find_ncc_model — Find the best matches of an NCC model in an image.
The operator find_ncc_model finds the best NumMatches instances of the NCC model ModelID in the input image Image. The model must have been created previously by calling create_ncc_model or read_ncc_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.
It should be noted that the NCC is very sensitive to occlusion and clutter as well as to nonlinear illumination changes in the image. If a model should be found in the presence of occlusion, clutter, or nonlinear illumination changes the search should be performed using the shape-based matching (see, e.g., create_shape_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 NCC model with create_ncc_model. A different origin set with set_ncc_model_origin is not taken into account here. 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).
The parameters AngleStart and AngleExtent determine 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 with create_ncc_model.
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.
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.
If the model exhibits symmetries it may happen that multiple instances with similar positions but different rotations are found in the image. If the model has repeating structures it may happen that multiple instances with identical rotations are found at similar positions 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.
The parameter SubPixel determines whether the instances should be extracted with subpixel accuracy. If SubPixel is set to 'false', the model's pose is only determined with pixel accuracy and the angle resolution that was specified with create_ncc_model. If SubPixel is set to '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 a high accuracy. Hence, SubPixel should usually be set to 'true'. Note that the subpixel accurate determination of the model's pose is only possible if the found instance lies at least 2 pixels away from the image border of the lowest used pyramid level. If the instance lies closer to the image border, its pose is only determined with pixel accuracy and the angle resolution that was specified with create_ncc_model, even if SubPixel is set to 'true'.
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 NCC model was created with create_ncc_model. If NumLevels is set to 0, the number of pyramid levels specified in create_ncc_model is used.
In certain cases, the number of pyramid levels that was determined automatically with, for example, create_ncc_model may 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 setting MinScore to a very low value to find all matches, it may be better to query the value of NumLevels with get_ncc_model_params and then use a slightly lower value in find_ncc_model. This approach is often better regarding the speed and robustness of the matching.
Optionally, NumLevels can contain a second value that determines the lowest pyramid level to which the found matches are tracked. Hence, a value of [4,2] for NumLevels means 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. 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.
The position and rotation of the found instances of the model is returned in Row, Column, and Angle. The coordinates Row and Column are related to the position of the origin of the model in the search image. However, Row and Column do not exactly correspond to this position. Instead, find_ncc_model returns slightly modified values that are optimized for creating a transformation matrix, that can be used for alignment or visualization of the model. (This has to do with the way HALCON transforms iconic objects, see affine_trans_pixel). The example below shows how to create the transformation matrix for alignment and how to calculate the exact coordinates of the found matches.
By default, the origin is the center of gravity of the domain (region) of the image that was used to create the NCC model with create_ncc_model. A different origin can be set with set_ncc_model_origin.
Additionally, the score of each found instance is returned in Score. The score is the normalized cross correlation of the template t(r,c) and the image i(r,c):
The NCC measures how well the template and image correspond at a particular point (r,c). It assumes values between -1 and 1. The larger the absolute value of the correlation, the larger the degree of correspondence between the template and image. A value of 1 means that the gray values in the image are a linear transformation of the gray values in the template:
i(r+u,c+v) = a * t(u,v) + bwhere a > 0. Similarly, a value of -1 means that the gray values in the image are a linear transformation of the gray values in the template with a < 0. Hence, in this case the template occurs with a reversed polarity in the image. Because of the above property, the NCC is invariant to linear illumination changes.
The NCC as defined above is used if the NCC model has been created with Metric = 'use_polarity'. If the model has been created with Metric = 'ignore_global_polarity', the absolute value of ncc(r,c) is used as the score.
Using the operator set_ncc_model_param you can specify a 'timeout' for find_ncc_model. If find_ncc_model reaches this 'timeout', it terminates without results and returns the error code 9400 (H_ERR_TIMEOUT).
To display the results found by correlation-based matching, we highly recommend the usage of the procedure dev_display_ncc_matching_results.
For an explanation of the different 2D coordinate systems used in HALCON, see the introduction of chapter Transformations / 2D Transformations.
find_ncc_model can be partially executed on OpenCL devices that support the cl_khr_global_int32_base_atomics OpenCL extension. Only the initial search for the model in the topmost pyramid level is done on the OpenCL device, while the tracking of matches is done on the host CPU. If the domain of the image to search in is substantially smaller than the size of the image, use crop_domain to reduce the amount of data that needs to be copied from the OpenCL device to the host CPU. Note that find_ncc_model using OpenCL may run either substantially faster or slower depending on a wide number of factors, so the only way to tell if using OpenCL is beneficial is by testing with images from the actual application.
Furthermore, note that the internally used memory increases with the number of used threads.
Input image in which the model should be found.
Handle of the model.
Smallest rotation of the model.
Default value: -0.39
Suggested values: -3.14, -1.57, -0.79, -0.39, -0.20, 0.0
Extent of the rotation angles.
Default value: 0.79
Suggested values: 6.29, 3.14, 1.57, 0.79, 0.39, 0.0
Restriction: AngleExtent >= 0
Minimum score of the instances of the model to be found.
Default value: 0.8
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
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
Maximum overlap of the instances of the model to be found.
Default value: 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
Typical range of values: 0 ≤ MaxOverlap ≤ 1
Minimum increment: 0.01
Recommended increment: 0.05
Default value: 'true'
List of values: 'false', 'true'
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
Row coordinate of the found instances of the model.
Column coordinate of the found instances of the model.
Rotation angle of the found instances of the model.
Score of the found instances of the model.
create_ncc_model (TemplateImage, 'auto', rad(-45), rad(90), 'auto', \ 'use_polarity', ModelID) find_ncc_model (SearchImage, ModelID, rad(-45), rad(90), 0.7, 1, \ 0.5, 'true', 0, Row, Column, Angle, Score) * Create transformation matrix vector_angle_to_rigid (0, 0, 0, Row, Column, Angle, HomMat2D) * Calculate true position of the model origin in the search image affine_trans_pixel (HomMat2D, 0, 0, RowObject, ColumnObject) * display the results dev_display_ncc_matching_results (ModelID, 'red', Row, Column, \ Angle, 0)
If the parameter values are correct, the operator find_ncc_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.
create_ncc_model, read_ncc_model, set_ncc_model_origin
find_shape_model, find_scaled_shape_model, find_aniso_shape_model, find_shape_models, find_scaled_shape_models, find_aniso_shape_models