Lecture Notes

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• Objects with faces
• Faces with fronts and backs
• Normals
• Sorting
• Display list

• surface properties,
• light sources,
• shadows,
• transparencies,
• reflectances

• Rendering
• In the academic environment, a great deal of time is spent completing final presentation renderings, where as in the workplace, only a fraction of this time is used to complete the same task.
• One of the first items to keep in mind is the following: modeling and rendering are very different processes, both in what they accomplish visually, and how they are accomplished as far as the computer is concerned.
• A good place to begin a discussion on the topic of rendering is with an item already covered in class: the relational database. We have discussed how this is a series of tables linked to each other, which, when used in conjunction with one another,   describe a model.
• For a moment, let one consider the table of points.
• The order of points in this table is very important. When considering the points which make up an object, the order in which these points were created (and thus stored in this table) describes the boundary of the object, and its direction.
• Note: While a 2D surface has height and width, it has no thickness. However, one of these two sides is the front, and the other is the back.
• To understand which is which, one first must have grasp a simple geometric rule known as the right hand rule: when looking at the right hand, the thumb can seen to indicate a hypothetical Z axis, perpendicular to the screen and coming out from the screen. The fingers of the right hand indicate positive rotation.  Thus counter clockwise rotation is positive; clockwise is negative. Therefore, if one imagines a square with the first point in the lower left hand corner of the screen, the second point in the lower right hand corner, the third point in the upper right, and the fourth in the upper left, one can imagine this counter clockwise movement of points, which tells one that this is the front of this surface.
• If the option is activated, projecting from the centroid of this face is an arrow which is called the normal. This indicates the front of the face. (To activate this option, under the menu items select display, followed by display options. In the resulting dialogue box which pops up, select view normals. In this same location, on can chose whether or not the direction of the faces should be shown).
• Another option is to view bounding volumes. This essentially takes the maximum and minimum x,y, and z values of each object, and constructs a box around it.
• Types of renderings.
• Form· Z has seven methods of renderings we will deal with in class: wireframe, quickpaint, hidden line, surface render, shaded render, open gl, and renderzone. (This list increases in image quality, wireframe being the most basic, and renderzone the most realistic).
• Quickpaint.
• Uses shades of colors and lines to define objects.
• The image is constructed from the rear of the view to the front, one element or face at a time.
• To do this requires a sorting process.
• The sorting process begins by minimizing the faces to be sorted.
• The normal of each face is compared to the line of sight. The line of sight is perpendicular to the screen with the object point (the center of interest) located in the center of the screen.
• The normal of each face can then be compared to the line of sight.
• If the angle formed by the normal and the line of sight is less than 90 degrees then the face will be seen. If the angle is greater than 90 degrees then the face is presumed to not be visible and the face can be excluded from the sorting process. This step will reduce the faces to be sorted by some 50%.
• A sorting algorithm is then used to determine the order in which faces are to be displayed. It relies on the location of the centroid of each face in the model relative to the eyepoint. The centroids (and thus the faces) may thus be sorted, from the farthest away from the eye point to the closest.
• Let’s take a quick look at how this is done:
• The data of the model is contained in the database.
• A z buffer sort yields a new list of information called the display list.
• The display list order is crucial.
• Items at the top of this list are the furthest from the eye point, and items at the bottom are the closest.
• Therefore the items at the top are rendered first (therefore, potentially behind other objects).
• This method, however, is not precise and yields inaccuracies in rendering at times. For example, if there are two objects on the screen, both are 5’ wide and 5’ high; one however is 20’ long and the other is 18’ long. One can imagine a case where the centroid of the longer box lies behind that of the smaller box, yet because of its length, the end face of longer one projects in front of the shorter. Based on the process described above, this case could yield very confusing results.
• The advantage of Quickpaint however is its speed…it is fast.