Operator Reference
find_shape_model (Operator)
find_shape_model — Find the best matches of a shape model in an image.
Signature
find_shape_model(Image : : ModelID, AngleStart, AngleExtent, MinScore, NumMatches, MaxOverlap, SubPixel, NumLevels, Greediness : Row, Column, Angle, Score)
Description
The operator find_shape_model finds the best
NumMatches instances of the shape model ModelID in
the input image Image. The model must have been created
previously by calling create_shape_model or
read_shape_model.
The position and rotation of the found instances of the model is
returned in Row, Column, and Angle.
Additionally, the score of each found instance is returned in
Score.
Input parameters in detail
Imageand its domain:-
The domain of the image
Imagedetermines 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 shape model withcreate_shape_model. A different origin set withset_shape_model_originis 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 thanMinScore(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 withset_system('border_shape_models','true')for all models or withset_shape_model_param(ModelID,'border_shape_models','true')for a specific model, which will cause models that extend beyond the image border to be found if they achieve a score greater thanMinScore. 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. AngleStartandAngleExtent:-
The parameters
AngleStartandAngleExtentdetermine the range of rotations for which the model is searched. If necessary, the range of rotations is clipped to the range given when the model was created withcreate_shape_model. In particular, this means that the angle ranges of the model and the search must overlap.Note that in some cases instances with a rotation that is slightly outside the specified range are found. This may happen if the specified range is smaller than the range given during the creation of the model.
AngleStartandAngleExtentare checked only at the highest pyramid level. Matches that are found on the highest pyramid level are refined to the lowest pyramid level. For performance reasons, however, during the refinement it is no longer checked whether the matches are still within the specified range. MinScore:-
The parameter
MinScoredetermines what score a potential match must at least have to be regarded as an instance of the model in the image. The largerMinScoreis chosen, the faster the search is. If the model can be expected never to be occluded in the images,MinScoremay be set as high as 0.8 or even 0.9. If the matches are not tracked to the lowest pyramid level (see below) it might happen that instances with a score slightly belowMinScoreare found.In case that the shape model has been extended by clutter parameters with
set_shape_model_clutterand thus 'use_clutter' is enabled,MinScoreexpects a second value which determines what clutter score a potential match must at most have to be regarded as an instance of the model in the image. The runtime using clutter parameters will be at least as high as the runtime without clutter parameters andNumMatchesset to 0. Changing this second value does not influence the runtime. NumMatches:-
The maximum number of instances to be found can be determined with
NumMatches. If more thanNumMatchesinstances with a score greater thanMinScoreare found in the image, only the bestNumMatchesinstances are returned. If fewer thanNumMatchesare found, only that number is returned, i.e., the parameterMinScoretakes precedence overNumMatches. If all model instances exceedingMinScorein the image should be found,NumMatchesmust be set to 0.When tracking the matches through the image pyramid, on each level (except the top 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 forNumMatchesset to 1 might be different from the match with the highest score returned when settingNumMatchesto 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.In case that the shape model has been extended by clutter parameters using
set_shape_model_clutter,NumMatchesalso considers the second value passed inMinScore. Matched instances will be in this case instances with a score greater than the first entry ofMinScoreand a clutter score smaller than its second entry. If the number of matched instances in the image is larger thanNumMatches, the bestNumMatchesinstances with respect to clutter scores are returned. Still,MinScoretakes precedence overNumMatchesandNumMatchesmust be set to 0 if all model instances fulfilling the conditions imposed byMinScoreshould be returned. Please note that using clutter parameters, when tracking the matches through the image pyramid, no matches are rejected. Thus the runtime using clutter parameters will be at least as high as the runtime without clutter parameters andNumMatchesset to 0. MaxOverlap:-
If the model exhibits symmetries it may happen that multiple instances with similar positions but different rotations are found in the image. The parameter
MaxOverlapdetermines 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 thanMaxOverlaponly the best instance is returned. The calculation of the overlap is based on the smallest enclosing rectangle of arbitrary orientation (seesmallest_rectangle2) of the found instances. IfMaxOverlap=0, the found instances may not overlap at all, while forMaxOverlap=1 all instances are returned. SubPixel:-
The parameter
SubPixeldetermines whether the instances should be extracted with subpixel accuracy. IfSubPixelis set to 'none' (or 'false' for backwards compatibility) the model's pose is only determined with pixel accuracy and the angle resolution that was specified withcreate_shape_model. IfSubPixelis set to 'interpolation' (or 'true') the position as well as the rotation are determined with subpixel accuracy. In this mode, the model's pose is interpolated from the score function. This mode costs almost no computation time and achieves an accuracy that is high enough for most applications. In some applications, however, the accuracy requirements are extremely high. In these cases, the model's pose can be determined through a least-squares adjustment, i.e., by minimizing the distances of the model points to their corresponding image points. In contrast to 'interpolation', this mode requires additional computation time. The different modes for least-squares adjustment ('least_squares', 'least_squares_high', and 'least_squares_very_high') can be used to determine the accuracy with which the minimum distance is being searched. The higher the accuracy is chosen, the longer the subpixel extraction will take, however. Usually,SubPixelshould be set to 'interpolation'. If least-squares adjustment is desired, 'least_squares' should be chosen because this results in the best trade-off between runtime and accuracy.Objects that are slightly deformed with respect to the model, in some cases cannot be found or are found but only with a low accuracy. For such objects it is possible to additionally pass a maximal allowable object deformation in the parameter
SubPixel. The deformation must be specified in pixels. This can be done by passing the optional parameter value 'max_deformation ' followed by an integer value between 0 and 32 (in the same string), which specifies the maximum deformation. For example, if the shape of the object may be deformed by up to 2 pixels with respect to the shape that is stored in the model, the value 'max_deformation 2' must be passed inSubPixelin addition to the above described mode for the subpixel extraction, i.e., for example ['least_squares', 'max_deformation 2']. Passing the value 'max_deformation 0' corresponds to a search without allowing deformations, i.e., the behavior is the same as if no 'max_deformation ' is passed. Note that higher values for the maximum deformation often result in an increased runtime. Furthermore, the higher the deformation value is chosen, the higher is the risk of finding wrong model instances. Both problems mainly arise when searching for small objects or for objects with fine structures. This is because such kinds of objects for higher deformations lose their characteristic shape, which is important for a robust search. Also note that for higher deformations the accuracy of partially occluded objects might decrease if clutter is present close to the object. Consequently, the maximum deformation should be chosen as small as possible and only as high as necessary. Approximately rotationally symmetric objects may not be found if 'max_deformation' andAngleExtentare both set to a value greater than 0. In that case, ambiguities may occur that cannot be resolved, and the match is rejected as false. If this happens, try to set either 'max_deformation' orAngleExtentto 0, or adjust the model such that symmetries are reduced. When specifying a deformation higher than 0 the computation of the score depends on the chosen value for the subpixel extraction. In most cases, the score of a match changes if 'least_squares', 'least_squares_high', or 'least_squares_very_high' (see above) is chosen for the subpixel extraction (in comparison to 'none' or 'interpolation'). Furthermore, if one of the least-squares adjustments is selected the score might increase when increasing the maximum deformation because then for the model points more corresponding image points can be found. To get a meaningful score value and to avoid erroneous matches, we recommend to always combine the allowance of a deformation with a least-squares adjustment. NumLevels:-
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 shape model was created withcreate_shape_model. IfNumLevelsis set to 0, the number of pyramid levels specified increate_shape_modelis used.In certain cases, the number of pyramid levels that was determined automatically with, for example,
create_shape_modelmay be too high. The consequence may be that some matches that may have a high final score are rejected on the highest pyramid level and thus are not found. Instead of settingMinScoreto a very low value to find all matches, it may be better to query the value ofNumLevelswithget_shape_model_paramsand then use a slightly lower value infind_shape_model. This approach is often better regarding the speed and robustness of the matching.Optionally,
NumLevelscan contain a second value that determines the lowest pyramid level to which the found matches are tracked. Hence, a value of [4,2] forNumLevelsmeans that the matching starts at the fourth pyramid level and tracks the matches to the second lowest pyramid level (the lowest pyramid level is denoted by a value of 1). This mechanism can be used to decrease the runtime of the matching. It should be noted, however, that in general the accuracy of the extracted pose parameters is lower in this mode than in the normal mode, in which the matches are tracked to the lowest pyramid level. Hence, if a high accuracy is desired,SubPixelshould be set to at least 'least_squares'. If the lowest pyramid level to use is chosen too large, it may happen that the desired accuracy cannot be achieved, or that wrong instances of the model are found because the model is not specific enough on the higher pyramid levels to facilitate a reliable selection of the correct instance of the model. In this case, the lowest pyramid level to use must be set to a smaller value.In input images of poor quality, i.e., in images that are, e.g., defocused, deformed, or noisy, often no instances of the shape model can be found on the lowest pyramid level. The reason for this behavior is the missing or deformed edge information which is a result of the poor image quality. Nevertheless, the edge information may be sufficient on higher pyramid levels. But keep in mind the above mentioned restrictions on accuracy and robustness if instances that were found on higher pyramid levels are used. The selection of the suitable pyramid level, i.e., the lowest pyramid level on which at least one instance of the shape model can be found, depends on the model and on the input image. This pyramid level may vary from image to image. To facilitate the matching on images of poor quality, the lowest pyramid level on which at least one instance of the model can be found can be determined automatically during the matching. To activate this mechanism, i.e., to use the so-called 'increased tolerance mode', the lowest pyramid level must be specified negatively in
NumLevels. If, e.g.,NumLevelsis set to [4,-2], the matching starts at the fourth pyramid level and tracks the matches to the second lowest pyramid level. This means that an instance of the shape model is searched on the pyramid level 2. If no instance of the model can be found on this pyramid level, the lowest pyramid level is determined on which at least one instance of the model can be found. The instances of this pyramid level will then be returned.If the
ModelIDwas adapted withadapt_shape_model_high_noisethe estimated lowest pyramid level will be used by default. However, the user can override the estimated lowest pyramid level by providing two values toNumLevelsand explicitly setting the lowest pyramid level. Greediness:The parameter
Greedinessdetermines how “greedily” the search should be carried out. IfGreediness=0, a safe search heuristic is used, which always 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. IfGreediness=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. ForGreediness=1, the maximum search speed is achieved. In almost all cases, the shape model will always be found forGreediness=0.9.
Output parameters in detail
Row,Column, andAngle:-
The position and rotation of the found instances of the model is returned in
Row,Column, andAngle. The coordinatesRowandColumnare related to the position of the origin of the shape model in the search image. However,RowandColumndo not exactly correspond to this position. Instead,find_shape_modelreturns slightly modified values that are optimized for creating a transformation matrix, that can be used for alignment or visualization of the model contours. (This has to do with the way HALCON transforms iconic objects, seeaffine_trans_pixel). The example below shows how to create the transformation matrix for alignment of the found matches and how to use it to display them in the search image.By default, the model origin is the center of gravity of the domain (region) of the image that was used to create the shape model with
create_aniso_shape_model. A different origin can be set withset_shape_model_origin. Score:-
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.In case that the shape model has been extended by clutter parameters using
set_shape_model_clutter, following the above valuesScorealso returns the clutter score of each found instance. If, for example, half of the clutter region is filled by clutter edges, the clutter score will equal 0.5. If, e.g., two instances are found, the score is 0.9 for the first instance and 0.8 for the second instance, and the clutter score is 0.2 for the first instance and 0.1 for the second instance,Score= [0.9,0.8,0.2,0.1] is returned. Please note that of all shape-based matching results, clutter scores are affected the most when a variation of illumination occurs.
Specifying a timeout
Using the operator set_shape_model_param you can specify a
'timeout' for find_shape_model. If find_shape_model
reaches this 'timeout', it terminates without results and
returns the error code 9400 (H_ERR_TIMEOUT).
Visualization of the results
To display the results found by shape-based matching,
we highly recommend the usage of the procedure
dev_display_shape_matching_results.
Further Information
For an explanation of the different 2D coordinate systems used in HALCON, see the introduction of chapter Transformations / 2D Transformations.
Execution Information
- Multithreading type: reentrant (runs in parallel with non-exclusive operators).
- Multithreading scope: global (may be called from any thread).
- Automatically parallelized on internal data level.
This operator supports canceling timeouts and interrupts.
Parameters
Image (input_object) (multichannel-)image → object (byte / uint2)
Input image in which the model should be found.
ModelID (input_control) shape_model → (handle)
Handle of the model.
AngleStart (input_control) angle.rad → (real)
Smallest rotation of the model.
Default: -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: 0.79
Suggested values: 6.29, 3.14, 1.57, 0.79, 0.39, 0.0
Restriction:
AngleExtent >= 0
MinScore (input_control) real(-array) → (real)
Minimum score of the instances of the model to be found.
Default: 0.5
Suggested values: 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
Value range:
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: 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: 0.5
Suggested values: 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
Value range:
0
≤
MaxOverlap
≤
1
Minimum increment: 0.01
Recommended increment: 0.05
SubPixel (input_control) string(-array) → (string)
Subpixel accuracy if not equal to 'none'.
Default: 'least_squares'
Suggested values: 'none', 'interpolation', 'least_squares', 'least_squares_high', 'least_squares_very_high', 'max_deformation 1', 'max_deformation 2', 'max_deformation 3', 'max_deformation 4', 'max_deformation 5', 'max_deformation 6'
NumLevels (input_control) integer(-array) → (integer)
Number of pyramid levels used in the matching
(and lowest pyramid level to use if
|NumLevels| = 2).
Default: 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: 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
Value range:
0
≤
Greediness
≤
1
Minimum increment: 0.01
Recommended increment: 0.05
Row (output_control) point.y-array → (real)
Row coordinate of the found instances of the model.
Column (output_control) point.x-array → (real)
Column coordinate of the found instances of the model.
Angle (output_control) angle.rad-array → (real)
Rotation angle of the found instances of the model.
Score (output_control) real-array → (real)
Score of the found instances of the model.
Example (HDevelop)
create_shape_model (ImageReduced, 0, rad(-45), rad(180), 0, \
'none', 'use_polarity', 30, 10, ModelID)
get_shape_model_contours (ModelXLD, ModelID, 1)
find_shape_model (SearchImage, ModelID, rad(-45), rad(180), \
0.5, 1, 0.5, 'interpolation', \
0, 0, Row, Column, Angle, Score)
* Create transformation matrix
vector_angle_to_rigid (0, 0, 0, Row, Column, Angle, HomMat2DObject)
* Display results
dev_display_shape_matching_results (ModelID, 'red', Row, Column, Angle, \
1, 1, 0)
Result
If the parameter values are correct, the operator
find_shape_model 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.
Possible Predecessors
create_shape_model,
read_shape_model,
set_shape_model_origin,
set_shape_model_clutter
Possible Successors
Alternatives
See also
set_system,
get_system,
set_shape_model_param
Module
Matching