Translating your CAD Model to a VRML Model

Transcription

1 Translating your CAD Model to a VRML Model While students' CAD models will be the basis of all visual materials displayed by the 3D Virtual Buildings Project, the models themselves will not be displayed in the repository. Given the bandwidth constraints which currently govern data transmission over the Internet, two formats will be employed to portray the project's urban landscapes. The first is QuickTime VR, which will be used to generate highly textured, detailed renditions of the cities on site. While the graphic quality of QuickTime VR is impressive, it nevertheless fails to provide full mobility to users. Viewers frame of reference is limited to the panorama or object in their screen. VRML, Virtual Reality Modeling Language, by contrast, provides full mobility, enabling viewers to move in any direction within the model. The shortcomings of this modeling format are twofold. Most VRML scenes offer only rudimentary portrayals of the objects they are meant to represent. This limitation is necessary since detailed models such as those produced by CAD software produce files too large for rapid transmission over the Internet. Such files would also preclude smooth navigation through the model once it has been downloaded into the computer. Second, the visual quality of VRML models compared to their QuickTime VR counterparts is poor. It is for this reason that both formats have been incorporated into the 3D Virtual Buildings Project. The shortcomings of one format will be compensated by resort to the strengths of its counterpart. The object of this tutorial is to take the user through the steps necessary to generate a VRML equivalent of an existing CAD model such as the Hope building. Like the regular tutorial, this lesson will be based on Diehl Graphsoft's VectorWorks to generate the model. Unlike the previous tutorial, this lesson assumes the reader has already completed the regular tutorial and has a working knowledge of VectorWorks. After the model is constructed, the reader will be expected to use AccuTrans, or an equivalent translation package, to translate his or her model into VRML format. As the reader progresses through this tutorial it is important that he or she realizes that: The purpose of constructing a VRML model is to produce a model that can be viably added to an urban landscape that will be transmitted via the Internet. This means that the memory size of your present CAD model, which likely is between 5 to 35 M of memory, must be reduced to a figure between 10 and 100K of memory. Only abstract the basic shapes necessary to produce a simple rendering of the building. Stated another way, stay away from complex shapes. As you can see in Figure 1, curved surfaces such as the canister above require the software to generate large numbers of polygons, and the more polygons you have, the more memory you use. You will also need to avoid producing objects of ornamentation, such as tiles, and polygon heady objects such as window frames, chimneys, and gables. Instead of reproducing an entire window unit, simply draw a plane to represent the window. Remember, the important task here is to produce a rudimentary facsimile of the model, if the user wants to see a detailed rendition, he or she can consult the QuickTime VR representation of the structure.

2 Figure 1: While constructing the VRML model it is important to keep the number of polygons in your model to an absolute minimum. The complex shapes in CAD models are produced by adding large numbers of simpler, smaller shapes together. Most CAD software packages, for example, generate curved surfaces by adding large numbers of small rectangles -- or polygons -- together. The software produces the curved effect by altering the angular orientation of each polygon, so that it is slighltly different from the polygon that preceded it. See Figure 1. Creating a VRML Model from CAD To illustrate how these points may be realized, this tutorial will take the user through the process that was followed to convert the Hope building shown in Figure 2. This tutorial will not be a step by step tutorial, simply a statement of points modelers should keep in mind while generating their own VRML models. Figure 2: Part A: Don't Reinvent the Wheel It's important to remember that this exercise is not meant to be labour intensive. You want to produce a good facsimile, but in short order, and the first way to do that is to build on what you know already. Use your existing CAD model as the basis for your new model. You already made measurements for your structure while constructing the CAD model, so needn't repeat that process. Use the information you already have to determine the position of windows in your new model As well, use copies of existing components wherever possible. If you need to modify the shape of a component to reduce its polygon, use the CAD component as a reference shape. 2

3 Part B: Use your Reference Shape As well, use the reference shape the same way you employed it with your CAD model. It will help you position the walls and roof of your structure. Figure 3: Part C: Simplify your model Figure 4 shows the VRML model which we generated from the CAD model shown in Figure 2 in order to demonstrate the degree of simplification that is required. 3

4 Figure 4: This step requires you to simplify in two ways. The first is deciding which objects are superfluous for your present needs. Note that there are no doors or windows on the first floor. Note also that there are no tiles on the columns, and that all window frames have been removed. All we want are basic shapes to portray the basic features of the structure. Simplification also requires you to alter the shape of the components that remain. Note for example, the structures from the CAD model and the columns of the original structure shown respectively in Figures 6a and 6b. Figure 6a: Figure 6b: 4

5 Figure 7a shows the modifications we made to the structures shown in Figure 6a. Note that all curved surfaces in the base and [buttress] have been removed and replaced with a flat surface. The ridge on the sign level has also been removed, and neither the sign level nor the base are wrapped around the columns, but simply traverse the length of Facade 1 in the VRML model. These changes substantially reduced the number of polygons required to model Level 1. Figure 7a: In Figure 7b note how the columns have been simplified. All tiles have been removed. The objects attached to the column on Level 3 have been removed, and the dimensions for the columns on all three levels have been made uniform. Note also that the arches and other ornaments on the surrounding windows have either been removed or reduced. Figure 7b: 5

6 Part D: Reduce Polygon Count As you generate your model it is important to realize that most VRML modelers regard CAD modeling packages as one of the worst possible options for generating VRML representations. One reason for this unfavourable attitude is that CAD packages produce far more polygons than required to produce a viable VRML model. One way to overcome this difficulty is to rely on a basic rule of thumb while constructing your model: if the shape you are constructing has any surfaces or polygons which will not be visible to the future viewer, delete them. Consider, for example, the roof of the CAD Hope building shown in Figure 8. Figure 8: Aside from possessing far too many polygons, the shape also has unnecessary sections in the back, underside, left and back edges and intersections points. To reduce the polygon count, we constructed a new roof first by copying the roof from our CAD model and pasting it in a new layer. We then switched to the "Right View" of VectorWorks and traced the shape indicated in Figure 9. Figure 9: Once the new roof was generated it looked like Figure 10. 6

7 Figure 10: While eliminating the curve of the roof has substantially reduced the number of polygons in our object, more can be eliminated, particularly the polygons indicated in Figures 11 and 12. Figure 11: 7

8 Figure 12: To remove the surplus polygons, we highlighted the entire object using the Selection Arrow. We then converted the object to polygons by opening the "Model" menu and selecting "Convert to 3D Polys". The object was then divided into individual polygons by opening the Organize menu and selecting "Ungroup." Once that was done we were in a position to move or remove individual polygons. Our roof appeared as in Figure 13 after the polygons were removed. Figure 13: 8

9 Converting the CAD Model to VRML Part A: Divide Model into Separate Parts Figure 14 shows the appearance of our model once it was completed in VectorWorks. Figure 14: While the structure is finished, we are still not in a position to convert the model to VRML format. If we were to do so, the conversion software would convert the entire structure into a single object and render it into a single colour, as shown in Figure 15. Figure 15: The rule of thumb with VRML, as with other 3D modeling formats, is one colour or texture is assigned to each object. The challenge at this stage of the modeling process is to find a way to divide the VRML model into separate objects so that the model will have multiple colours. The first step in this process is to divide your CAD model into separate files. Each file will contain all the objects in your model that will have a particular colour. For example, Figure 16 shows the sections of the model which we decided to render brown. 9

10 Figure 16: To generate a new file: Open the File menu and select "New" to generate a new file. Open the Page menu and select "Layer scale" to ensure that the scale of your new file matches that of your VRML model. If you want the scale to be the same as your original CAD model, select 1" = 20'. Adjust the view in your screen so that it is the same as the file containing your VRML model. For example, if the file containing your VRML structure is presently in the "Right Isometric" of VectorWorks, you will need to ensure that your new file is in the same position. Open the Window menu to return to the file containing your VRML model. Select the objects you want to copy from your old file to your new file. Activate the "Copy" command in the Edit menu. Open the Window menu to return to your new file. Open the Edit menu and select "Paste in Place." Figure 17 shows what you should see in your screen. 10

11 Figure 17: Part B: Export Model from VectorWorks Once you've divided your model into separate files, you will be in a position to export them from VectorWorks. To do so, open the File menu and move the Selection Arrow down to "Export". After the new menu emerges select "Export DXF/DWG". Here we are converting our model into *.dxf format, the format associated with AutoCAD modeling software. As a format, it is useful for exporting and importing 3D models into a variety of software packages. Once the "DXF DWG Export Options" panel opens, ensure: That in the section marked "2D/3D Conversions" that the "Export 2D and 3D Objects" option is selected. Also ensure that the box "Triangulate to preserve 3D Fills" is checked. That in the section "Invisible Classes are:" that the selection "Not Exported" is chosen. That in the section marked "Objects in Invisible Layers are:" that the selection "Not exported" is chosen. Once you've ensured that all correct parameters have been selected, hit "OK". After the "Write DXF File to" panel appears, select a name for your file, such as the colour "brown", and then hit "Save". Hit "OK" when the "Map Line Weights to Color" panel appears. 11

12 Save your file and close VectorWorks. Part C: Convert File to VRML Open AccuTrans, the conversion package we will be using to convert our *.dxf model into VRML: Open the File menu and move the selection arrow to "Open". After the new menu emerges move the selection arrow to "DXF (*.dxf) and select it. Open your file. After the model appears in your screen open the File menu again and select "Save As". After the "Save Object as" panel emerges, proceed to the "Save File Type" section of the panel. Open the selection menu by pressing, and move your selection arrow to "VRML 2.0 (*.wrl). Once you've entered the correct file type, hit the "Save" button in the lower left corner of the panel. Enter a name for your VRML file. Again, we suggest the name of your file match the colour of the objects you are representing in this file. Eg. "brown.wrl." Once you've saved your file, press the "Close" button in the lower left corner of the panel. Answer "Yes" when the program asks if you want to clear your work and layer attributes. Close AccuTrans. 12

13 Part D: Adjust VRML Code Figure 18 indicates what your file should look like after AccuTrans has generated the file. Figure 18: As you can see from the diagram, the model looks incomplete. Several polygons appear to be missing. They aren't. It's important to remember in many computer modeling formats if an object or polygon is not rendered, it will appear invisible to a viewer using a browser such as the one displayed in Figure 18. The problem with the model in its present state is that only one side of the polygon is rendered. If you are looking at the polygon from the right direction you will be able to see it. If you are observing it from the opposite direction, the polygon will be invisible. Our objective here will be to change the code so that the polygons are rendered on both sides. To change the code, you will need to open brown.wrl or other files into a software package that will enable you to read the code, such as "Notepad" or "Microsoft Word". Any software package capable of reading a text file will do. Once you've opened your file, code similar to what you see in Figure 19 should appear in your screen. 13

14 Figure 19: Your object here will be to alter a line of code at the end of the file. Move the scroll bar on the right side of your screen to the bottom. Figure 20 indicates what you should see. Figure 20: Change the line indicated in Figure 20. Erase the word "TRUE" and write the word "FALSE" in block letters. Save 14

15 your file, and open it in your browser. Figure 21 indicates what you should see in your screen. Figure 21: Part E: Alter the Colour of your Model To alter the colour of your object, we will again need to adjust the code. Open your file in Notepad, Microsoft Word, or equivalent package: 1. Note at the top of your file that there is code which reads: "DEF VP1 Viewpoint". This code is intended to establish points of view -- coordinates -- for viewers to observe the model in a browser. It's a command designed to enable viewers to rapidly view their object from above, from the right, left or the back. 2. While interesting, the "DEF VP1 Viewpoint" command is not our primary concern, we are only using it as a reference point. Scroll down until you see "DEF VP6 Viewpoint". Below it you should see the code indicated in Figure

16 Figure 22: 3. The first line refers to "DEF VP6 Viewpoint" indicated above. The second line refers to the line of code which we will need to modify. 4. Colours in VRML are generated by mixing three basic colours: Red, Green and Blue. The first number refers to Red, the second to Green, the third to Blue. 5. For each figure, you will need to enter a figure between 0.0 and 1.0. For example, you can enter 0.75 as a value for Blue. 6. The colours you generate will depend on the values you enter for each colour. In the file above, AccuTrans entered the value to produce the colour white. If you enter you will generate the colour black. 7. Figure 23 provides a list of colours and the values you will need to enter to realize them in your VRML model. 16

17 Figure 23: Colour: Red Green Blue Black Blue Green Light Blue Red Violet Yellow White Grey Brown Realize that the colours listed above are not the only ones you have at your disposal. If the colour produced is not what you want, you can increase the amount of say red or blue to alter the shade of your colour. For example, to produce the brown used in our VRML Hope building, we entered instead of the values listed in the table above. Figure 24 was the result. 17

18 Figure 24: Part F: Combine VRML Files to Produce Model Once you've encoded the requisite colour to each component of your model, you will be in a position to combine them to produce the final model. To start, open Notepad, Microsoft Word, or an equivalent package. Type the code indicated in Figure 25. Figure 25: Once you've typed the title of your program save your document. Enter any name you wish in the "File Name" text entry field, but do not save the file as a text or Word File. In the "Save as type" text entry bar 18

19 choose "All files", and before you save your file write the file extension *.wrl to the name of your file. For example, if you want to use the word "Combine", write "Combine.wrl" in the "File Name" text entry field. Doing so will enable your browser to read your file as a VRML file. To generate your model, you will need to do two things. First, ensure that your new file is in the same directory as the other files associated with your model. In other words, "Combine.wrl" needs to be in the same directory as "brown.wrl." Second, you will need to use the Transform command indicated in Figure 26 to recombine the various files -- such as "brown.wrl" -- into a single file. Figure 26: Type the code in exactly as you see it in Figure 26. You will need to use this sequence of code for each file that you want to import into your finished model. For example, if you want to add "Blue.wrl" to your file, you would add the code shown in Figure

20 Figure 27: Once you've added the indicated sequence of code for each file that you wish to incorporate into your final model, save your file and close Notepad or the equivalent package. Open your new file in your browser. Figure 28 indicates what you should see. Congratulations! You've just completed your VRML model. Figure 28: 20