Computer Graphics (CS 4300) 2010S: Example Exam 2 Problems
Exam 2 covers lectures 15 through 22.
- Transform a given 3D point by a given
homogeneous transformation matrix. - Give a sufficient set of conditions that must hold for a homogeneous transformation matrix to be a pure rotation.
- If a homogeneous transformation matrix is known to be a pure translation, what must be true of its upper-left
sub-matrix? - Give one advantage and one disadvantage of the Euler angle representation for 3D rotations.
- Let
be a 3D rotation matrix. Show how to compute 
. - Let
be a 3D rotation in axis-angle form. Give two representations of the inverse rotation. - What is the purpose of Rodrigues’ rotation formula?
- Let be a 3D rotation in axis-angle form. Show how to construct an equivalent unit quaternion.
- Show how to rotate a 3D point
by a unit quaternion 
. Then prove that composition of two rotations represented as unit quaternions and 
is equivalent to quaternion multiplication. In your setup, which rotation is applied first, or ? - Give one advantage and one disadvantage of the unit quaternion representation for 3D rotations.
- Assume you have available a routine that computes quaternion multiplication. Explain how you can use this routine to (a) compute the dot product and (b) compute the cross product of two 3D vectors.
- Let
. Consider the set 
of all values of that satisfy the algebraic equation 
for real nonzero constants 
. What kind of geometric object is ? - Let and
. Rewrite the equation in vector form, using and 
. Explain the geometric significance of . - Give an algebraic equation that must hold for all and only the points that lie on a sphere with center
and radius 
. - Let be the implicit form of a plane in 3D, with real nonzero constants. What is the unit normal to a given point on the surface?
- Let be the center and be the radius of a sphere. What is the unit normal to a given point on the sphere?
- Give one mathematical way to define a line in 3D.
- Let
be 3D points that are the vertices of a triangle. Let 
be the barycentric coordinates of a point in the plane of the triangle. Show how to compute the Cartesian coordinates of in terms of and . (Assume that at 
; at 
; and at 
.) - Let be 3D points that are the vertices of a triangle. Show how to compute a unit normal
to the triangle such that the vertices appear in CCW order exactly when points towards you. - Give two fundamentally different ways that a ray and a sphere in 3D can intersect in a single point. Draw diagrams for each.
- We studied a method to find the intersection of a ray and a sphere that involved solving a quadratic equation. What is the geometric significance of the discriminant of this equation being zero? Less than zero?
- Give the parametric equation for a point along a ray starting at
with direction 
. Let 
and 
be two solutions to this equation corresponding to the intersections of the ray with a sphere. Which is closer to ? Let 
correspond to the intersection closer to . Give expressions for (a) the radius of the sphere and (b) the surface normal of the sphere at that intersection point in terms of , , , and the center of the sphere.
- Give two uses in graphics for an algorithm that computes ray/triangle intersections.
- Draw a simple scene graph showing the relationship between a world frame
, an object frame 
, and a camera frame 
. Let 
be a homogeneous transform from object frame to world frame, and similarly let 
be a homogeneous transform from camera to world frame. Show how to compose these two to produce 
, the homogeneous transform from object to camera frame. - Sketch the appearance of a set of train tracks heading into the distance, viewed obliquely, in (a) parallel and (b) perspective projection.
- Show how to transform a 3D point
by a projective transformation matrix 
.
- Assume you are using the painter’s algorithm to render a set of triangles as viewed from a camera in 3D. This involves sorting the triangles according to their depth in camera frame, but you would like to do this sorting only when required, because it is relatively time consuming. What are two events that would require re-sorting?
- The painter’s algorithm requires sorting
triangles by their depth in camera frame. (a) Give one way to assign a single “depth” value to a triangle with 3D vertices in camera frame (assume that the camera’s viewing direction is 
in camera frame). (b) Assuming you do not know any upper bound on , what is the worst-case running time for an optimal sorting algorithm for the triangles in big-O notation, in terms of ? - Sketch a view of three 3D triangles
that are an occlusion cycle, i.e., some part of 
is in front of 
, some part of 
is in front of , but some part of is in front of . This is one example of a case where the basic painter’s algorithm fails. Give a second example of another situation, in this case involving only two triangles, where the basic painter’s algorithm also cannot produce a correct rendering. - Given pseudocode for a basic triangle rasterization algorithm as we have studied, pencil in a set of modifications that would resolve occlusions using the z-buffer method. (Assume that the camera’s viewing direction is in camera frame.)
- Write pseudocode that calculates the “minimum”
and maximum 
corners of an axis-aligned bounding box for a list of triangles 
. - Given the “minimum” and maximum corners of an axis-aligned bounding box
, show one method to compute the center and radius of a bounding sphere that encloses . - Let
be a list of 
triangle meshes 
, where every 
is an individual 3D triangle (mesh 
contains 
triangles). Let be a 3D ray. Assume you have algorithms
that takes in a triangle mesh and returns a bounding sphere 
for that mesh
that returns true if intersects triangle 
and false otherwise
that returns true if intersects sphere and false otherwise
Write two pseudocode procedures
that is called (by other code that you do not have to write) whenever any triangle in any mesh is added, removed, or changes position, and that can create or update a global datastructure to be used to accelerate picking (it is up to you to define this global datastructure)
that is called whenever the user has clicked the mouse (by other code that you do not have to write) with a corresponding pick ray, and that returns one of the meshes 
that was intersected by that pick ray, if any, or 
otherwise (if more than one mesh is intersected, you may return any of them).
For full credit, you must not call on any triangle in a mesh unless 
would also return true. - Define frustum culling, backface culling, and occlusion culling.
- Given three 3D points
that appear in CCW order from some camera viewpoint, show one method to determine if another 3D point is on the near side or the far side of the plane through with respect to the camera. - You are given a sketch of a 3D triangle with vertices that is cut by a plane such that
are on one side of the plane and is on the other side, splitting the original triangle into one trapezoid 
and one smaller triangle 
. The intersection of the plane with the triangle is a line segment with endpoints 
where 
is on the side of the triangle between and and 
is on the side between and 
Assume that from some camera appear in CCW order. (a) What are the vertices of the trapezoid as they would appear to the same camera in CCW order? (b) What are the vertices of the smaller triangle as they would appear to the same camera in CCW order? (c) Split into two triangles by sketching in one of its diagonals, and give the vertices of these triangles in CCW order as they would appear to the same camera.
- You are given a table with four columns labeled “point”, “directional”, “spot”, and “ambient”, referring to the four types of lights that we have studied, and four rows labeled “position ”, “direction
”, “angle 
”, and “intensity 
”. Fill in the table with checkmarks to show which properties are needed to define each type of light. - You are given a sketch of the side-on view of a small patch of a 3D surface at a point . A point light is also shown at a location
and a camera at location 
. Sketch in (a) the outward pointing surface normal unit vector at ; (b) the unit vector 
from towards the camera; (c) the unit vector 
from towards the light. (d) Show the math to compute and from the given data. (e) Assume that the ambient and diffuse colors of the surface are given as RGB triples 
and 
, respectively. Write pseudocode to compute the apparent color of the surface as it would appear from the camera, according to the Lambertian shading model we have studied, including both ambient and diffuse terms. Use the same scalar intensity for the light in both terms. This code should assume that , , and are already available, along with and . The code should be correct for all geometric situations, not just the one pictured. In particular, make sure to correctly handle the case that is behind the surface. - We studied a shading model that included separate ambient, diffuse, and specular components. Consider a scene with a single sphere illuminated by one ambient light with intensity
and one directional light with intensity 
. The ambient light only contributes to the ambient shading term; the directional light contributes to the diffuse and specular shading terms, but not ambient. Several pictures are given of the scene with different settings of the following variables
or 
or 
- specular shading enabled or disabled
Mark the value of each of those three variables that must have been in effect to produce each picture. - List at least three real-world light effects that are not typically implemented in a basic rasterization framework, such as OpenGL, that is based on the z-buffer method and the Lambertian and Blinn-Phong shading equations we have studied. [Hint: some such effects are typically implemented in ray tracers; also, with extra work, at least some of these effects can be added to a rasterizer, especially when texture mapping is available.]
- You are given the terms “rasterization” and “ray tracing” in a left column and the terms “object-order rendering” and “image-order rendering” in a right column. Draw two lines, each connecting a term in the left column with the synonymous term in the right column.
- Assume you have
- a list of “surface” objects in world frame
- a function
that can create a 3D ray in world frame that starts at the location of a camera and goes through image plane pixel 
- a function
that returns a scalar 
that is either the nearest intersection of a ray with a surface , or 
if the ray does not intersect the surface - a function
that sets the color of pixel to the ambient color of surface .
Write pseudocode for a basic ray tracer that sets each pixel in a 
canvas to the ambient color of the nearest surface, or to black if no surface is intersected by a ray from the camera through that pixel. - Assume you have
- a list of “surface” objects in world frame
- a point light at location in world frame
- a function
that returns a scalar that is either the nearest intersection of a ray with start point and direction with a surface , or if the ray does not intersect the surface (i.e. if 
then the intersection point is 
; note that and are both given in world frame, and that does not necessarily need to be a unit vector)
Write pseudocode that determines if the point light is in shadow at a given point on some surface 
. - (a) State the mirror reflection law. [Hint: you may want to use the phrases “angle of incidence” and “angle of reflection”.] (b) You are given a diagram showing a small patch of a 3D surface viewed side-on. The outward pointing surface normal at a point is shown as a unit vector . A unit vector from towards a camera is shown as . Give the math to compute
, the component of perpendicular to the surface, and 
, the component of parallel to the surface. These should be set up so that 
. Sketch in 
and 
. (c) Give the math to compute 
, a unit vector in the direction that a ray from the camera would reflect after bouncing off the surface at according to the mirror reflection law. You may use and in your calculation of . Finally, sketch in . - You are given pictures of two solid objects
and , along with four additional pictures that are the result of applying boolean constructive solid geometry (CSG) operations to and . Label each of the latter four pictures with the corresponding operation that was performed, i.e. 
, 
, 
, or 
. - List at least three real-world lighting effects that are not captured in the standard Lambertian and Blinn-Phong shading equations, and that are not typically implemented (without using texture mapping) in a rasterization framework like OpenGL, but that are not hard to implement in a ray tracer.
- When implementing texture mapping, it is necessary both to handle the case where a large part of a texture image (i.e. many texels) are mapped to a single image pixel (texture minification), as well as the case where a single texel is mapped to multiple image pixels (texture magnification). We have studied the MIP-Mapping technique, which pre-computes a set of scaled-down versions of the original texture image. Does MIP-Mapping help with texture minification, magnification, or both? Explain a scenario where MIP-Mapping could greatly accelerate rendering.
- List at least two real-world lighting effects that are not captured in the standard Lambertian and Blinn-Phong shading equations, and that are not typically implemented in a rasterization framework like OpenGL, but that are often added (at least to some approximation) in practice by careful use of texture mapping. [Hint: some common strategies have two-word names of the form “___ mapping”.]