1 Introduction 3. 2 Previous work Available tools Similar projects Cracking in the real-world and cinema...

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2 Contents 1 Introduction 3 2 Previous work Available tools Similar projects Cracking in the real-world and cinema Technical background Houdini SOPS, DOPS and VOPS Implementation Approach Houdini cracking tool Overview and context Crack generation Noise Boolean operation Displacement Colouring along the crack Propagation and animation Debris Approaches Shattering Simulation User interface Optimisation and known problems Results Conclusion 22 Appendix 23 References 25 2

3 Chapter 1 Introduction The creation real-world phenomena is a very common task in CGI and visual effects. From tornados to avalanches, from apocalyptic tsunamis to earthquakes. For the CGI Techniques module, cracks as they occur in earthquakes have been investigated further. A user-friendly tool has been built to allow easy creation and artistic control of a crack on a given surface. This report documents the background research and explains technical aspects. Furthermore, the key problems during implementation are explained and the solutions presented. Andy Abgottspon 3/25

4 Chapter 2 Previous work 2.1 Available tools A lot of the techniques and tools used in the visual effects industry remain a secret and are never published. Companies develop their own sets of tools to integrate into their pipeline and keep the specifics of their toolchain hidden. Because of this, the main source of background reading for this project were online tutorials and forums. One of the main sources used was the Houdini Hip Tricks Volume 1 DVD (Castaneda 2011) and its chapter about the multi-purpose ground cracking tool. Many insights and best practices were gained from this and implemented in the final tool. Another useful resource were the odforce forums where people were discussing different approaches in creating cracks (odforce 2011). 2.2 Similar projects Two master s theses by previous MSc students offered a good starting point for Houdini tool development and techniques related to this project. Peter Claes thesis about controlling fluid simulations contains a good introduction to Houdini s node-based structure and DOPS (Claes 2009). Goffredo (2010) gives useful information about the use of shattering in cinema as background for his shatter tool. Andy Abgottspon 4/25

5 2.3 Cracking in the real-world and cinema Cracks are omnipresent in CGI and are heavily used throughout a variety of films. In many cases, these effects are achieved with the aim to create a pleasant visual effect rather than a realistic physics simulation. In nature, cracks most typically occur as a result of earthquakes (see Figure 2.1). Figure 2.1: Reference picture of an earthquake crack in New Zealand (Probaway 2010) In films and trailers, these cracks appear in various different styles. For example, the trailer to Ice Age s Scrat s Continental Crack-up (see Figure 2.2) contains a multitude of scenes featuring cracks. The same is true for the movie 2012 (see Figure 2.3). Andy Abgottspon 5/25

6 Houdini Cracking Tool Figure 2.2: Various scenes containing cracks in the trailer to Scrat s Continental Crack-up (Scratś Continental Crack-Up 2010) Figure 2.3: Poster of the movie 2012 ( ) Andy Abgottspon 6/25

7 Chapter 3 Technical background 3.1 Houdini The 3d software package Houdini was chosen to develop a tool for easy, artist-friendly crack creation. Due to Houdini s open node based architecture, existing nodes can be easily viewed, modified and/or extended. This is a major advantage and also provides a clean visual representation of all operations and functions. SideFX describes their node-based system as follows: Nodes are the basis of everything that Houdini does. When you create geometry, rig a character, and so on, Houdini is creating nodes behind the scenes. The nodes record the actions you took and the parameters you used. You can go back in time and change parameters and selections of existing nodes to edit the history of how your scene was created. (SideFX 2011b) 3.2 SOPS, DOPS and VOPS Inside Houdini, there is a variety of node types. This project mainly relies on surface operators (SOPS) and dynamic operators (DOPs). SOPS create and manipulate geometry (curves, surfaces, metaballs, and so on) and live inside a Geometry container object. DOPS set up the conditions and rules for dynamics simulations (SideFX 2011b). In this project, rigid body dynamics were used to simulate shattering as further explained in Section Additionally, vertex operators (VOPS) are a graphical way to perform low-level mathematical functions that are then executed as VEX (Houdini s vertex expression language), as used for many applications like shaders or for this project the displacement (see Section ). Andy Abgottspon 7/25

8 Chapter 4 Implementation This chapter describes the development of the tool as well as the different approaches and techniques chosen. 4.1 Approach The basic idea was to create a tool that gives artistic control over the cracking of a surface and allows configuration of a variety of aspects such as depth, thickness, level of detail, displacement, crack animation, etc. The tool should also use the appropriate methods and nodes within Houdini rather than re-inventing existing practices. There are two main parts that had to be addressed: the first part deals with the actual generation of the crack based on a curve that is provided by the user/artist. Several operations like noise are applied to the original curve before the crack is cut out of the base geometry (also provided by the user) and further refined (see Section 4.2.2). The second part is the debris, consisting of generating shattered geometry on the inside of the crack and then simulating their fall using rigid body dynamics as shown in Section Houdini cracking tool Overview and context A Houdini Digital Asset (HDA) was created using the identifier "aa_crack_tool". It is included in the digital asset library file in the otl folder which can be installed via File -> Install Digital Asset Library. Andy Abgottspon 8/25

9 The tool lives inside a geometry node and is grouped into certain parts internally, as seen in the network via in Figure 4.1. Regarding the pipeline, this tool is typically used to create an effect which can then be rendered out in various passes onto a compositing package like Nuke. Figure 4.1: Network view of the final Houdini Digital Asset showing the main groups: terrain, create curve, moving ground, shatter, cut out and texture inside, grouping and subdivision, sweeping, displacement and finalising. Andy Abgottspon 9/25

10 4.2.2 Crack generation Noise The starting point for the crack is a curve provided by the user. This curve can have an arbitrary number of points. In the process, the curve is resampled using the Resample SOP as shown in Figure 4.2 (b). Figure 4.2: Generation of the crack based on a curve provided by the user In the two further steps, Fractal and Jitter SOPS are used to achieve the final effect as shown in Figure 4.2 (d). The parameters Fractal Scale and Jitter Seed for these nodes can Andy Abgottspon 10/25

be influenced in the user interface (see Section 4.2.5). 4.2.2.2 Boolean operation Next, the crack is cut out of the user-provided surface. To achieve this, the curve from Section 4.2.2.1 is processed to form a tube that is then subtracted from the base geometry (grid) as shown in Figure 4.

11 be influenced in the user interface (see Section 4.2.5) Boolean operation Next, the crack is cut out of the user-provided surface. To achieve this, the curve from Section is processed to form a tube that is then subtracted from the base geometry (grid) as shown in Figure 4.3. Figure 4.3: A tube is created using the PolyWire SOP and then subtracted from the grid using a boolean operation (Cookie SOP) The thickness of the tube created by the PolyWire SOP is controlled by the following expression for the parameter wire radius of the tool s create_tube_from_points node: ch ( ".. / tube_radius " ) 2 chramp ( ".. / t h i c k n e s s ", 1/$NPT ($PT+1), 1) 3 chramp ( ".. / crack_animation ", i f ($F > ch ( ".. / nb_of_frames " ), 1, 4 1/ ch ( ".. / nb_of_frames " ) $F), 1) This expression returns the tube radius at any given point of the curve. It takes the general tube radius into account which is simply a multiplier specified in the user interface. Further- Andy Abgottspon 11/25

12 more, the thickness can also be controlled in a ramp (see Figure 4.4) to allow for a uniform, linear or completely arbitrary thickness along the curve. Figure 4.4: Ramp parameter giving control over the crack thickness The chramp expression takes ramp path, position and component index as parameters (SideFX 2011a). The position is expected to range from 0-1, so the current point number $PT has to be scaled to that range by multiplying it with 1 / $NPT (number of points). We also have to add 1 since points start at 0 in the case of our curve. For example, in a curve with 10 points, the fifth point number ($PT = 4) will return the position of 0.5 (1/10*(4+1) = 0.5) in the ramp. The same applies for the animation part, which is explained in more detail in Section Displacement Before deforming the geometry along the crack, detail is added. In order to avoid subdivision of the entire base geometry and thus creating a lot of unused additional geometry, only the parts along the crack are subdivided as shown in Figure 4.5. Andy Abgottspon 12/25

Figure 4.5: Extrusion and grouping based on scaled tube from the Polywire SOP (left). Subdivisions and displacement along the crack edges (right).

Noise is generated and multiplied by the imported colour (how this colour is assigned is explained in Section 4.2.

This causes points to be relocated in a random fashion that can be controlled using parameters like frequency, offset, roughness and max octaves. Figure 4.

13 Figure 4.5: Extrusion and grouping based on scaled tube from the Polywire SOP (left). Subdivisions and displacement along the crack edges (right). The displacement is performed in a VOP network as shown in Figure 4.6. Noise is generated and multiplied by the imported colour (how this colour is assigned is explained in Section ) before being added to the original point position. This causes points to be relocated in a random fashion that can be controlled using parameters like frequency, offset, roughness and max octaves. Figure 4.6: VOP network showing displacement using a Anti-Aliased Noise and an input colour to control the amount Colouring along the crack To provide additional visual control, the colour of the edges along the crack can be controlled. This colour information is at the same time used for the displacement, making sure only areas close to the crack get displaced. Andy Abgottspon 13/25

The Sweep SOP was used to distribute a simple red line along the crack as shown in Figure 4.8. In order to add some sort of turbulence, the expression in Figure 4.7 was used.

14 The Sweep SOP was used to distribute a simple red line along the crack as shown in Figure 4.8. In order to add some sort of turbulence, the expression in Figure 4.7 was used. The first part of the expression creates a value using the sin function together with pow to expand the high and low values. This is then multiplied with a random value and finally added to the second part which is the base thickness as explained in Section Figure 4.7: Expression influencing the scale of the sweeping operation Andy Abgottspon 14/25

Figure 4.8: Sweep SOP distributing lines along the curve (top). The colour information is then passed onto the geometry using an AttributeTransfer node. 4.2.

This is achieved by using the Carve SOP to clip the curve based on the following expression (carve_crack_animation node, parameter Second U ): 1 i f ($F < ch ( ".. / nb_of_frames " ), 2 chramp ( ".

15 Figure 4.8: Sweep SOP distributing lines along the curve (top). The colour information is then passed onto the geometry using an AttributeTransfer node Propagation and animation The tool by default animates the crack from bottom to top. This is achieved by using the Carve SOP to clip the curve based on the following expression (carve_crack_animation node, parameter Second U ): 1 i f ($F < ch ( ".. / nb_of_frames " ), 2 chramp ( ".. / crack_animation ", 1/ ch ( ".. / nb_of_frames " ) $F, 1), 3 1) The if-statement makes sure the animation does not repeat after the specified number of frames are passed. In this case, 1 is returned which is equivalent to no clip. Similar to Section , chramp is used to read the animation value from the ramp in the user interface Debris Approaches Dynamics in Houdini are a great way to simulate and add realism to an effect. However, simulations are usually very expensive in terms of computation allow only a limited amount of artistic control. Andy Abgottspon 15/25

One way is to attach every piece of geometry to a particle and use traditional POP (particle operator) networks to control them.

16 Particles can be driven by a variety of forces like drag, wind, fans and in addition be affected by attractors. For rigid bodies in RBD simulations, a lot of these options are unavailable and although forces like gravity can be applied, they can not be positioned. There are several approaches to solve this. One way is to attach every piece of geometry to a particle and use traditional POP (particle operator) networks to control them. Unfortunately, this makes interaction between the shattered pieces more complicated. Another alternative is to use dynamic groups to control which elements are affected by forces like gravity. For the crack, it is desirable that the pieces of geometry start falling from the centre along the crack. To achieve this with groups would be rather difficult, which is why an approach using an additional piece of geometry was chosen (see Figure 4.9). Figure 4.9: Approach using a specifically carved object to control the falling. The ground object is moving back (left), causing the remaining geometry to fall (right). Using this particular shape, the pieces in the centre start falling first Shattering Before the simulation can run, the main geometry inside the crack needs to be pre-shattered. For this, Houdini s Shatter shelf tool was used which "cuts a model along a jagged line multiple times to create pieces you can then use with the RBD Fractured Object or RBD Glue Object" (SideFX 2011c). Andy Abgottspon 16/25

4.2.4.3 Simulation Houdini uses DOP (dynamic operator) networks to define the setup of a rigid body dynamics simulation. A network as shown in Figure 4.

17 Simulation Houdini uses DOP (dynamic operator) networks to define the setup of a rigid body dynamics simulation. A network as shown in Figure 4.10 was built, connecting the various components of the simulation with each other and the appropriate solvers. The merge node specifies in which way objects affect each other (uni-directional, mutual, etc.). Gravity was applied as the only force. However, this force can be scaled up to simulate different conditions and make objects fall quicker. Figure 4.10: DOP networks showing simulation of the moving ground object with the pre-shattered pieces It is also worth pointing out that the friction of the moving_ground object in Figure 4.10 was set to 0. This causes the crack pieces to stay at their original position without moving back. The friction value can be changed if this effect is desired User interface The user interface is kept as simple as possible, by only focussing on the essential settings (see Figure 4.11). Ramps gives a lot of artistic control over the timing of the animation and the thickness of the crack. Furthermore, high-res can be turned off for quicker performance Andy Abgottspon 17/25

18 and less cooking operations. For more information about the settings, please refer to the Help file (see Appendix). Figure 4.11: User interface of the cracking tool with tabs for general settings, crack size and attributes as well as rendering Optimisation and known problems Using the appropriate nodes to delete attributes, groups and geometry after they have been used had a major impact on memory consumption and performance. It is therefore advisable to be very careful and delete these as soon as they are not needed anymore (see Figure 4.12). Andy Abgottspon 18/25

Figure 4.12: Deleting all groups after they are being used The tool relies on boolean operations to subtract the crack from the extruded base geometry.

Figure 4.13). Figure 4.13: Problem with boolean operator (Cookie SOP) using high amount of jitter and resampling 4.

19 Figure 4.12: Deleting all groups after they are being used The tool relies on boolean operations to subtract the crack from the extruded base geometry. This however causes problems in some cases of particularly high jitter values or when the resample length is very low and therefore a lot of points are added (see Figure 4.13). Figure 4.13: Problem with boolean operator (Cookie SOP) using high amount of jitter and resampling 4.3 Results Here are some of the results that can be achieved by using the cracking tool in various setups. There is a lot more potential on the rendering side. Particles could be added and the choice Andy Abgottspon 19/25

20 of textures plays a major role as well. For these examples however, the focus shall remain on the pre-visualisation of the cracking shape and shatter rather than creating final renders. Figure 4.14: Preview showing stylised crack (high-res flag turned off) Figure 4.15: Preview showing crack with displacement using the high-res settings Andy Abgottspon 20/25

17: Preview showing RBD simulation (Number of Chunks = 500).

21 Figure 4.16: Preview showing crack with texture assigned to the input geometry (grid) Figure 4.17: Preview showing RBD simulation (Number of Chunks = 500). The timing between the moving ground and the animation however still needs improvements when compared to the simple demo in Figure 4.9. Andy Abgottspon 21/25

22 Chapter 5 Conclusion Given the very short time frame for research and implementation, the tool fulfils the main requirements of creating an artist-friendly tool for crack generation. Features like the rigid body dynamics were not originally planned but still turned out to be a useful addition to achieve certain visual goals. There are several areas for future work. The approach for simulation as described in Section is still very new and would require more experimentation to have a certain number of pre-defined shapes for different aesthetics. User feedback could also be very useful to further improve usability and combine these inputs in the development of new tools and/or extension to the existing one. In terms of GUI, more control could be added for specific attributes that the user desires more control, e.g. more rendering features (particles, dust, etc.) and more sophisticated cracking patterns. Finally, performance can always be improved, especially when dealing with large amounts of shatter objects and a high level of subdivision. For some specific cases, a tool that does not rely on boolean operations might be desirable, due to the nature and limitations of these operations (see Section 4.2.6). Andy Abgottspon 22/25

23 Appendix Help section for the Houdini Digital Asset. Houdini 11 Nodes Surface nodes Search Find Cracking Tool surface node The cracking tool can be used to create and animate cracks based on a curve that can be provided. Additional settings allow shattering and RBD. Parameters General On this page Parameters General Crack Size Crack Attributes Rendering Inputs High-Res Turn on subdivision and higher resolution. The amount of subdivision can be defined under Crack Attributes! Bricker Polygon Size. Debris (RBD simulation) Number of Chunks Crack Animation Duration (in Frames) Creates geometry inside the crack that is then used for rigid body dynamics. The degree of shatter for the inside of the crack. This ramp determines the animation of the crack. Timespan over which the crack is animated. Crack Size Crack Depth Depth of the crack. This is the amount by which the base geometry gets extruded. Tube Radius Thickness The basic width of the crack. This ramp can be used to have better control over the thickness along the curve. Crack Attributes Andy Abgottspon 23/25

24 Bricker Polygon Size Turn on subdivision and higher resolution. The amount of subdivision can be defined under Crack Attributes! Bricker Polygon Size. Resample Length Defines how long the maximum segment in the curve is. The curve gets resampled to have more detail before the jitter and fractal are applied. Fractal Scale Jitter Seed Amount of fractal noise. Random seed for the jitter operation. This can be used to generate different results or help solve artifacts that might occur with the cookie operator. Rendering Amplitude Amplitude of the color around the edges. Frequency Frequency of the color distribution along the curve. Crack Color Color of the crack along the edges. Texture inside Inside Material Scale Allows texturing the inside of the crack. Material assigned for the inside of the crack. Scale of the texture applied. Offset UV offset of the texture applied. Inputs Curve Curve with arbitrary number of points to form the base of the crack. Grid Base grid or surface. Andy Abgottspon 24/25

25 References 2012 (2009), Centropolis Entertainment. Film. Directed by Roland Emmerich. Castaneda, A. (2011), Houdini Hip Tricks Volume 1, cmivfx. Claes, P. (2009), Controlling fluid simulations with custom fields in houdini. Available from: [Accessed 10 May 2011]. Goffredo, E. (2010), A tool for procedural destruction in houdini. Available from: [Accessed 10 May 2011]. odforce (2011), Animating a crack up a wall in houdini. Available from: [Accessed 10 May 2011]. Probaway (2010), Earthquake in new zealand. Image. Available from: z ealand q uake b ridge s t.jpg[accessed10may2011]. Scratś Continental Crack-Up (2010), Blue Sky Studios. Film. Directed by Steve Martino and Mike Thurmeier. SideFX (2011a), Houdini 11 documentation chramp expression function. Available from: [Accessed 10 May 2011]. SideFX (2011b), Houdini 11 documentation nodes. Available from: [Accessed 10 May 2011]. SideFX (2011c), Houdini 11 documentation shelf tools shatter. Available from: [Accessed 10 May 2011]. Andy Abgottspon 25/25

Images from 3D Creative Magazine. 3D Modelling Systems

Images from 3D Creative Magazine 3D Modelling Systems Contents Reference & Accuracy 3D Primitives Transforms Move (Translate) Rotate Scale Mirror Align 3D Booleans Deforms Bend Taper Skew Twist Squash