create_uncalib_descriptor_model — Prepare a descriptor model for interest point matching.
The operator create_uncalib_descriptor_model prepares a descriptor model of an image region, which is passed in the image Template, that can be used for descriptor-based matching. By a subsequent call of find_uncalib_descriptor_model a projective 2D transformation (homography) from Template to a search image can be obtained. The center of gravity of the domain region in Template is used as an origin of the model. In contrast to create_calib_descriptor_model, no calibrated camera is needed and hence the result of a subsequent matching is a 2D projection.
The descriptor model describes a set of points of interest. It stores their locations and discriminative descriptions of their local gray value neighborhood. The interest point extraction is parametrized by DetectorType, DetectorParamName and DetectorParamValue. The respective descriptor around the interest points is parametrized by DescriptorParamName and DescriptorParamValue. The parameter Seed seeds the random number generator, which is used during the construction of the descriptor implemented with randomized ferns. The returned ModelID is a reference to the generated descriptor model. This model can be used for an efficient detection of instances of the learned template with find_uncalib_descriptor_model, allowing perspective transformation between model and search image. As descriptor-based matching relies on the existence of stable and discriminative points of interest, the object to be detected must be textured, but not in a repetitive way.
As mentioned, the detector is used for the extraction of stable interest points within the image. By the parameter DetectorType, it is possible to select the interest point operator to be used. Currently, points_lepetit, points_harris, and its binomial approximation points_harris_binomial are supported ('lepetit', 'harris', 'harris_binomial'). In case of templates or search images with a low contrast, one of the harris point operators should be used. Depending on the selected DetectorType, appropriate parameter names and values can be set in DetectorParamName and DetectorParamValue.
Possible parameter names for DetectorParamName and corresponding default values are:
[3, 1, 15, 30, 'interpolation']
[ 0.7, 2.0, 0.08, 1000]
[ 5, 15, 0.08, 1000, 'on']
Further details on the meaning of those parameters can be found in the descriptions of points_harris, points_harris_binomial and points_lepetit, respectively. If an empty tuple is passed or a parameter is not provided in DetectorParamName, the above mentioned default values are taken.
While adjusting the operator parameters it should be aimed that a set of 50 to 450 feature points is extracted (depending on the texture and size of Template), which are uniformly distributed over the template's ROI. Therefore, it is recommended to run the point operator of choice in advance and visualize the results by gen_cross_contour_xld. In most cases it is sufficient to use the default setting.
A point descriptor is a classifier, which builds characteristic descriptions of the gray-value neighbourhood for interest points. Currently the descriptor is implemented with the so-called randomized ferns, which learns the polarity of the gray-value differences of pairs of pixels certain at random locations in the surrounding area of the interest point. The descriptor is later used in find_uncalib_descriptor_model to classify interest points in the search image, or in other words: to identify (match) potential model points in the search image.
The descriptor needs to store only the projectively stable interest points (which will appear in many projective views of the template). To assess the stability of the interest points, a simulation is performed: Template undergoes many affine transformations and points that can be extracted in most of the views are considered stable. The affine transformations are a good approximation of the projective transformations within a local neighborhood of the interest points.
The following descriptor parameters can be set with DescriptorParamName and DescriptorParamValue:
Descriptor size parameters:
depth of a classification fern. A deeper randomized fern can better discriminate interest points. However, the memory requirement for the fern grows by a factor 2 to the power of 'depth'. Typical values are [5 .. 11], default value is 11.
number of fern structures used. Using more ferns increases the recognition robustness, however it also increases the runtime of the matching. If the memory needed for the descriptor should be small, many ferns with only a small depth should be used (e.g., 'number_ferns'=150, 'depth'=5). If detection speed is more important, fewer ferns of larger depth should be used (e.g., 'number_ferns'=10, 'depth'=11). Typical values are [1 .. 150], default value is 30.
side length of a quadratic neighbourhood describing a point of interest. Too large values of this parameter can influence the run time. Typical values are [15 .. 33], default value is 17.
Summing up, the parameters 'depth', 'number_ferns' and 'patch_size' allow a transparent control over detection robustness, speed and memory consumption.
switches on or off the projective transformations during the simulation phase. When switched on, the robustness of the model is improved and objects with bigger tilts can be found. When switched off, training time can be significantly reduced and the model is still able to recognize projectively invariant objects. Possible values are ['on', 'off'], default value is 'on'.
minimal angle of rotation around the normal vector of Template. Typical values are [-180 .. 0], default value is -180.
maximal angle of rotation around the normal vector of Template. Typical values are [0 .. 180], default value is 180.
minimal scale of Template. Typical values are [0.1 .. 1.0], default value is 0.5.
maximal scale of Template. Typical values are [1.0 .. 3.5], default value is 1.4.
The parameters 'min_rot', 'max_rot', 'min_scale', and 'max_scale' allow to manually set which affine transformed views of the template are used to train the descriptor. Setting these parameters is useful to reduce training time especially in combination with the parameter 'tilt'. Be aware that these parameters have direct influence on the results of find_uncalib_descriptor_model and therefore have to be set carefully. If, for example, the range of rotation is restricted from 'min_rot' = -10 to 'max_rot' = 10 it is not possible to find views of Template rotated by an angle outside that scope. A limited training range requires less ferns / ferns' depths to find Template. In such a case the number and depth of the ferns can be further reduced, which optimizes the model.
Please note that depending on your hardware, the processing time for training the randomized ferns can vary between several seconds to some minutes. Therefore, a once trained model can be saved and loaded with write_descriptor_model and read_descriptor_model.
The parameters and the location of the final descriptor points can be determined with get_descriptor_model_params and get_descriptor_model_points.
create_uncalib_descriptor_model stores the detector type, detector parameters, and descriptor parameters, which are used in every succeeding call of find_uncalib_descriptor_model. The reference point (origin) of the model is the center of gravity of the template's ROI. Its coordinates can be changed by set_descriptor_model_origin.
Input image whose domain will be used to create the model.
The type of the detector.
Default value: 'lepetit'
List of values: 'harris', 'harris_binomial', 'lepetit'
The detector's parameter names.
Default value: 
List of values: 'alpha', 'check_neighbor', 'mask_size_grd', 'mask_size_smooth', 'min_check_neighbor_diff', 'min_score', 'radius', 'sigma_grad', 'sigma_smooth', 'subpix', 'threshold'
Values of the detector's parameters.
Default value: 
Suggested values: 0.08, 1, 1.2, 3, 15, 30, 1000, 'on', 'off', 'none', 'interpolation'
The descriptor's parameter names.
Default value: 
List of values: 'depth', 'max_rot', 'max_scale', 'min_rot', 'min_scale', 'number_ferns', 'patch_size', 'tilt'
Values of the descriptor's parameters.
Default value: 
Suggested values: 0.5, 1.4, 11, 21, 30, -180, 180, 'on', 'off'
The seed for the random number generator.
Default value: 42
The handle to the descriptor model.
points_lepetit, points_harris, reduce_domain
V. Lepetit and P. Fua: “Keypoint Recognition using Randomized Trees.“
IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 28,
Nr. 9, pp. 1465-1479, 2006.
M. Ozuysal, P. Fua, and V. Lepetit: “Fast Keypoint Recognition in Ten Lines of Code.“ In Proceedings of Conference on Computer Vision and Pattern Recognition, 2007.