Int J Adv Manuf Technol (1998) 14:269-279 1998 Springer-Verlag London Limited

The hlterrtationa~ 3o.rnal of

Advanced fflanufacturing Technologu

Creating Machinable Textures for CAD/CAM Systems

Chow Kin Yean,* Chua Chee Kai,t Terry Ong* and Lin Fengt *Delcam International PLC, Birmingham, UK; and +Nanyang Technological University, Singapore

Texture is app,!ied on three-dimensionally modelled surfaces in computer graphics to enhance visual effect. This research focuses on the development of three-dimensional textured sur- faces which are suitable for manufacturing. Three approaches for creating the three-dimensional texture are presented.

The first approach is to process a design from either an artist's sketch or an image from a two-dimensional scan. The second approach uses a three-dimensional scanned texture. Both these approaches depend on the quality of the scanned image and are more tedious than the third approach, which is to convert texture using parameters and is a more direct approach. In the user interface design, two custom-made forms are developed to cater for both regular and irregular textures.

The case studies have shown that the textures created are not only good for a visual effect, but are also machinable. The development work is incorporated into the ArtCAM system which is a specialised CAD~CAM system that is capable of generating three-dimensional shapes from two-dimensional art- work.

Keywords: ArtCAM; Artwork; Computer-aided design and computer-aided manufacturing (CAD/CAM); Three-dimen- sional texture

1. Introduction

The use of the computer has greatly improved the speed of computing complex and complicated numerical problems. With the integration of the computer in manufacturing systems, production time has been reduced significantly. The evolu- tion of CAD/CAM (computer-aided design/computer-aided manufacturing) systems has enabled more complex parts to be modelled and manufactured. CmTently, most of the parts mod- elled are of a complex shape and the representation of the complex shape is mainly by three-dimensional complex sur-

Correspondence and offprint requests to: Dr Chua Chee Kai, School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798.

faces e.g. Bezier surfaces Ill, NURB surfaces [2] and G ~ surfaces [3].

In computer graphics, texture is added to the complex surface to make the part appear more realistic. The incorporation of texture on the modelled surface enhances the visual effect of the modelled part. This improves the graphics simulation of the part as a real object. Currently, most texturing techniques are similar to either solid texturing [4] or bump mapping [5]. These texturing techniques do not provide a three-dimensional representation of the textured surface. This implies that the textured surface modelled cannot be manufactured, that is, the texture itself cannot be machined. Therefore, there is a need to create a "real" three-dimensional texture which can be machined.

There are two main categories of texture - "macrotexture" and "microtexture" [6]. "Macrotexture" refers to surface primi- tives that are governed by specific placement rules, whereas "microtexture" refers to surfaces that are just beyond our perceptual resolving power. "Microtexture" is a texture that is most likely to be formed from machining or finishing processes. The range of surface texture specified by ANSI B46.1 is 0- 50 txm. This surface texture is called "microtexture". The range of surface finish obtained by a milling process is 1-200 ~m according to British Standard BS 1134 Part 1. Using a rigid NC (numerical control) mill machine [7], a surface tolerance of 10 Ixm can be easily achieved [8]. The texture under research and development in this work falls in the "macrotexture" group.

2. Approaches to Create 3D Textured Surface

There are three approaches to create surface texture. The texture created from any of these approaches is mapped onto a three-dimensional surface to form a textured surface within the software ArtCAM. The three approaches differ primarily in the source of the texture. The source of the texture refers to the initial form of the texture representation. The first source is from a two-dimensional image. The second source is from a sample textured surface, which is three-dimensional, e.g. produced by finishing processes such as sand blasting. The third source is derived from a custom-made form defined with

270 K. Y. Chow et al.

several texture parameters. Figure 1 shows the flowchart of the three approaches for creating the textured surface.

2.1 From Two-Dimensional Image to Texture

The two-dimensional textured image is obtained either by scanning an existing textured surface or from the creation of a computer graphics artist. The two-dimensional textured image must be a grey-scaled image. This is because the development of the image to the three-dimensional relief surface in ArtCAM is made in relation to the colour intensity of the image. This approach requires two steps for transforming the source to the texture. The first step is to obtain a textured image that involves the scanning process or the drawing process. The second step is to create a relief image which is derived from the colour intensity of the image.

2.2 From Sample Three-Dimensional Scanned Surface to Texture

The sample textured surface is obtained from finishing pro- cesses like sand blasting and chemical etching. The three- dimensional information of this textured surface is acquired by a three-dimensional laser digitiser. The laser digitiser outputs the surface information as a point cloud data. This point data is then converted to the data format of ArtCAM which forms the texture. This approach requires three steps. The first step is to create the textured surface. The second step is to digitise the textured surface. The final step is to convert the point data obtained from laser digitiser to the data format of the texture.

2.3 Using Custom-Made Forms to Create Texture

The custom-made form is a map that contains several para- meters used to define the characteristics of the texture. The parameters of texture are next transformed into the data format

Sample Textured Surface Custom Form with Texture Parameters

-A~CAM

E 3D Scanner

i 3D Point Data I I

A~Read +

I 3D Relief Sutthce [

ArtCAM

Texture

Mapping

Textured Surface

2D Scanner PaintBrush

2D Textured Image t 1

ArtCAM

1 3D Relief Surface I

ArICAM I

I 3D Surface ]

Fig. 1. Approaches to creating 3D texture surfaces.

of the texture in ArtCAM. This approach requires only one step in transforming the source to the texture.

2.4 Evaluation of the Three Approaches

The first approach - from a 2D texture image - involves two steps in obtaining the textured surface. The entire process of creating the texture is purely software-based. The design of the textured image is dependent on the skill of the artist. Alternatively, the textured image can be extracted from the sample images found in an application software such as Corel Draw. However, not all textured images can be converted into texture. This is because the conversion of the image to textnre is made on the basis of the colour intensity. Some textured images might not be suitable.

The second approach - from a sample textured surface - involves three steps. This approach is more cumbersome than the first one. As the creation of the sample textured surface is time consuming, the time taken for the conversion from the sample surface to the texture is longer than for the first approach. The second approach can obtain good textural infor- mation from a laser digitiser. Thus, the texture created from the source is more reliable than for the first approach, but the accuracy of the textural information is dependent on the accu- racy of the laser digitiser.

The third approach - from custom-made forms - involves only one step. In terms of conversion time, this approach is the fastest. It provides a more direct way for obtaining texture in comparison to the other two approaches.

3. Relief Generation Process by ArtCAM

The generation of a textured surface on a 3D shape can be achieved using the software ArtCAM developed by Delcam International. The textured surface developed is based on the software library of ArtCAM. The textured surface can be wrapped onto the three-dimensional relief generated from 2D artwork. Figure 2 shows the flow process of converting 2D artwork into a 3D relief.

The development of 3D relief by ArtCAM starts from 2D artwork. The artwork data can be either in vector format or bitmap format. The supported vector formats are "eps", "dxf' and "pcx". The supported bitmap formats are "bmp", "gif', "jpg" and "tif'. The next step is to assign colour to the artwork; this is generally done with the artwork in vector format. With the bitmap, some manipulation such as colour reduction and colour linking is carried in the ArtCAM software. The reduction of colour is to ensure the generation of smooth 3D surfaces.

The next step is to associate a unique shape profile to each colour. There are three types of shape profile: flat profile; slope profile; and curve profile [9]. Several parameters are associated with these profiles such as the sloping angle for the slope angle and the profile height for the flat profile. After the profile is assigned to each colour, the 3D relief is then gener- ated.

From the generated 3D relief, a cutter tool path can next be generated. The generated cutter tool path can be displayed

Artwork in vector data Artwork in bitmap format

~ Colour assigned to the artwork

Bitmap manipulation

Associate shape to each cotour

Generate 3D relief from artwork

i, Toopath Generation

Toopath Simulation

[ - -~> Final Product

Fig, 2. Relief generation by ArtCAM.

and rendered. This can be simulated as the actual machined relief. ArtCAM is ideal for making both the male and female mould for mould production and 3D engraving shape.

Figure 3 shows the artwork and the generated relief for the logo of the Jaguar car. The relief is generated from the profiles associated with the colour. There several advantages in using ArtCAM for logo production. First, the logo design can be reproduced exactly. ArtCAM is useful for repeated work or the maintenance of moulds. Secondly, the 3D surface finish can be visualised before proceeding with the cutting of the material. This will save production time, cost and material and,

Creating Machinable Textures ./'or CAD~CAM Systems 27]

therefore, it is able to gain early customer acceptance of design. Lastly, ArtCAM can quickly create a 3D product from 2D artwork.

4. Application of Texture from Textured Image

Of the three approaches for creating 3D textured surfaces, the second approach, where a sample textured surface is used, simply involves converting the 3D scanned data to ArtCAM data for texture mapping. As the texture is created from computer graphics, creating a textured surface from a textured image is a more general approach. The general process of converting a textured image into a 3D texture is shown in Fig. 4.

From the textured image, the colour paiette of the image is checked, if the image is not in the grey scale, the image is converted into the grey scale format. Next, the user will be prompted for the maximum height of the texture to be gener- ated. Using the maximum height, the height scale is evaluated from subdividing the maximum height by the number of colours of the image.

The next step is to assign the computed height scale to colours. Then, the texture relief is generated from the colour. Finally, the texture relief is mapped onto the base relief to form the textured relief.

Figure 5 shows 2D artwork and the generated base relief. The artwork is created by typing in English letters and Chinese characters. The letters and characters are in vector format. By filling the outline of the letters and characters with different colours, the bitmap artwork is created (as shown in Fig. 5). A different profile shape is assigned to a different colour. With

Fig. 3. Artwork and generated relief of the Jaguar logo.

272 K. Y. Chow et at.

I Textured image [

I Check for image colour palette type I

o

Convert Mmage ~o grey sca e Yes

Sub-divide maximum height

I Assign height scale to each colour I

I Generate texture relief I

,L I Map texture relief to base relief I

Fig. 4. Converting textured image to texture relief.

the shape associated to each colour, the base relief is generated as shown in Fig. 5. With the bitmap textured image as shown in Fig. 6, a texture relief is generated.

The final part is created by mapping the generated texture relief onto the base relief. Figure 7 shows the relief of the

final product. The texture is excluded from the lettering of the relief, as intended.

With this final relief, the tool path can be generated with a specified cutter type and size. Figure 8 shows the machined part of the final relief. The texture on the machined part (Fig. 8) is exactly the same as the computer-generated relief (Fig. 7).

5. Design Considerations for Custom- Made Forms

The custom-made form for textured surfaces is designed for two types of texture: regular and irregular. Several factors need to be considered in the design of the custom form for the texture. These factors will govern the internal representation and the external interface of the texture custom form and concentrate mainly on practicality and feasibility.

5.1 Simplicity

The texture form should contain the optimal number of para- meters to represent the texture efficiently. The number of parameters must be as small as possible so as not to confuse the user. Parameters must not be inter-dependent, that is, the value of one parameter must not be dependent on the value of another. If this is not possible, the interrelationship between the parameters must be kept to the minimum. This is to ensure that the user can easily understand the custom-design form and thus be able to apply the form to create the texture.

Fig. 5. Two-dimensional artwork and generated relief. Fig. 6. Textured image.

Crea~ing Machinable Textures .liar CAD~CAM Systems 273

Fig. 7. Computer-generated relief.

5.2 Easy Computation

The computation of the textm'e information must not be too time consuming. The longer the time taken to compute the texture, the slower would be the design of the part. Conse- quently, the time-to-production would be prolonged, and ulti- mately this wo~ald reduce the competitiveness of the product. Therefore, the computation of the texture should be as fast as

possible, However, the calculation must be complex enough for the representation of texture. This implies that the calculation of the texture representation must be optimised.

5.3 Flexibility

The texture custom-design form must be able to create different shapes for the texture pattern. However, this must be carried

Fig. 8. Machined part.

274 K. E Chow et aL

out without compromising the time or complexity of the design. This is to ensure that the design time for the texture would not unacceptably increase the overall design time of-the pro- duct. The custom-design form must also permit ease of modifi- cation of the shape of the texture pattern. With the above two features, the form will be flexible in both the design and modification aspects.

5.4 Fine Representation of Texture

The custom form must be able to reveal the fine details of the texture on a surface, and allow them to be displayed on the computer screen. In addition, this fine detail must be machinable on a CNC (computer numerical control) machine, so that it can be machined into a tangible product.

6. Regular Texture

The result of the regular texture designed is illustrated on the base relief generated from the artwork as shown in Fig. 9. With the artwork, the base relief is generated as shown in Fig. 10. This relief is a smooth dome-like surface with some lettering on it. The base relief is not textured.

The custom-made form of a regular texture is designed based on the four design factors. The form is internally a 2D 10 x 10 grid map which is analogous to a 2D textured image. Each value in the grid represents the height of the textured surface. There are two additional parameters associated with this texture map. These two parameters are the spacing and the height scale. The spacing defines the distance between each grid point in the texture map. The height scale shows the height. The user interface for the form is shown in a dialogue

box in Fig. 11. This user interface is in the form of a dialogue within a developed software programme. The programme developed is part of the development framework of the software (ArtCAM TM) developed by Delcam International PLC.

The shape of the textured pattern can be changed by assigning numerical values to the grid of the texture map. Inputting of numerical values to the texture map and changing of the numerical values are done frequently during the design stage of the texture pattern. Thus, a set of templates is created to assist the user to create pattern, which can speed up the design and creation of the pattern. In addition, these templates provide some guidance to the user on how to use the form. The created templates include the round, sphere, null, square, triangle, diamond and the prism blocks. The round block template creates cylindrically shaped texture patterns. The square block template produces rectangular shaped texture pat- terns. The diamond block template creates diamond-shaped base block texture patterns. The sphere block template creates semi-spherical shaped texture patterns. Figure 11 shows the relief mapped by the generated texture pattern created by the sphere block template, The prism block template creates a square based pyramid texture pattern. All the shapes designed for the texture patterns are tile-mapped onto the surface.

By combining the generated relief (Fig. 10) and the textured mapped relief (Fig. 11), the final relief is created as shown in Fig. 12. The creation of the final relief is achieved by merging the higher height value of the two reliefs. Figure 13 shows the machined part of the relief. The machined regular texture is similar to the modelled relief. Thus, the regular texture created is machinable.

Fig. 9. Artwork for final relief,

Creating Machinable Textures for CAD~CAM Systems 275

Fig. 10. Generated relief.

Fig. 11. Dome-like relief mapped with regular texture.

7. Irregular Texture

Apart from regular textures, the other form of texture is the irregular texture. For natural objects, most of the textures appear in irregular forms. Some examples of irregular texture are animal skin, marble, stone and wood surfaces. Irregular texture does not exhibit pattern properties like placement rules and texture element shapes.

Figure 14 shows the artwork of a dinosaur. The ~xue shape of the dinosaur is not generated from the artwork. The artwork can only generate a relief which has a 3D image of the dinosaur. Figure 15 shows the generated relief of the dinosaur. The skin of the dinosaur is smooth, but generally, the skin of a dinosaur is not smooth. Therefore, creating a texture for the skin of the dinosaur is essential to make the relief model as realistic as possible.

The design factors for the custom-made form of irregular textures are similar to those of regular textures. The dialogue

276 K. Y. Chow et al.

Fig. 12. Final relief mapped with regular texture.

Fig. 13. Machined model of the final relief.

box, as shown in Fig. 15, is the user interface for custom- made forms of irregular textures. The form consists of four parameters, namely, the map size, the height range, the random seed value and the texture-generating method. The generated texture is basically a square relief with height values. The map size is the dimension of the square relief in real units. The height variation of the texture relief is very fine. The height range value is between zero height and the maximum allowable

height of the relief. The height range parameter of the form is the maximum allowable height of the texture relief.

The generation of irregular textures requires random fields. The random seed value parameter of the form is the initial value for the random field. The value of the seed parameter can be a number or a character. Currently, two types of texture have been developed. These are the lime stone and wood grain texturing, more will be added in future.

Creating Machinable Textures for CAD~CAM Systems 277

Fig. 14. Artwork of a dinosaur.

Fig. 15. Generated relief of the dinosaur.

With the map size value set to 10 mm and the height range value set to 0.5 ram, a texture relief is generated. The random seed value for the generated texture relief is "monster". The texturing selected is the wood grain, The generated texture relief is to simulate the skin of the dinosaur. Figure 16 shows the generated texture relief for the dinosaur.

Using the texture relief as shown in Fig. 16 as the skin texture, the texture is mapped onto the generated relief of the

dinosaur. Figure 17 shows the final relief of the dinosaur with skin texture mapped on. The skin texture is mapped around the dinosaur model except for the belly portion.

As compared with the relief without texture (shown in Fig. 15), the relief with texture (Fig. t7) looks more realistic. Fig. 18 shows the machined model of the dinosaur, which looks very much like the computer displayed model, Thus, the irregular texture created fulfils both requirements - good visual display and machinability.

278 K. E Chow et al.

Fig. 16. Textured relief for the dinosaur.

Fig. 17. Dinosaur relief mapped with texture.

8. Conclusion

Texturing is very popular in computer graphics for the realistic display of 3D surfaces. There is a need to produce textured surface with 3D information for manufacturing purposes. Three approaches for creating 3D textured surfaces have been presented. The last approach, using custom-designed forms with texture parameters, is the fastest and most direct.

Acknowledgements

This report is part of the documentation on the development project of the software ArtCAM TM. This development project is a joint project of Delcam International, the School of Mechanical and Production Engineering of Nanyang Techno- logical University and the GINITIC Institute of Manufacturing Technology. This S$2 M development project is partly spon-

Creating Machinable Textures for CAD~CAM Systems 279

Fig. 18. Machined model of the dinosaur.

sored by the National Science and Technology Board, Singa- pore. The authors would like to express their appreciation to Delcam International for approving the use of the library and development framework of ArtCAM TM software.

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