1 Lecture 11 Scene Modeling

2 Multiple Orthographic Views The easiest way is to project the scene with parallel orthographic projections. Fast rendering. Adjusting the distances. However, it is not easy to understand the scene. Although a single point may be unambiguously located from three mutually perpendicular orthographic projections, multiple points and lines may conceal one another when so projected.

3 Multiple Orthographic Views

4 Axonometric Projection Easy to change size of distance

5 Perspective Projection Depth clues Difficult to change distances

6 Depth Cueing

7 Depth Clipping Further depth information can be provided by depth clipping. The back clipping plane is placed so as to cut through the objects being displayed. Front clipping plane may also be used. By allowing the position of one or both planes to be varied dynamically, the system can convey more depth information to the viewer. Back-plane depth clipping can be thought of as a special case of depth cueing: In ordinary depth cueing, intensity is a smooth function of z', in depth clipping, it is a step function.

8 Depth Clipping

9 Color and Texture Simple vector textures, such as cross-hatching, may be applied to an object. These textures follow the shape of an object and delineate it more clearly. Texturing one of a set of otherwise identical faces can clarify a potentially ambiguous projection. Texturing is specially useful in perspective projections, as it adds yet more lines whose convergence and foreshortening may provide useful depth cues.

10 Color and Texture Color may be used symbolically to distinguish one object from another. Color can also be used in line drawings to provide other information. For example, the color of each vector of an object may be determined by interpolating colors that encode the temperatures at the vector's endpoints.

11 Colored Vectors

12 Visible Line Determination The most effective way to show depth relations. Display of only visible (i.e., unobscured) lines or parts of lines. Thus, objects that are to block others must be modeled either as collections of surfaces or as solids. Only surfaces, bounded by edges (lines) can obscure other lines. Thus, objects that are to block others must be modeled either as collections of surfaces or as solids.

13 Visible Line Determination: Wire Frame

14 Visible Line Determination: Surfaces with ambiental light By analogy to visible-line determination, visible-surface determination or hidden-surface removal, entails displaying only those parts of surfaces that are visible to the viewer.

15 Illumination and shading A problem with the previous slide is that each object appears as a flat silhouette. Next step toward achieving realism is to shade the visible surfaces. Individually shaded polygons with diffuse reflections

16 Interpolated shading Interpolated shading is a technique in which shading information is computed for each polygon vertex and interpolated across the polygons to determine the shading at each pixel. This method is especially effective when a polygonal object description is intended to approximate a curved surface.

17 Interpolated Shading Gouraud shading

18 Curved Surfaces

19 Texture mapping

20 Multiple lights

21 Transparency and Reflection

22 Stereo Effects The principle of stereopsis or depth perception due to binocular vision depends on each eye seeing a slightly different view of a scene or object. It is due to the distance between eyes.

23 Stereo Effects It can be done in computer graphics rendering two versions of the scene with the rendering camera offset from the first view by an appropriate interocular distance. As a result there are 2 images:

24 Stereo Effects How to view the image? Cross-eyed viewing, the left eye image is placed to the right of the right eye image. Then the viewer crosses his/her eyes until three images are perceived.

25 Stereo Effects How to view the image? In parallel viewing the left eye image is placed to the left of the right eye image. The trick is to relax one's gaze as if looking at a distant object beyond the image, i.e., without convergence.

26 Stereo Effects How to view the image? Using red/blue or red/green glasses (anaglyph). The red lens is placed over the left eye and blue/green over the right. By color encoding the two views of the image and superimposing them, each eye sees only it's intended image. However, this method suffers from altered color balance and for strong red or blue colors in the original, the stereo effect is not as great. This method also suffers from "ghosting" in which the opposite eye view is sometimes faintly perceived by the wrong eye.

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29 Stereo Effects How to view the image? The method which yields the best effect is with liquid crystal shutter glasses in conjunction with a monitor operating in interlaced mode. In interlaced mode, a full frame consists of two fields in which one field is made up of all the even lines and the other field is made up of all the odd lines. With this technique, the right and left eye views of the scene are interlaced so that one view corresponds to the even lines and the other view the odd lines. Each lens of the shutter glasses alternates between opaque and transparent states in synchronization with the monitor so that one eye only sees the even lines and the other eye only sees the odd lines. The result is a full color stereoscopic display.

30 Stereo Effects

31 Stereo Effects