Non-Photorealistic Rendering Techniques for Motion in Computer Games

1. Non-Photorealistic Rendering Techniquesfor Motion in Computer Games Michael HallerChristian HanlJeremiah DiephuisUpper Austria University of Applied Sciences, Media Technology and Design, Hagenberg (ACM Computers in Entertainment, 2004)

2. Introduction • Computer graphics has long been defined as the quest to achieve photorealism • Programmable hardware makes rendering photorealistically not only possible, but allows us to produce stylized renderings • Users expect realistic behavior in worlds that are rendered photorealistically

3. Varieties of Realism – Ferwerda[2003] • Physical Realism • the virtual objects provide the same visual simulation as the real scene • Photorealism • the image produces the same visualresponse as the scene • Functional Realism • the image provides the same visual information as the scene

4. Functional Realism • NP pictures can be more effective at conveying information • Is there a way to express motion ? • Are there any “rules”, especially for games, that enhance the users’ perceptions ? • We don’t usually depict the motion of objects in computer-generated dynamic images－especially in real-time rendered dynamic images • Although motion is essential, it is not usually visualized

5. Related Work the most important for still image • Motivation • Walt Disney Studio: 12 principles of animation [1930] • guide production and creative discussions as well to train young animators better and faster • help us to create more believable characters and situations, ex: bouncing ball • McCloud[1993] has also analyzed the abstract illustration of motion in comics • motion lines • multiple images

6. Motion Lines Techniques • Most common one for visualizing motion in comics • This Technique can express high degree of movement, and emphasizes the quality of the illustration’s dynamics • Its usage in a dynamic environment would enhance the users’ perception as well • Fun factor: the attention of the user becomes more focused on motion

7. Multiple Images • Create a “blurred” object on the screen, often seems to be more realistic than the normal movement of the object without the blur effect • When objects move very fast, our eyes expect to “see” something (still images,contours), even if the object if off the screen • Very complex motions can be emphasized with this technique because users have more time to analyze movement

8. Squash-and-Stretch [Disney] • A combination of the two techniques, together with Disney’s squash-and-stretch technique, will achieve the best results for visualizing motion in real-time • The deformation of an object due to itsspeed is a very important technique in enhancing visual perception • The squash-effect exaggerates the objectand the stretch-effect anticipates the collision

9. Our Approach • The motion of objects can be classified into the following three groups • squash-and stretch • multiple images • motion lines • The three methods can by combined to enhance the graphical behavior of motion

10. Requirements & Limitations • Detailed geometric data for all objects we want to apply the technique to • The object’s geometric data has to be modifiable • The position of the object has to be available at all times • Only past motions can be considered, future motions cannot

11. Squash-and-Stretch • Adjust the style of motion • (direction, speed or acceleration of movement as well as the rigidity of the object) • Maximum speed, vmax • Maximum acceleration, amax • Maximum speed scale, kvmax • Maximum acceleration scale, kamax • Minimum acceleration scale, kamin • User defined and assigned to a specific object • The desired results can be achieved easily by varying the parameters maximum stretch minimum squash

12. Squash-and-Stretch stretch stretch squash result scaling parameter • Speed-dependent scaling parameter

13. Deformation • Simulating Cartoon Style Animation [Chenney 2002, University of Wisconsin at Madison] • The stylistic choices we made in defining our deformations are: • The deformations should be volume preserving • Each object has a natural set of deformation axes, including one principle axis • These axes define a deformation coordinate system with the x-axis aligned with the “forward” direction for the object

14. Deformation * s = kresult • The deformation is controlled by a single parameter, s, which is the scaling coefficient along the principle axis • Scale the other dimensions equally according to our volume preserving requirement

15. Squash-and-Stretch

16. Multiple Images • Snapshots of the moving object are taken for every constant time interval that can be configured • The easiest form of multiple images if to draw the whole object several times (real-time) • A texture containing all contour replications is drawn in the background • Replication rate, n: the amount of contour replications generated per second • A common style of multiple images is the continuous decrease in contour replications

17. Multiple Images

18. Multiple Images

19. Motion Lines - Requirements • Schulz [1999] calls for a couple of requirements for the motion lines in still image • The upper and lower motion lines relative to the direction of motion have to be at the same level as the maximum extent of the object • Object’s geometrical data (vertices) → starting points of motion lines

20. Motion Lines - Requirements • Motion lines are to be placed on significant region of the object • The number of motion lines has to be configurable • The space between motion lines must not be too constant

21. Motion Lines - Requirements • Accumulations of motions on certain regions of the object are not allowed • Motion lines are to begin after the object and are not allowed to cut the object • The length of motion lines has to be configurable

22. Motion Lines • To determine the starting points of the motion lines, the positions of the vertices are transformed into screen coordinates

23. Motion Lines • To get the starting points more easily, the object is rotated to the motion direction • Now the limiting vertices can be determined by comparing their coordinates of the y-axis

24. Motion Lines • To get the starting points for the motion lines in between, the object is split into equally large strokes • In every stripe, the vertex with the lowest x-coordinate is used as a starting point

25. Motion Lines • A particle system is used to draw the motion lines • particle emitters: correspondingto the number of motion lines,each of these emitters releases a defined number of particles • life cycle: a particle lasts for a predefined period (properties of the particle are changing) • position of the emitter is updated every program cycle

26. Implementation • OpenGL & Cg, Halflife model as file format • Squash-and-Stretch: vertex shader • //1. Set the position toe the model originfloat3 modelOrigin = float3(0, 0, 0);modelOrigin = Vertor Transform ( modelOrigin, boneMatrix);position.xyz -= modelOrigin;//2. Rotate the object based ob the direction of the motionposition.xyz = VectorRotate ( position.xyz, motionMatrix );//3. Apply the scalingposition.x *= motionScaleX; //k_resultposition.y *= motionScaleYZ; //k_normposition.z *= motionScaleYZ; //k_norm//4. Bring the object to its original orientationposition.xyz = VectorIRotate ( position.xyz, motionMatrix);//5. Move the object back to the original positionposition.xyz += modelOrigin;

27. Implementation • Multiple images • It is necessary to switch between the two projection modes • Orthogonal Projection Mode • it is possible to use the two-dimensional screen coordinates • update and display the texture containing the contour replications • Perspective Projection Mode • render the 3D object

28. Implementation • Motion Lines • Properties of emitter • current position • duration of the life cycles of particles • length of the motion lines is defined through the duration of the life cycle • Every property of the particle (e.g., line width, color) has a start value and an end value

29. Results

30. Results

31. Result

32. Result

33. The End