Sculptor: A Three-Dimensional Computer Sculpting System

Leonardo

Sculptor: A Three-Dimensional Computer Sculpting SystemAuthor(s): Mike KingSource: Leonardo, Vol. 24, No. 4 (1991), pp. 383-387Published by: The MIT PressStable URL: http://www.jstor.org/stable/1575512 .

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ARTIST'S ARTICLE

Sculptor: A Three-Dimensional

Computer Sculpting System

Mike King

C omputer solid modelling systems are arbitrary object aimed mainly at commercial animation, engineering, pro- overlapping spl duct design or architectural users, and are usually based on dimensions. Th a polygon-mesh description of objects. There are two main the user to ci methods of constructing objects described in this way: con- dimensional (3 structive solid geometry (CSG), in which complex forms are successively add created by the intersection of 'positive' or 'negative' primi- any radius at arl tive volumes; and surface modelling, in which surfaces are (chosen by the created in three dimensions using sets of control points. Further manip Artists and sculptors have used these systems, however, as scaling, rotati

modelling techniques in these systems are better suited to copying, mirrorii other applications. In addition, 'photo-realistic' rendering are capabilities techniques are often inappropriate for the way that artists Perspective viev work. Artists have traditionally applied a personal feel for object, or scene

depth, perspective, light and shadow; this is not possible a number ofobj with computer graphics systems, as these systems generally and the viewing use the laws of optics in the precise renderings of objects. be adjusted for The rendering software can only be tuned to certain degrees spectives. by the artist, so the main area of expression lies with the Sculptor was

modelling. The modelling in Sculptor was designed as an al- ceived as a sim ternative to these common techniques that are more suited ling system, wh to engineering. Sculptor is more closely related to CSG than test bed for inve to surface modelling in that objects are 'built up' only by the graphics media intersection of positive volumes, starting with a single sphere system for gene as the basic building block, or primitive, my students an(

design concepts ing system calle

THE SYSTEM AND ITS ORIGINS [1]. Because Sc

Sculptor allows the interactive creation and editing of this results in un

Fig. 1. Sculptor screen layout, showing three orthogonal views of an object under construction in first-angle projection. The user drives the system through the menus, using a mouse or a tablet and puck, then, through the small boxes to the right of the plan view, selects one or more groupings with which to work.

to the more gec elling systems. representation < Knowlton [3] to

example, the a SOFT compute:

Sculptor is at modelled objec This system em} circles are used

expense of this other kinds of F which consists

plan and side vi that inform the

In order to

large number o allows a maxil

s composed of heres in three e system allows

ABSTRACT reate a three- 3D) object by The author describes Sculp- iing spheres of tor, a three-dimensional computer bitrary positions modelling system that allows the user) in space. use of sculptural methods for the

culations, such construction of objects. These objects can be subsequently realised

ng, translating, as physical sculptures through the ng and deleting, use of a contour-slicing routine, of this system. though the major use of the system sing of a single has been in the production of com- es composed of puter images of three-dimensional

'scenes'. The origins, structure and ects, is possible, usage of the system are described, parameters can followed by a discussion of the close or far per- author's personal exploration of its

possibilities.

originally con-

ple 3D model- ich I built as a

stigating user interface design in interactive . More recently I have used Sculptor as a

rating a very personal style of imagery, and d I have used some of Sculptor's interface s in a two-dimensional (2D) painting/draft- d ICAS (Integrated Computer Art System) ulptor uses the sphere as its only element, lusually organic-looking imagery, in contrast )metrical imagery of conventional 3D mod- The system uses the techniques of object and display described by Badler et al. [2] and ) achieve the organic shapes, rather than, for

pproach adopted by Wyvill et al. with the r program [4]. ble to provide three concurrent views of the t, due to the unique symmetry of the sphere. ploys a first-angle projection system in which to represent the spheres; the computational projection is negligible compared to using primitives. Figure 1 shows the screen layout, of first-angle views of an object (elevation, ews) along with menus and various symbols user of the system's status.

create objects of reasonable complexity, a f spheres are needed (at present the system mum of around 20,000). Unfortunately,

Mike King (educator), City of London Polytechnic, 100 Minories, London EC3N 1JY, U.K.

Received 6 February 1989.

? 1991 ISAST Pergamon Press plc. Printed in Great Britain. 0024-094X/91 $3.00+0.00 LEONARDO, Vol. 24, No. 4, pp. 383-387, 1991 383



FILES -GET OBJECT

ORG GRPN3 AS ONE GRUP

-SAVE OBJECT

SCULPT -SELECT GROUP -ADD SPHERES

X ONLY ZONLY YONLY ANY

-RING

XCNLY

-CYLINDER -CYLINDER X ONLY ZONLY YONLY ANY

-DEL SPHERE -DEL GROUP

ATTRIBUTES -SELECT MULTI

SCALE ENLARGE 10% REDUCE 10%

-ROTATE X-AXIS Y-AXIS Z-AXIS

-MOVE GROUP

X ONLY ZONLY YONLY ANY

-VISIBILITY

GROUPINGS -SELECT GROUP -COPY GROUP

x ONLY Z ONLY YONLY ANY

-MIRROR GROUP

CHORNTAL VERTICAL DEPTH

-TRANSFER -MERGE

VIEWING SELECT MULTI RENDER

SNGLE MULTI

HATCHED

SMOOTH OUTUNE

Fig. 2. Sculptor menus. The operations for the Sculptor system are arranged in five menus that group related operations together. Variations of each operation are set by the switches ar-

ranged below the operation titles on the screen.

editing of an object becomes very diffi- cult once the sphere count reaches more than even 100. To counter this

problem, Sculptor employs a technique called hiding, in which spheres are made invisible (although not lost from

memory). In order to do this in a prac- ticable way, spheres are added to the current object in groups (the system allows 12 groups). When a current

group is selected for editing, the re-

maining groups are 'hidden' to facilitate the editing process. Hence objects are usually created or sculpted in parts, with the spheres in each part grouped together. Various operations allow ungrouping, regrouping, transferring and merging of existing groupings. Groups can also be loaded from and saved to memory, thus allowing the user to build up libraries of sculptural 'com-

ponents'. The blocks of little squares to the right of the plan view in Sculptor's screen layout show a tree-structure of

groupings that allow the user to select and make visible any combination of the twelve available groups (see Fig. 1).

Operations for object manipulation are grouped in five submenus; each

operation with variations has its own submenu of 'switches' that appear whenever that operation is selected. The main menu is always visible; submenus are called up from the main menu and appear below it; switches appear below the submenu (see Fig. 1). The tree-structure of menus, submenus and switches is shown in Fig. 2.

Kenneth Knowlton's paper, "Com-

puter-Aided Definition, Manipulation, and Depiction of Objects Composed of

Spheres" [5], outlines the algorithm I have used in Sculptor for object representation and rendering. I have modified the rendering algorithm in several ways to provide a choice of finished imagery. The illusion of solid

objects is created using two types of

filled disc, the facade and the outliner, which are sorted by depth and drawn in order from the farthest to the nearest. The outliner is always drawn as a filled black disc, painted directly into the frame buffer (overwriting pixels). Originally the facade was drawn in OR mode, which allows previously drawn

highlights to 'shine' through; now an alternative technique is used involving the comparison of colour values for

brightness-brighter colours are always drawn over darker colours.

Different rendering styles are achieved by drawing different types of

facade: a white disc for 'outline', and a series of a maximum of 256 discs with

smoothly interpolating colour values, resulting in the 'smooth' rendered

option. These two types of rendering

are completed in different amounts of time; 'outline' is the fastest, and 'smooth' is the slowest. While the 'smooth' rendered version is usually used for the final result, the 'outline' version is useful for a quick 3D visual- isation to guide the user in the creation of the object.

These rendering algorithms are an

approximation, in that no 3D data is used for drawing a given sphere, but are

performed much more quickly than any true 3D rendering could be.

Sculptor, written in the C program- ming language, runs on an IBM-com-

patible personal computer driving a

Digisolve Ikon frame-store. The frame- store has its own 68000 processor, with Hitachi ATRTC graphics chip and

graphics commands in firmware, and is

capable of 768 x 576 resolution with 8 bits per pixel. I have also adapted it to run on the Hercules Graphics Station card, which is controlled by the TIGA

graphics processor and is significantly faster than the Ikon.

CONSTRUCTING

OBJECTS (MODELLING) While researching creative computer media in the visual arts at the Royal College of Art in London [6], I iden- tified the principle of synthesis from

primitives as a useful concept. All the operations provided by Sculptor allow

Fig. 3. Stages in the construction of a chair. (a) A column of spheres has been added using the elevation view and the spheres constrained to align their centres vertically. (b) A smaller column has been added in front of the previous column, using the side view; then both columns copied using the copy group command. (c) Cross pieces are added; each new element is added as a new 'group' to facilitate correction of any errors. (d) The 'finished' chair includes part of a torus (ring of spheres).

t?. , h- .

384 King, Sculptor: A Three-Dimensional Computer Sculpting System

. . Jf t. _ S . -



Fig. 4. Morphl, computer graphics, 1988. Three views of an object rendered on parallel-processing equipment. A solid marble texture was mapped onto the surface; the various characteristics of the texture are controlled by parameters that set, for example, the width and base colour of the veins. The third view has shadows added by a ray-tracing technique (this adds considerably to the rendering time). (Photo: Ian Curington) for the construction or synthesis of some end-result from this (single) primitive, the sphere. While consider-

ing the processes involved, I proposed a distinction between arbitrary and algorithmic synthesis from primitives. Arbi- trary synthesis from primitives implies a

sequence of operations on the object determined solely by the artistic dis- cretion of the user, while algorithmic synthesis from primitives implies a

sequence of operations determined by a set of rules embodied in the machine as an algorithm. The initial choice of rules or starting conditions is still arbitrary, however, in the sense that the artist has complete control over this choice. There is a relationship between algorithmic synthesis and different

types of geometry [7]. Sculptor pro- vides several geometrical operations, or geometrical primitives, that are simple examples of algorithmic synthesis from primitives: cylinder, cone, arc and torus. In the first two cases, spheres are added, small distances apart, along a straight line specified by the user, while in the other cases, spheres are

added along a circular arc, again specified by the user. With object construction using Sculptor, arbitrary and

algorithmic synthesis feature in different proportions in different examples, with different effects on the end-result. Figure 3 shows the construction of a chair, and illustrates how much of the construction work arises from copying previously sculpted groups of spheres. The cylinder and part of a torus are also used in the construction.

Many types of objects can be con- structed using the shortcuts that Sculp- tor offers the artist. However, a more figurative or biomorphic kind of piece requires a similar amount of effort to any traditional medium. Figure 4 shows some different renderings of an object that required this kind of approach. These renderings involving the map- ping of 3D surface textures were carried out on high-performance parallel processing machines by Ian Curington [8], using my stored data file describing the object.

The chair's construction (Fig. 3) was modelled mainly through arbitrary

Fig. 5. William Latham, Form II, computer graphics, 1988. William Latham has been the only artist, other than myself, to use the system. Latham saw the possibilities of ex- ploiting Sculptor to implement some of his ideas on rule-based evolution of form (Form Synth). (Photo: Ian Curington )

synthesis from primitives, with some algorithmic synthesis (using the defini- tions made above), while the amorphous shape of Fig. 4 involved only arbitrary synthesis from primitives. Wil- liam Latham extends the concept of algorithmic synthesis from primitives towards a rule-based system for evolving complex forms called 'Form Synth' [9], and the results are different again. Wil- liam Latham used Sculptor to realise some of his early ideas, and although the execution of the rules within his system was carried out 'manually', there is the potential to drive Sculptor from an expert system that formalises the rules by which he works. Figure 5 is an

example of his use of the Sculptor system, again rendered with 3D texturing by Ian Curington.

I have extended the use of Sculptor with a contour-slicing routine, which will take any Sculptor object (i.e. any object composed of spheres) and plot out a series of cross-sections. The reverse process, that of converting a set of surface points or contours into a set of spheres to define the same object, has been described by O'Rourke and Badler [10]. However the techniques I have used are not a simple reversal of their process, which in any case was restricted to certain object types. Briefly, the contour-slicing technique involves extracting a set of circles for each slice through the object, plotting their points of intersection and creating an ordered sequence of 'exposed' arcs that make up the contour. The routine looks likely to benefit from a spatial-subdivision approach, which re- peatedly divides up the 3D volume sur- rounding the object into sub-volumes and discards those that are empty.

The main purpose of developing the contour-slicing routine was for constructing solid sculptures from lamina

King, Sculptor: A Three-Dimensional Computer Sculpting System 385



Fig. 6. Morphl, plywood laminae, 12 x 8 x 8 cm, 1988. This view shows Morphl as a physical sculpture. The upper part of the figure shows Morphl as a series of contour- slices in orthogonal projection. The lower figure shows the corresponding sculpture made from plywood laminae. Individual contours (100 of them in this case) are plotted, cut out and stuck to sheets of

plywood; the laminae are then cut out and pinned or glued together to form the sculpture. Alignment of successive slices is secured by the use of a grid of numbered register marks overlaying the contour plot.

defined by the contour-slices. Figure 6 shows a sectioned version of the object seen in Fig. 4, in which 100 slices were drawn of an object in an orthogonal projection; the figure also shows the

resulting sculpture made from plywood lamina. The individual contours were

plotted out, stuck onto plywood sheets, cut out with a fretsaw, then nailed and

glued together. Other sculptures have been created as maquettes (models for larger sculptures) using expanded polystyrene sheets, which can be

rapidly cut to the contour using a hot- wire cutter.

The idea of producing sculpture from computer-generated contour slices is not new-Robert Mallary has pro- duced sculptures in marble from contour plots (described by Ruth Leavitt [11]), and, more recently, Mark Dun- hill showed series of sculptures in the Cleveland Gallery (UK) computer art

exhibition [12]. I have so far only con- structed a few small sculptures by this method and have made no attempt to smooth the stepped surfaces, but it is

possible to construct sculptures on a

large scale (several metres high) and then smooth the surface by either re-

moving or adding material.

SCULPTOR: ADVANTAGES, DISADVANTAGES AND

POTENTIALS

The advantages of Sculptor have the same basis as do other creative com-

puter media (such as word processing, paint systems and drafting systems): the infinite editability of the medium. (This in turn requires a special kind of

discipline, in particular, a strong sense of purpose in the face of potentially valuable serendipity.) Using a com-

puter medium is economical in terms of both materials and mess, although this advantage is offset by the cost of computer purchase or access-time.

Sculptor offers the artist a system for

constructing maquettes, with the ability to view the created object from any angle on the screen and, finally, the

option of creating a physical sculpture from lamina, once the development is

complete and satisfactory. As with any sculpting medium, there

are limitations with Sculptor; the only primitive provided with this system, the

sphere, is not appropriate for all forms of sculpture. The type of objects that are natural to build with this system are bulbous, amorphous and, to some

people, rather disquieting or repulsive. Sculptor, like any 3D computer

graphics system, is not as easy to use, in terms of control over the medium, as, for example, a paint system. Paint

systems are more immediate because movement of a mouse or stylus can be seen to add colour directly to the screen in response to the hand, in a simulation of traditional painting techniques; 3D packages involve the complexities of several different views of the same

object while providing very few ana-

logues to conventional non-computer techniques. Sculptor, however, is easier to use than some other interactive solid-

modelling systems because of the use of the sphere as the sole primitive, but this also limits the range of objects that can be created. To enable a wider range of

objects to be created, e.g. those with flat

planar surfaces, the system would have to be extended. The present contour- slicing routine could be used as a

'polygoniser', to yield a planar-polygon data structure for the objects that would allow, for example, the slicing of objects and other more conventional CAD techniques such as subtracting volumes or creating volumes of translation or revolution. This is the most likely route to making Sculptor more universally useful as a modelling tool in which this

system would just form part of a range of techniques, along with CSG and surface modelling, that could be used by artists. A sculptor could model crude organic volumes using Sculptor techniques, run the data through a

'polygoniser', and continue modelling with the more conventional tools of 3D CAD.

Fisher and Masters describe a large sculpture that they designed and built with the aid of computers, though the

technique differed as they used a more traditional CAD system [13]. However, part of their exploitation of the com-

puter lay in the display of the sculpture in a computer simulation of the build-

ing in which it hung. This kind of dis-

play is also possible with Sculptor, which could provide a computer image of an object to be used within an en- vironmental simulation system, either

landscape, architectural or both. Another proposed development for

Sculptor involves the use of a 3D input device (locator), such as a wand. This would make it easier to work in three dimensions, because one would not need to constantly cycle between elevation, plan and side views in order to

specify the 3D position of objects. Sculptors are used to walking around a

sculpture, in order to gain a 3D under-

standing of what they are creating. A single view, which can take from minutes to hours to render, is frustrat-

ing and restricts creativity. A speed-up in rendering time to bring about real- time or near real-time rotations of the rendered objectwould be helpful. New, faster personal computers, and the use of the Hercules card mentioned earlier, have already improved the speed of the system. Faster hardware will undoubtedly be used as it becomes available, thus enabling the system to

respond more rapidly to the flow of ideas.

A PERSONAL EXPLORATION In my recent work with Sculptor I have moved away from constructing single sculptures to building perspective scenes, in which I use relatively simple

386 King, Sculptor: A Three-Dimensional Computer Sculpting System



_ w- mw Fig. 7. Untitled, inkjet print, 250 x 200 mm, 1988. This perspective Fig. 8. Untitled, inkjet print, 250 x 200 mm, 1988. This perspective scene was created by composing several Sculptor renderings using scene was created by blending two different views of the same set a paint system. The transparent effect of the bars is the result of of objects. Using more than one perspective setting in one image is averaging the pixel values of the two images, while the shadow is a a way of reducing the tyranny of the mathematically precise view- shifted, flood-filled copy of one element of the scene. The image ing of 3D computer graphics. (Photo: Hugh Lacey) was output to an ink-jet printer capable of 256,000 colours. (Photo: Hugh Lacey)

shapes to create a world into which the viewer is drawn. Although the system is limited in terms of geometry and

topology, I find that I am painting in three dimensions, directly manipu- lating weight, volume and juxtaposi- tion, following vague spatial intuitions. I have always been drawn to the exploration of perspective in computer graphics, and I find that Sculptor has allowed me to do this in a satisfying way. I take output from the Sculptor system into a conventional paint system, then cut and paste to build up more complex scenes, such as in Fig. 7, where I have taken three separate Sculptor images and combined them carefully, using similar perspective settings. The bars in the foreground were added semi- transparently to suggest a missing vanishing point. Figure 8 shows two renderings of the same scene with different perspective settings, blended together with a shaded background. Although the mathematics of perspective can be limiting to the artist in 3D graphics, the use of several possible viewpoints in one scene, as in this image, can provide new areas of exploration for artists. In some early religious paintings, multiple viewpoints within one scene were used very expressively to give some elements greater importance than others; this has not been explored much in 3D computer graphics.

Color Plate B No. 3 shows a Sculptor scene rendered by Ian Curington, and is typical of recent work with Sculptor. The scene is shown with no perspective, but illustrates some further use of algo-

rithmic synthesis from primitives, in which the rows of spheres have been added using simple loops. I plan to develop the algorithmic side of the system further using fractal techniques or 3D grammars [14]. I have recently had access to the source code of a ray-tracer (courtesy of Richard Wright, my col- league at City Polytechnic), and have adapted this to render Sculptor objects. This takes some of the softness out of the original range of renderings, but has the great advantage of adding shadows, which I find very helpful in the exploration and definition of volume and space.

CONCLUSION

Sculptor started out as a research tool for developing interfacing ideas, but has developed into a system for creating a rather personal type of imagery. There are possibilities for widening the scope of the system, both in terms of making it useful for a wider community of users, and in terms of making physical sculptures from the computer models. It may also represent one of many efforts by artists to write specialised software for their own use or, alternatively, one of many efforts of software writers to pursue the creative possibilities of a system that they have written.

References and Notes

1. M. R King, "Development of an Integrated Computer Art System", in N. Magnenat-Thalmann and D. Magnenat-Thalmann, eds., New Trends in Computer Graphics, Proceedings of the CG International 1988 (Berlin: Springer-Verlag, 1988) pp. 643-652.

2. N. I. Badler, J. O'Rourke and H. Toltzis, "A Spherical Representation of a Human Body for Visualizing Movement", Computer Graphics and Ap- plications, Procadings of the ISEE, 67, No. 10 (Los Alamitos: IEEE Computer Society, 1979) pp. 1397- 1403.

3. K. Knowlton, "Computer-Aided Definition, Manipulation and Depiction of Objects Composed of Spheres", Computer Graphics, 15, No. 1, 48-71 (April 1981).

4. G. Wyvill, C. McPheeters and B. Wyvill, "Data Structure for Soft Objects", The Visual Computer 2, No. 4, 227-234 (1986); B. Wyvill, C. McPheeters and G. Wyvill, "Animating Soft Objects", The Visual Computer2, No. 4, 235-242 (1986).

5. Knowlton [3].

6. M. R. King, Computer Media in the Vtsual Arts and their User Interfaces, unpublished doctoral thesis, Royal College of Art, London, 1986.

7. M. R. King, ~Towards an Integrated Computer Art System", in R.J. Lansdown and R. A. Earnshaw, eds., Computers in Art, Design and Animation, Proceed- ings of the 1986 Conference at the Royal College of Art (London: Springer-Verlag, 1989).

8. I. Curington, "A Normal-Buffer Vectorised Sur- face Shading Model", in Carlo E. Vandoni, ed., Eurographics '85 (Amsterdam: North-Holland, 1985) pp. 365-374. Curington completed these renderings (under the author's direction) atAmaz- ing Array Productions, London.

9. W. Latham, "Form Synth, The Rule-Based Evo- lution of Complex Forms from Geometric Primi- tives", in Lansdown and Earnshaw [7]. Latham is now developing the Form Synth System at IBM UK Research Centre.

10. J. O'Rourke and N. Badler, "Decomposition of Three-Dimensional Objects into Spheres", IEEE Transactions on Pattern Analysis and Machine Intel- ligence 1, No.3,295-305 (July 1979).

11. R. Leavitt, Artist and Computer (New York: Har- mony Books, 1976).

12. S. Chettle, "Art and Computers", exh. cat. (Middlesborough, England: Cleveland Gallery, 1988).

13. R.N. Fisher and R.J. Masters, "Computer- Aided Sculpture: Visual and Technical Considera- tions", Leonardo, 18, No.3, (133-143) 1985.

14. King [7].

King, Sculptor: A Three-Dimensional Computer Sculpting System 387



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Sculptor: A Three-Dimensional Computer Sculpting System