Perspective multiple perspective, non-linear projection Multiple perspective JANUSZ SZCZUCKI Multiple perspective Non-linear perspective Getty Images Outline • Representation systems (projection and perspective) • Alternative perspectives in Computer Graphics (multiple perspective, non-linear) Animation Nonlinearly projected n∗ Karan Singh† ersity of Toronto Plate 2. A multiprojection still life containing 10 camera groups took phive urper hite usdg ns. chbrown hebldg mevan ed erde roStreet. a views, ese green. he cts ed — ee- on- onwdel of about an hour to create with our system. The impressionist style painting was created in a post!process using Hertzmann’s [7] image processing algorithm. (a) Multiple Oblique Projections Figure 1: A nonlinear projection rendering from Ryan, designed with our interactive(b) system True Perspective Projection Plate 3.Multiple oblique projections create an artificial sense of perspective, but still allow some area comparisons. In (b) the pink building’s rooftop area (arrow) is exaggerated. In (a) it correctly artists suchtoas of linear perspective appears bePicasso about thebroke samefrom size asthe theconfines gray rooftop next to it. to integrate the temporal view of a scene as a nonlinear projection. Linear perspective has the primary advantage of being a simple and approximate model of the projections associated with both real cameras and the human visual system. The model also provides simple, consistent, and easily understood depth cues to the spatial relationships in a three-dimensional scene. From a mathematical standpoint, the pinhole camera model is a linear transformation that provides an efficient foundation for current graphics pipelines within which rendering issues such as clipping, shadowing, and illumination are well understood. While a linear perspective view is • Glassner - multiple perspective rendering • Agrawala - multiple perspective projection • Singh - non-linear projection (based on multiple perspectives and based on distortions of geometry) alternate views. This results in an appearance discrepancy between the local regions of the nonlinearly projected image and the linear perspective projections used by the animator to define the nonlinear projection. We introduce two methods for incorporating the multiple views into illumination calculations, and compare these with the single view illumination model. While both are appropriate, we argue for the use of the model that is both more predictable with respect to controlling multiple linear perspective cameras and has Wyvill and Mcnaughton 1990; Glassner 2000] render scenes correctly, but can be difficult to control by artists and are not well suited to interactive rendering. Multi-perspective panoramas capture three-dimensional camera paths into a single image [Wood et al. 1997; Rademacher 1999; Peleg et al. 2000]. While these approaches render correctly, they provide little control over varying the importance and placement of different objects in a scene and are also not well suited to interactive manipulation. Levene describes a framework for incorporating multiple non-realistic projections defined as radial transformations in eye-space[Levene 1998]. Illumi- Representation Systems Consider the problem of representing or depicting a scene in a lower dimension - projection. Singh, RYAN Object-based • Parallel Systems - orthographic, oblique, axonometric, View-based • Perspective - linear perspective (single-point, many- Mixed • Mixed systems and multiple perspectives in a single Figure 3: Preproduction artwork for Ryan incorporating an artistic combination of projection techniques Figure 4: A preproduction composite of multiple nonlinear deformations and isometric projections. 130 point), non-linear perspective/projection. representation. Parallel Projection In this case, orthogonals (projection rays in third dimension) are parallel. Borromini • Object-centric, viewer independent. • Used in early art, Eastern art, mechanical and architectural drawings, used in modern art (cubism, surrealism, expressionism). Perspective Bruce MacEvoy, 2004 Linear single-point perspective Non-linear perspective • View point-centric. • Linear vs. curvilinear: uses vanishing curves instead of vanishing lines or points. Mixed Systems School of Athens, Raphael Breakfast 1914, Juan Gris • Multiple oblique projections and multiple perspectives combined into one drawing system. Done intentionally for different reasons. 3D Representation • Nonlinear and multiple perspective in 3D... time dimension or spatial location as view point can be multiple or non-linear. Frank O. Gehry MIT Strata Center Alternative Projections/Perspectives to CG staple (single-point perspective) • Alternatives are: mixed systems (religious art), multiple projections (cubism), multiple perspectives (Hockney, camera path drawings), curvilinear perspectives (imax, wide-angle, fish eye), non-linear and 3D warps. • Used especially for expressive or representational reasons (i.e. divorce from viewer, make unreal, give best view). Applications "Space Module" Kasa Usterhjusa • Projection on large surfaces (reduce distortion). • • • • Emmersive environments. • Expressive CG imagery and animation. • Simultaneous views of scene and data. IBR (warping, mapping). View-independent rendering. Better representation of data and local regions. Non-linear Perspective Projections in CG Cubism and Cameras, Glassner RYAN, Coleman and Singh • Lots of work being done: image warping, 3D projections, multiperspective panoramas. Multiperspective Panoramas for Cel Animation, Wood et al • Multi-projection rendering. Cubism and Cameras... : Glassner • “Suppose you could take a camera - lens, film, and all - and stretch it like a blob of Silly Putty. You could wrap it around people, simultaneously capturing them from all directions.” • Cubist Camera: presents many interpretations and points of view simultaneously. Cubism and cameras... : Glassner • Glassner combines multiple non-linear perspectives to make the imagery seamless and continuous. The nonlinearity of the perspectives allows them to be merged more easily. • Implemented as a material plug-in that alters the ray by an ‘eye’ surface and a ‘lens’ surface. Example of nonlinear ray tracing. • Nonlinear raytracing handles lighting, but can cause artifacts. occlusion relationship Artistic multiprojection rendering: Visibility Ord Agrawala, Zorin,4.1Munzer • With single linear pr A tool for creating multi-projection (of a multiple perspectives) images and animations. points in 3D space tha • that locallie on Given scene geometry, UI to position and master cameras. Algorithm for multi-projection that solves occlusions. Algorithm must: resolve visibility, constrain cameras (choose best projections or perspectives), and perform interactive rendering. A the C B Artistic multiprojection rendering: Agrawala, Zorin, Munzer (a) Single projection master camera view (b) Multiprojection with depth compositing only (c) Multiprojection with occlusion constraints and depth compositing • Each scene object is assigned to a local camera. • Visibility is difficult because of inconsistent depth ordering. Fig. 2. To reduce the distortion of the column in the single projection image (a) we alter its projection as shown in figure 5(c). In the multiprojection image, the point on the column (triangle) and the point on the floor (circle) coincide. With depth-based compositing alone (b) the floor point “incorrectly” occludes the column point since it is closer to the master camera. However, the column occludes the floor in the master view. Applying this object-level occlusion constraint during compositing yields the desired image (c). Use a ‘master camera’ and object-based occlusion It may be tempting to resolve visibility for the multiprojection image by directly comparing constraints. local depth values stored with each image layer. The rendering algorithm would then be quite simple: we could add the local camera projection to the modeling transformation matrix for each object and render the scene using a standard z-buffer pipeline without resorting to layer based compositing. However, this approach would lead to objectionable occlusion artifacts. Suppose our scene consists of a vase sitting on a table. If we simply add the vase’s local camera projection into its modeling transform, in most cases the vase will intersect the table. Our algorithm handles these situations more gracefully by using the master camera to impose visibility ordering while employing local cameras to provide shape distortion. In our example, the master camera would be the original projection, in which the vase and table do not intersect, and the local projection would affect only the shape of the vase without affecting visibility. • Camera Constraints: for use in animations, based on best camera placement or movement for local scenes (object size, fixed-view, fixed-position, direction and orientation). Artistic multiprojection rendering: Agrawala, Zorin, Munzer Plate 2. A mu about an hour painting was processing alg white bldg brown bldg van • • • Plate 1. Our reconstruction of Giorgio de Fixes distortions, creates ofsurrealist Chirico’s Mystery and Melancholy a Street. The thumbnails show the 5 local camera views, toony styles.geometry highlighted in green. with attached and Plate 2. A multiprojection still lifedisjoint. containing 10 camera groups too Good when objects about an hour to create with our system. The impressionist style painting was created in a post!process using Hertzmann’s [7] imag Doesn’t solve lighting and shadow processing algorithm. problems. Plate 3.Mult perspective, A fresh perspecive: Karan Singh • Creates images from a nonlinear perspective by combining the perspectives of multiple cameras. • Different from Agrawala because resulting image of each object is potentially influenced by all cameras. Figure 6: UI Framework (directional influence) • Can create more continuous multi-perspectives to actually attain a ‘non-linear’ perspective. A fresh perspecive: Karan Singh (e) (f) (g) Figure 9: Case Study I Figure 10: Case Study II: Camera configuration [4] S. Carpendale. A framework for elastic presentation space. PhD. Dissert., Simon Fraser University, 1999. [5] J. Dorsey, F. Sillion and D. Greenberg. Design and Simulation of Opera Lighting and Projection Effects. Computer Graphics, 25(4):41–50, 1991. [6] J. Foley, A. van Dam, S. Feiner, J. Hughes. Computer Graphics, Principles and practice. Addison-Wesley, Chapter 6:229–284 ,1990. [7] C. Fu, T. Wong, P. Heng. Warping panorama correctly with triangles. Eurographics Rendering Workshop,1999. [8] E. H. Gombrich. Art and Illusion. Phaidon, 1960. [9] J. Gomez, L. Darca, B. Costa and L. Velho. Warping and Morphing of Graphical Objects . Morgan Kauf(a) (b) mann, San Francisco, California, 1998. (c) • Interactive and familiar approach. • Can weight cameras based on distance from object cent squares are the 8 viewports. Some of them are horizontally scaled by -1 to unravel the object better but are also responsible for the twisted behavior of the orange and royal blue edges. The relative depth of the purplemagenta-pink corner’s viewport is changed to make the cube projection appear turned inside out in Figure 11b. Note that the silhouette of the edges remains unchanged. Changing the depth on the green-limegreen-blue corner’s viewport in Figure 11c results in a 2D projection of a 3D projection of a tesseract (a common Escher motif, held by the jester in his work titled Belvedere). 4 [10] E. Gröller. Nonlinear ray tracing: visualizing 11: Case Study II strange worlds. The Visual Computer,Figure 11(5):263– 276, 1995. [11] M. Inakage. Non-Linear Perspective Projections. Proceedings of the IFIP WG 5.10), 203–215, 1991. [12] J. Levene. A Framework for Non-Realistic Projections. Master of Engineering Thesis, MIT, 1991. [13] D. Martin, S. Garcia and J. C. Torres. Observer dependent deformations in illustration. NonPhotorealistic Animation and Rendering 2000 , Annecy, France, June 5-7, 2000. [14] N. Max. Computer Graphics Distortion for IMAX and OMNIMAX Projection. Nicograph ’83 Proceedings 137–159. [15] S. Peleg, B. Rousso, A. Rav-Acha and A. Zomet. Mosaicing on Adaptive Manifolds. IEEE Transactions on Pattern Analysis and Machine Intelligence, 22(10):1144–1154, 2000. [16] P. Rademacher and G. Bishop. Multiple-Center-ofProjection Images. SIGGRAPH, 199–206, 1998. [17] P. Rademacher. View-Dependent Geometry. Computer Graphics, 439–446, 1999. [18] T. Sederberg and S. Parry. Free-form deformation of solid geometric models. Computer Graphics, 20:151–160, 1986. [19] S. Seitz and C. Dyer. View Morphing: Synthesizing 3D Metamorphoses Using Image Transforms. Computer Graphics, 21–30, 1996. [20] Edward Tufte. Envisioning Information. Graphics Press, 1990. [21] D. Wood, A. Finkelstein, J. Hughes, C. Thayer and D. Salesin. Multiperspective Panoramas for Cel Animation. Computer Graphics, 243–250, 1997. [22] George Wolberg. Digital Image Warping. IEEE Computer Society Press, 1992. or viewing direction of camera (localizes effect of camera). Conclusion We have presented a new interactive approach for exploring and rendering 3D objects. Our chief contribution is an intuitive way for artists to experiment with a 3D subject and subsequently convey it expressively in a 2D rendering. Aside from its applicability to non-photorealistic and painterly rendering, the model has wider applications in scientific visualization, where the limitations of the traditional linear perspective are well recognized [20]. Interaction of illumination models with such non-linear projections is fertile area for future research. In conclusion, the approach presented in this paper marks a step towards overcoming the limited expressive potential of existing projection models. We hope that this work will motivate further discussion and open the door to an interesting new type of computer generated imagery. • Does not handle illumination issues and does not control global scene coherence. References [1] M. Agrawala, D. Zorin and T. Munzner. Artistic Multiprojection Rendering. Eurographics Rendering RYAN: Rendering your animation nonlinearly projected • Nonlinear projection system that integrates (a) (b) (c) (d) into the conventional animation workflow. • Interactive techniques to control and render scenes using nonlinear projections. (e) (f) (g) (h) • A linear combination of linear perspectives. Figure 15: Ryan Bathroom Set RYAN: Rendering your animation nonlinearly projected 1. Distorts scene geometry so under linear perspective appears nonlinearly perspective. 2. Provides interactive authoring of nonlinear projections with scene constraints and linear perspective cameras. 3. Addresses nonlinear projection’s effect on rendering and illumination. In a mixed perspective scene, the goal is to keep qualities of global coherence and local distortions of geometry and shading result from the changes in perspective. RYAN: Rendering your animation nonlinearly projected • Boss camera is the traditional linear perspective. Lackey cameras represent local linear views. • Lackey camera deforms objects (in scene space) so that through the boss camera, they have view properties of the lackey, depending on weight of lackey for the objects. • Incorporate the multiple views of the lackey cameras into the illumination calculations. (a) Pillar, Rt (lackey view) (b) Constraint deformed pillar, Rt , Rf (boss view) RYAN: Rendering your animation nonlinearly projected Figure 7: Constraint setup The resulting spatial constraint matrix1 is: (a) With constraints Con = (Cartesianize(Rf Ci Mi Vi )) −1Cartesianize(Rt CbMbVb) (3) The resulting deformation transform for the lackey camera with a constraint is similar to Equation 1, but with the constraint matrix appropriately inserted: Ai = Ci Mi Vi (Con)(CbMbVb) −1. (4) • Constraints maintain global Con is most often a per object constraint defined for all lackey cameras, but it can also be global for all objects or even defined on a selective basis per object per lackey camera. Figure 7 demonstrates the use of a position constraint on a pillar seen from an alternate point of view. Figure 7a shows the original pillar geometry from the lackey camera’s point of view, as well as a reference frame Rf that indicates a positional constraint on the geometry. Figure 7b shows the column deformed to have the projective appearance of the lackey camera’s point of view, but seen from the boss camera, which is located to the right of the lackey camera. Without application of the constraint, the column would appear at the same location in screen space in each view. The additional reference frame Rt indicates the deformed position of the constrained point, and the constraint effects the image space transformation necessary to hold the column in place relative to Rf . For complex objects it might be necessary to define multiple constraints. Points on the object are constrained to proximal reference frames. Formally stated, a set of constraints Con1, ..,Conm are defined using frames Rf 1, .., Rf m and Rt1, .., Rtm. The constraint matrix Con(P) for a point P is defined using frames Rf (P) and Rt (P). Rf (P) and Rt (P) are computed as weighted interpolations of frames Rf 1, .., Rf m and Rt1, .., Rtm, respectively. The weight for constraint j is inversely proportional 2 to the Euclidean distance Rt (lackey view)R . We (b)precompute ConstraintApre deformed from P(a) toPillar, the origin of frame i = Ci Mi Vi fj pillar, R , R (boss view) t f −1 and Aposti = (CbMbVb) to represent the deformation of a point P, combining Equations 2 and 4 as: coherence (and stop walls from collapsing). (b) Without constraints Figure 6: Removal of scene constraints: wall and ceiling collapse into scene 2.1 Constraints Agrawala et. al.[2002] demonstrate that for multiple linear projections, it can be desirable to constrain objects in space to preserve their relative position and size in a composited scene. They handle these constraints with a translation and scale in screen space after the object has been projected. Singh[2002] allows a user to control the relative position and size of camera projections through viewport transformations within the canvas. Figure 6 shows the importance of constraints in our system. The removal of sceneconstraintscausesthetableon theleft to undergo a large vertical translation due to the differing positions of the lackey (a) With constraints • Camera weights restrict influence. • Chained lackeys (in-betweens) for better interpolation between boss and lackey and for better illumination blending. P" = P+ Figure 7: Constraint setup P(w ∑ iP((Aprei )(Con(P))(Aposti ) − I )). n i= 1 The resulting spatial constraint matrix1 is: 2.2 Camera Weight Computation −1 (5) Figure 5b, theisgeom isolated from the projection model,In and projection disa camera5viewpoint. Note As a comparative example, Figure shows three var thewith viewer illumination for an object viewed tworeflecting cameras.spo T rectly illuminated by bos spo has been deformed such that when viewed from the tocombination the presenceofofprojecti the lac it appears as an equally weighted (a) Camera setup (b) Boss camera view by whichview. the illu boss camera’s view and to the methods lackey camera’s incorporated. In Figure 5 shows the layout of the cameras, the undeformed geom lationThe between boss ar two spotlights used for illumination. virtualthe camera in the illumination calcul an interpolated viewpoint. two modified In Figure 5b, the geometry is illuminated withhighlights respect t respectone to directly both thei camera viewpoint. Note the two with highlights: resulting the viewer reflecting spotlight 1 are andblended, one halfway to thein close to appe rectly illuminated by spotlight 2,arewith noenough illumination e this illumination techniq to the presence of the lackey camera. Figures 5c and 5d (b) Boss camera view ation ofthe images such as s methods by which the illumination from lackey camera incorporated. In Figure 5c a virtual camera representing a lation between the boss and lackey cameras is used as the 4 theImplementat (c) Nonlinear projection (d) Nonlinear projection (corin the illumination calculations for entire object, result (wrong shadows and shadrect shadows and shading) two modified highlights. In Figure 5d, the object is ill This section describes ing) with respect to both the boss and lackey cameras, and tht ten as a plug-in to the a are blended, resulting in four attenuated highlights, alth the interface the system are close enough to appear as single stretchedtohighlight. Figure 10: Shadows Singh[2002]. defor this illumination technique to stylized shaders The allows fo rated into ation of images such as seen in Figure 9. Maya is then p e next section). RYAN: Rendering your animation nonlinearly projected • Use original geometry so shading is not based on the deformed geometry. • Illuminate by blending illumination of boss and lackey cameras, or set a single view point for lighting. 3.3 (d)Illumination Nonlinear projection (cor- 4 Implementation 4.1 User Interface RYAN: Rendering your animation !"#$%&!'()'*+(,&"-. nonlinearly projected ! References A GRAWA L A , M ., Z ORI N , D., A ND M UNZNER, T. 2000. Artistic multiprojection rendering. In Proceedings of Eurographics Rendering Workshop 2000, Eurographics, 125–136. A L EX A , M . 2002. Linear combination of transformations. In Proceedings of the 29th annual conference on Computer graphics and interactive techniques, ACM Press, ACM, 380–387. BA RR, A . H. 1986. Ray tracing deformed surfaces. 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Salesin - Multiperspective Panoramas for Cel Animation, Proceedings of SIGGRAPH 97 • Glassner, Andrew S., "Cubism and Cameras: Free-form Optics for Computer Graphics", Microsoft Research, January 2000. • http://www.gettyimags.com • EECS MIT http://www.eecs.mit.edu • http://www.archcenter.ru/eng/news/fluidarh/default.asp • Janusz Szczucki - Multiple perspective. Mala Gallery, Warsaw, Poland, January 2000