课程名称︰计算机图形 课程性质︰选修 课程教师：欧阳明 开课学院：电资学院 开课系所︰资工所、网媒所 考试日期（年月日）︰2013.11.18 考试时限（分钟）： 试题 : Interactive Computer Graphics Mid-term Exam, 18 Nov. ,2013 1. (5%) Suppose that we render Bezier patches by adaptive subdivision so that each patch can be subdivided a different number of times. Do we maintain geometric continuity along the edges of the patches? Draw a picture and explain your answer. Ans: If we have two patches that share an edge and subdivide only the patch on one side of this edge, we can create a crack. The middle shared endpoint on the subdivided patch does not have to lie on original edge. We can either create an extra triangle from the original endpoints and this middle point to fill the crack or we can triangulate the unsubdivided patch to meet the new subdivided edge, i.e. we replace the unsubdivided patch by a set of triangles that use three of the edges of the patch and the subdivided edge. 2. (10%) Clipping Clip the line segment from (-1, 4) to (2, -1) to the rectangle defined by (0, 1) and (1, 0). Use a parametric technique to find line intersections. You can give the parameter values for the starting and ending points of the clipped lines (instead of the actual coordinates) if you want to. You do not need to follow any particular algorithm - just show that you can find where the line should be clipped. 3. (15%) Frustum Clipping Define the frustum of vision to be centered on the z-axis with the viewpoint (eye) at (0, 0, -d) and with the window on the x, y-plane having its center at (wex, wey, 0), width=2*wsx and height=2*wsy. The far clipping plane is to be parallel to the window and intersect the z-axis at (0, 0, f). (a) (6%) Give the equations (in the window coordinate system) of the six clipping planes. Adjust the signs in your equations (if necessary) so that a positive value denotes "visible" and a negative value denotes "invisible". (b) (9%) How to derive the perspective matrix appropriate for this formulation of the problem. Explain your answer in steps, for example, step 1, step 2, etc. 4. (15%) BSP tree Figure 1: http://i.imgur.com/O7w1j5h.png i. (8%) Construct the Binary Space Partitioning (BSP) tree of the model in Fig 1. Please use the right part of node 3 as the root. Split any line segment as you wish, and mark them as a, b, c, d, etc., then choose the smallest polygon as the next sub-root (children) when you have multiple candidates. ii. (7%) From the BSP tree in (a), derive the display sequence in terms of the viewing position "the eye", where the eye is at the back-side of (behind) polygon 3. 5. (10%) Transformation Let M denote the matrix to transform object space to world space, V denote the matrix to transform world space to camera space. There is a normal vector N but is described in object space. Please derive a matrix in terms of M and V that transforms N to camera coordinate N'. You can define other variables to make the derivation more understandable. 6. (10%) Curves and Surfaces Figure: http://i.imgur.com/vw3dTjk.png (1) (5%) Please give a brief comparison (advantages) between Bezier curve and B-spline. (2) (5%) If we have control points, P1, P2, …, P5, how to draw a curve through start point P1 and end point P5 using B-spline (that is let the curve pass through P1 and P5)? Write down the shortest control points sets to draw. (just like (P1, P2, P2, P3, P4, P5)) Ans: (2) (P1, P1, P1, P1, P2, P3, P4, P5, P5, P5, P5, P5) 7. (15%) Ray tracing (1) (5%) What is the shadow ray in Ray Tracing, why is it needed? (2) (5%) Which part is the most computational expensive in Ray Tracing? Why? (3) (5%) Consider a ray passing through a sphere. Find the point on this ray closest to the center of the sphere. For example, a ray through (0, 0, 0) and (1, 1, 1) passing through a sphere with a center of (3, 4, 5) with a radius of 10, find the point on this ray closest to the center of the sphere Ans: (3) The line from the center of the circle to the closest point on the ray must be perpendicular to the ray. Thus, if the ray is written as p = p_0 + td and the circle has radius r and center p_c, we can solve d · (p_0 + td - p_c) = 0 for t. We can then check the distance between this point and the center. If it is greater than r, the ray misses the sphere. 8. (5%) What is your term project for this semester? What are the technical difficulties involved in the project? (You can refer to the project listing). 9. (15%) Illumination Models A popular illumination model is called Phong model, which is developed in 1975. The formula of this model for each color channel is (please refer to textbook): _ _ _ _ n I = I_a k_a O_d + f_(att) I_P [k_d O_d (N · L) + k_s (R · V) ] (1) (5%) Could you give two examples of materials that definitely can not be approximated by the Phong model? And simply explain why. (2) (5%) A general depiction of illumination model is called bidirectional reflectance distribution function (BRDF) (please refer to textbook, bidirectional reflectivity): I = f_(BRDF)(L, V) The idea of this function is that given a viewing and lighting (single light source) direction, the BRDF function returns its reflectance (or precisely, the ratio between the incident flux and reflected intensity). Please design a platform to capture BRDF of materials. You will have the following equipments: 1. Spheres of different materials 2. A digital camera 3. A light source with a 1DOF rotational arm that allows you to move the light direction horizontally. (3) (5%) The function I, if stored as a database, can be quite big (much larger than 1 Giga bytes). This can be a problem for ordinary PC graphics card in real-time graphics. Can you think of a way to compress the data of function I? (Assume viewing and lighting direction is in division of one degree, say in spherical coordinate system, per sample in theta and phi direction.). --

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