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）︰2016.05.03
考试时限（分钟）：
试题 :
Interactive Computer Graphics Mid-term Exam, May 3, 2016
1. (15%) BSP Tree
Figure: http://i.imgur.com/WwdSRzO.png
A. Construct the Binary Space Partitioning (BSP) tree of the model in
Fig. 1 below. Please use the node "a" as the root. Split any line
segment as you wish, and mark them as b.1, b.2, b.3, b.4, etc. Please
choose smaller numbers/alphabets as the sub-tree root node.
B. From the BSP tree in (a), derive the display sequence in terms of the
given viewing position in this figure.
2. (10%) When using the Cohen-Sutherland line clipping algorithm, how do we
check the outcodes to see if a line can be trivially accepted or rejected?
Ans: Each coordinate of a line is assigned a 4-bit outcode according to its
location relative to the left, right, top, and bottom screen edges.
● First bit set to 0 if: ycoord < yscreen-min
● Second bit set to 0 if: ycoord > yscreen-max
● Third bit set to 0 if: xcoord < xscreen-min
● Fourth bit set to 0 if: xcoord > xscreen-max
If both of a line's outcodes are 0000 (logical OR), trivially accept.
If a logical AND of the lines outcodes is not zero, trivially reject.
3. (3%) (OpenGL) What does glFlush do?
4. (4%) (OpenGL) Consider this sequence of calls:
glColor3f(1., 1., 1.);
glColor3f(0., 0., 0.);
glVertex3f(1., 1., 1.);
glVertex3f(2., 2., 2.);
● What color is the vertex (1, 1, 1)?
● What color is the vertex (2, 2, 2)?
5. (5%) (OpenGL) Suppose that you have 1000 triangles that can be arranged
into a triangle strip. How many vertices would you have to specify to
OpenGL if you use the fact that they can be arranged into a triangle strip?
6. (5%) (OpenGL) What would look different in the resulting image if you
changed the 1.0 to -1.0 in the following line.
glm::lookAt(eye, center, vec3(0, 1.0, 0))
7. (10%) The following is the pseudocode of Floyd-Steinberg error diffusion
dithering.
for each y from top to bottom
for each x from left to right
oldpixel = pixel[x][y] ○─→○ 3/8
newpixel = floor(oldpixel / quant_number) * quant_number │╲
pixel[x][y] = newpixel ↓ ↘
quant_error = oldpixel - newpixel ○ ○ 2/8
pixel[x+1][y] := pixel[x+1][y] + quant_error * 3/8 3/8
pixel[x][y+1] := pixel[x][y+1] + quant_error * 3/8
pixel[x+1][y+1] := pixel[x+1][y+1] + quant_error * 2/8
Approximate such a matrix using Floyd-Steinberg error diffusion if we have
elements of the form 2xN only (i.e. 0, 2, 4, 6, 8, 10, etc.), give the
result of the 2x2 upper left matrix.
Ans: ┌ 3 5 * ┐
│ 7 1 * │
└ * * * ┘
8. (10%) Painter's Algorithm
(1) (5%) Which of the following scenes would have troubles in the
Painter's Algorithm? Explain your answer briefly. (Each rectangle
is a single primitive.)
Figure: http://i.imgur.com/KNZiZMo.png
(2) (5%) Determine the painting order in Painter's Algorithm.
Figure: http://i.imgur.com/V3KkehA.png
9. (20%) There are nice properties about Ray Tracing.
A. (7%) Please describe the strength and weakness of Ray Tracing, using
the theory of "The Rendering Equation"
Equation: http://i.imgur.com/W1EfstM.png
B. (5%) Using the rendering equation, please explain the weakness of
Phong shading.
C. (8%) Modified Distribution Ray tracing. In the photo below, there are
very bright spots under the glass sphere, called caustics. One way to
improve the previous ray-tracing is to combine the rays (light packets)
shooting from the lights, and store the (location, direction) information
at the light-surface intersection points, called photon maps.
For caustics, we only store the photon map when rays hit the highly
specular surface or pass through a transparent object and finally
reach a diffuse surface. After the photon maps are created, we can
use ray-tracing to shoot rays from the eye until it hits the surfaces
with the photon maps. Why is this method successful in solving the
Rendering Equation? Please give your own algorithm describing the
previous solution.
Figure: http://i.imgur.com/YPxKHhi.png
10. (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).
11. (13%) 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) ]
where L is light direction, V is viewing direction.
(a) (4%) Could you give two examples of materials that definitely can not
be approximated by the Phong model? And simply explain why.
(b) (4%) A general depiction of illumination model is called bidirectional
reflectance distribution function (BRDF) (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.
(c) (5%) The function I, if stored as a database, can be quite big
(much larger than 1.0 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.).