Re-engineering Point Clouds

Transcription

1 form Z Plugin Contents 0 Introduction: Re-engineering point clouds and objects 3 1 Reading in point cloud data: the point cloud file translator 4 2 The Re-engineer tool Riverside Drive Columbus, Ohio formz@autodessys.com auto des sys Product Information & Support (614) INC TEL : (614) FAX : (614)

2 April 2003 COPYRIGHT: auto des sys, Inc., All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, transcribed, transmitted, or translated into any language in any form by any means without the written permission of auto des sys, Inc. TRADEMARKS: form Z, RenderZone, and RadioZity are registered trademarks of auto des sys, Inc. LightWorks is a registered trademark of LightWork Design Limited. ACIS is a registered trademark of Spatial Technology, Inc. Apple, Macintosh, Power Macintosh, and the MacOS logo are registered trademarks or trademarks of Apple Computer, Inc. Microsoft, Windows, and the Windows logo are registered trademarks or trademarks of Microsoft Corporation. All other products mentioned in this document are registered trademarks or trademarks of their respective owners.

3 0 Introduction: Re-engineering point clouds and objects We call point cloud a collection of unthreaded geometry points. Point cloud data is typically generated by 3D scanning devices (digitizers) or 3D software. Especially when it is generated by digitizers, it consists of points lying on the surface of some 3D object. Such objects are typically digitized so that a computer model can be generated for them, from which a physical object can subsequently be constructed, by using a rapid prototyping or milling machine. The process of building a physical object from another existing physical object, frequently in a different scale, is often called reverse engineering. In form Z, this process can be accomplished by using the Re-engineer tool, which can be applied not only to point cloud data but also to other already structured form Z objects for the purpose of restructuring them. When applied to a point cloud, the process starts with importing the respective data and building a form Z object that consists of points only. This object is picked after a sampling density parameter has also been set. When applied to an already structured form Z object, the respective object is selected first and it is sampled using a user specified sampling density value to obtain a set of points analogous to point cloud data. Once this data is available, the first step is to create a dense mesh that depicts the topology of the unstructured set of points. If an optimized mesh is desired, the dense mesh is reduced to a simpler mesh with fewer faces. This is done by eliminating expendable edges one by one. There is also a boundary factor that can be specified by the user. It controls the quality of the surface boundary. Instead of a mesh, the option to generate a nurbz object is also available. When selected, the original dense mesh is partitioned into rectangular regions and for every rectangular region a nurbz surface is fit to best approximate the points in that region. All the nurbz surfaces thus generated are then joined together to form one form Z object. 3

4 1 Reading in point cloud data: the point cloud file translator Before the Re-engineer tool can be applied, point cloud data needs to be available in the program. While such data can be generated in form Z from existing objects, it makes practrical sense only when it is imported from 3D scanners, so that objects may be re-engineered from it and eventually manufactured. Such data can be imported into form Z by using the PTS file translator that reads point cloud data and creates form Z objects from it. To import the content of a PTS file, from the File menu, select the Import item. This invokes the standard File Open dialog from where the name of a file with point cloud data can be selected. One form Z object will be created from each point cloud file. This object will consist of a number of individual coordinate points that are topologically disconnected. After an object has been generated in form Z, a variety of its tools can be used to manipulate it, before a mesh or other type of object is created from it, including breaking it down to smaller groups of point clouds. The name of the point cloud file should end with a.pts extension and the data in the file should be formatted as follows: The first line should contain an integer n representing the number of data points in the file. The second line should contain a floating point number d representing the sampling density of the point cloud data. See the discussion on the Sampling Density option later in this section for more details. The remainder n lines in the file contain triplets of decimal numbers representing the x y z values of the points in the point cloud data. On each line the three numbers are separated by spaces. An example of a point cloud data file is shown in Figure Figure 1.1: The point cloud file format. 4

5 2 The Re-engineer tool This tool can be used to accomplish a variety of reengineering tasks. It can be used to thread a collection of points into a facetted object. It can be also used as a reduction tool to simplify form Z objects or as a conversion tool to convert form Z meshes into NURBS objects. Re-engineer This tool can be applied to an arbitrary set of points, after they are imported into form Z and transformed into an object that will be referred to as a point cloud object. Or it can be applied to a regular form Z object that will be referred to as a structured object. It can produce a facetted mesh, a NURBS, or a point cloud object. Which of these options will apply is selected from the Re-engineer Options dialog, shown in Figure 2.1, which is invoked directly from the tool. Figure 2.1: The Re-engineer Options dialog. Both the prepick and the postpick method of selection can be used with this tool. When using the postpick method, with the Re-engineer tool active, click on the object. This will invoke the Re-engineer Preview dialog (Figure 2.2) and will display the new object generated according to the Generate options currently selected in the dialog. With the prepick method, use the Pick tool to preselect any number of objects and then, with the Re-engineer tool active, click anywhere on the graphics window. The program will proceed with the execution of the operation and will invoke the Re-engineer Preview dialog once for each of the selected objects. This operatrion is always executed at the Object topological level. If another topological level is active at the time the ooperation is applied, the level is ignored. The operation can be applied to either point cloud objects or structured objects, which can be solid or surface objects, facetted, smooth, or nurbz objects, plain, primitive, metaformz, or patches. 5

6 Figure 2.2: The Re-engineer Preview dialog. Both the Re-engineer Options and the Re-engineer Preview dialogs contain a group of options labeled Generate. These determine what kind of an object will be generated from the selected data. There are four options. Point Cloud: When this option is on, a point cloud object is created from the selected object, provided it is not a point cloud object already, in which case this option has no effect. A point cloud object is an object that consists of a collection of geometry points and has no topology. That is, the points are not linked with other points to form facets. Mesh: When this option is selected, a triangulated mesh is created from a point cloud object or a structured object. This mesh typically has a large number of faces and is usually not a good fit to the data. The fit can be improved by using the next option. Optimized Mesh: When this option is selected, a triangulated mesh with lesser number of faces is created from the selected objectr and the mesh is a better fit to the data. Thanks to this option, the Re-engineer tool can also be used as a mesh simplification tool for any form Z object. The number of faces in the mesh is reduced by deleting the expendable edges and creating new faces while not losing any significant features of the original mesh. This option is complemented by the following two suboptions.

7 Optimization Level: This number determines how many times the optimization process should be applied. A higher value results in a smaller number of faces and longer processing time. The default value is 4. Recommended range for this value is between 1 and 4. Boundary Factor: This slider bar or the value entered in its field, which is a percentage, determines the quality of the boundary edges of the optimized mesh. As the value gets higher, the boundaries are recovered more faithfully at the cost of more processing time and more faces. The default value is 50%. The Optimized Mesh options are illustrated in Figures 2.3 and 2.4. a b c d Figure 2.3: (a) The point cloud data containing 12,772 points. (b) With the Mesh option on, a mesh of 12,860 faces is generated. With Optimized Mesh on and Optimized Level set to (c) 1 and (d) 4, 1,415 and 974 faces are generated, respectively. a b c d e Figure 2.4: (a) The original object to be re-engineered. (b) Point cloud data derived from the original object. (c) New object generated with the Mesh option on. Objects generated with Optimized Mesh on and Boundary Factor set to (d) 30% and (e) 100%.

8 Nurbz: When this option is selected, a set of nurbz surfaces are fit to the point cloud data and are joined together to create a smooth object. It is generally not possible to fit a single nurbz surface to an arbitrary point cloud or an arbitrary mesh. An initial tentative mesh is fit to the point cloud data. This initial mesh is next subdivided into rectangular regions and nurbz surfaces are fit to these regions. The individual nurbz surfaces can be stitched together if so desired. Number Of Optimizations: This option is used to specify the number of optimizations to be performed in each surface fitting iteration and it controls the quality of the surfaces generated. The higher the value, the better will be the fit and the slower the process. Recommended range for this value is between 1 and 4. The default value is 4. Max Number Of Iterations: The surface fitting process performs a number of iterative steps in order to meet the specified Error Tolerance (see below). This option can be used to stop the process by specifying the maximum number of iterations. The surface fitting process will stop after performing this number of iterations even if the error tolerance is not met. Range for this value is 1 through 4. The default is 1. Error Tolerance: This option can be used to specify the maximum error allowed while fitting the nurbz surfaces to the point cloud or to the form Z mesh. The error is measured as the root mean square distance (average of all the distances between each point and the nurbz surfaces) between the points or the mesh vertices and the nurbz surfaces. The surface fitting process stops when the error tolerance is met or it has performed the maximum number of iterations specified by the Max Number Of Iterations criterion. A lower error tolerance gives a better fit to the data by producing more nurbz surfaces at the cost of longer processing times. If the slider is placed at the rightmost end of the bar, it means that the maximum error allowed is 0.1 inches. Higher percentage values can be typed in the percentage box next to the slider bar to allow error tolerances higher than 0.1. The slider is placed at 50% by default, which means the maximum error allowed is 0.05 inches. Clean Mesh Tolerance: Thin long triangular faces in the mesh can cause the nurbz surface fitting process to fail. This parameter can be used to delete such faces before the fitting procedure starts. The slider bar is mapped on to a float value between 0.0 and That is, when the slider is at 100%, the value of this parameter is 0.03; when at 50%, the value is 0.015, and so on. This value represents the ratio of the shortest edge of the triangular face to its perimeter (sum of all three edges). Faces with a ratio smaller than this value are eliminated. Note that a 0.0 value corresponds to a triangular face one edge of which has 0 length or coincident end points. A 0.03 value corresponds to a triangle the shortest edge of which is 1/30 of its perimeter. The slider is placed at the leftmost end of the bar by default.

9 Fairness Tolerance: The final nurbz surfaces may have unwanted wiggles in order to represent the data as faithfully as possible. This parameter helps to control the amount of wiggles on the surfaces. Usually noisy data produces surfaces with more wiggles that are hard to eliminate. There is no linear relationship between this slider and the quality of surfaces; that is, higher values may not necessarily yield better results. This is a parameter that has to be set by experimentation. In some cases, the wiggles may persist for any value of this parameter because of the nature of the data and the other parameter settings. The slider is placed at the rightmost end of the bar by default. Preserve Boundary: When this option is on, boundaries are recovered more accurately at the cost of more processing time and more nurbz surfaces. Split At Discontinuities: When this option is on, the nurbz surfaces are split at G1 discontinuities. Splitting produces more nurbz surfaces. If not split, a lesser number of nurbz surfaces will be created, but subsequent operations like parallel, Booleans may fail. Stitch Surfaces: When this option is on, the nurbz surfaces are stitched together. Examples of re-engineered nurnz objects are shown in Figures 2.5 and 2.6. a b c Figure 2.5: (a) The point cloud data to be re-engineered. Generating (b) a Mesh and (c) a Nurbz using default settings for all other options.

10 a Figure 2.6: (a) The point cloud data to be re-engineered. (b) Generating a Nurbz using default settings, except for Max Number Of Iterations, which is set to 4. Sampling Density: This parameter can be thought of as the minimum distance between two sample points that are known to be on the surface. In order for the surface fitting process to work, it is important that, for every sample point, there exists at least one other sample point within the sampling density distance. If the user specified sampling density is less than the actual sampling density of the point set, the resulting surface will contain holes. If, on the other hand, it is more, the surface may look like a convex hull. When the point cloud data is read from a file and a point cloud object is constructed in form Z, the sampling density is also read from the second line of the file and stored as an attribute with the new object. From there it is read by the Re-engineer tool, when it is applied to c point cloud object. Sampling density can be initially obtained from a scanning device. When the Re-engineer tool is used on already structured form Z objects, a sampling density should be specified by the user. The object will be meshed according to the specified sampling density. The vertices of the mesh are taken as point samples for the nurbz creation process. The lower the sampling density, the more the samples, and more the number of nurbz surfaces. Sample Object At Specified Density: This tool requires enough sample points at the specified sampling density to generate nurbz surfaces. When this option is on, the selected object is sampled at the specified sampling density to obtain enough points and then fed to the nurbz creation process. b 10

11 Hole Filling: These options are used to specify the hole filling method during the surface fitting process. Examples are shown in Figure 2.7. None: When this option is selected, the holes in the point cloud data are not filled. All: When this option is selected, all the holes in the point cloud data are filled. All Less Than: When this option is selected, all the holes with diameter less than the value specified are filled. a b c d Figure 2.7: (a) The original surface object to be re-engineered. Mesh with Hole Filling set to (b) None (no holes filled), (c) All (all holes filled), and (d) All Less Than 36 (only holes of diameter less than 36 filled). 11

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