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
Typical range of values: 0
≤
MaxOverlap
≤
1
Minimum increment: 0.01
Recommended increment: 0.05
NumLevels (input_control) integer(-array) → (integer)
Number of pyramid levels used in the matching
(and lowest pyramid level to use if
|NumLevels| = 2).
Default value: 0
List of values: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Greediness (input_control) real → (real)
“Greediness” of the search heuristic (0: safe but slow; 1: fast but matches may be missed).
Default value: 0.9
Suggested values: 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
Typical range of values: 0
≤
Greediness
≤
1
Minimum increment: 0.01
Recommended increment: 0.05
GenParamName (input_control) string-array → (string)
The general parameter names.
Default value: []
List of values: [], 'angle_change_restriction', 'angle_step', 'aniso_scale_change_restriction', 'scale_c_step', 'scale_r_step', 'subpixel'
GenParamValue (input_control) integer-array → (integer / real / string)
Values of the general parameters.
Default value: []
List of values: [], 'least_squares', 'least_squares_high', 'least_squares_very_high', 'none'
HomMat2D (output_control) hom_mat2d(-array) → (real)
Homographies between model and found instances.
Score (output_control) real-array → (real)
Score of the found instances of the model.
If the parameters are valid, the operator
find_planar_uncalib_deformable_model returns the value TRUE. If
necessary an exception is raised.
create_planar_uncalib_deformable_model,
read_deformable_model
find_planar_calib_deformable_model
Matching