Description
Introduction
GLSL shaders make it possible for us to create some amazing lighting effects in real-time
computer graphics. These range from photorealistic lighting to artistically inspired nonphotorealistic rendering, as featured in games like The Legend of Zelda: The Wind Waker and
Team Fortress 2. In this assignment, you will implement a GLSL shader that can produce both
realistic per-pixel lighting, “toon shading” as shown above, and a variety of other effects. You
will also implement another shader that adds silhouette edges (the black outlines seen
above) to complete the cartoon effect.
In this assignment, you will learn:
• how to calculate realistic and artistic per-pixel lighting in real time,
• how to modify geometry on the fly to create viewpoint-dependent effects such as
silhouette edges, and
• how to implement and use your own shader programs.
Requirements
There are three requirements for this assignment:
1. Per-pixel Phong shading using the standard (Blinn-)Phong lighting model.
2. Flexible shading using texture images.
3. Drawing outlines using silhouette edge.
These correspond to “C”, “B”, and “A” level work respectively. You will have to implement
one shader program (i.e. a pair of vertex and fragment shaders) to satisfy the first two
requirements, and a second shader program to satisfy the third.
Per-pixel Phong shading
During the next few class sessions we’ll work on some shader programs that calculate
ambient, diffuse, and specular lighting using per-vertex (Gouraud) shading. Your job is to
extend the concepts and programs we develop in class to implement per-pixel Phong
shading with the same lighting model. You should be able to build this by extending the
shaders that we discuss and develop in class.
Implement a shader program that performs all the calculations to accurately calculate the
Blinn-Phong lighting model for each pixel. The lighting terms must vary per-pixel based on
the normal and the light position, as well as the various material properties (such as the
specular exponent). You should implement this shader following the lighting model
equations discussed in class. For the specular component, you may use either the half-vector
or the reflection vector method, but document your choice in the README file.
None of the relevant shader input parameters (light color, position, material reflection
coefficients, etc.) should be hardcoded as constants in your shaders. Although we will
probably hardcode some of these values in the examples we do in class, you should take the
time to do this the right way in your assignment. Pass these parameters from your C++
program to your shader program using the program.setUniform() method.
Flexible shading using texture images
Once you have Phong shading working (including ambient, diffuse, and specular lighting),
adapt your shader as follows. Rather than setting the final color based on the intensity of
light you calculate for the Blinn-Phong model, you instead use this intensity value as a lookup
into a texture and use that to compute the final color. A texture used in this way is typically
called a “ramp”. With this strategy, you’ll be able to get a wide range of different lighting
effects just by switching the texture you use for input.
Suppose we use the dot product in the diffuse term, n · ℓ, to look up the texture, so that if its
value is −1, we pick the color from the leftmost pixel in the texture; if it is 1, we pick the color
from the rightmost pixel; and similarly in between. If we use standardDiffuse.png (see
below), which is zero in the left half corresponding to negative n · ℓ, and increases linearly
from 0 to 1 for positive n · ℓ, then we’ll get back the standard diffuse lighting term.
standardDiffuse.png
But, if we use toonDiffuse.png, we’ll get something that looks like a cartoon, as if an
artist were shading using just 3 colors of paint.
toonDiffuse.png
Note that this is the same type of lighting effect you see in many games, including The Legend
of Zelda: The Wind Waker and Team Fortress 2 (below). Wind Waker uses a very simplified light
model. In this example, it looks like there are just 2 values used in the shading: each surface is
either in bright light or dark. Team Fortress 2 is a bit more subtle: it reduces the brightness
variation in lit areas without completely flattening them out. You can read more about this in
Mitchell et al.’s article “Illustrative Rendering in Team Fortress 2”, linked in the “Further
Reading” section.
Inside your Phong shading program, you will have equations that calculate the intensity of
reflected light for ambient, diffuse, and specular. For the diffuse portion, the key quantity will
be n · ℓ, which should range from −1 to 1. This is the value that you want to use to lookup the
lighting color to apply from the texture ramp. If the value of n · ℓ is −1, then you want to use
the color on the leftmost side of the texture, and if it is 1, then you want to use the color on
the rightmost side of the texture. That means your texture coordinates for this lookup will be
(0.5(n · ℓ) + 0.5, 0), because texture coordinates only go from 0 to 1. (You could actually use
any value you want for the y coordinate, since the color only varies from left to right.) After
calculating these texture coordinates, you can get the color from the texture image using the
GLSL built-in function texture() as discussed in class.
You need to add two separate texture lookups to your shader, one for the diffuse component
and one for the specular component. For the specular component, we need to clamp h · n to
positive values anyway before taking the exponent, so you should directly use the intensity
max(h · n, 0)
s as the texture coordinate without rescaling. Note that adding these texture
lookups to your code will change the Blinn-Phong shader that you made in requirement 1.
You do not need to include an explicit option to draw in the old Blinn-Phong lighting mode,
because using the provided standardDiffuse.png and standardSpecular.png
textures should reproduce the original appearance.
Silhouette edges
There are lots of different ways to draw
silhouette contours on 3D shapes. We will use
a simple method described by Card and
Mitchell, linked under “Further Reading”. For
each edge of the triangle mesh, we check
whether it lies on the silhouette, that is, on
the boundary between the triangles facing towards the camera and the triangles facing away
from it. If so, it is a silhouette edge, and we will draw it as a thick black line segment to create
the outline of the shape.
Drawing a thick line segment takes a little bit of work in OpenGL 3 and above, since the
function glLineWidth() is no longer officially supported. Instead, we will have to draw
the line segment as a quadrilateral whose width is the desired thickness. Since we don’t know
in advance which edges will be silhouette edges and which will not, we will create a zerowidth quadrilateral for every edge. In the vertex shader, we will check whether the vertex is
part of a silhouette edge, and if so, displace it by the desired thickness. Thus, silhouette edges
will be drawn as thick quadrilaterals, while all other edges will be drawn as quadrilaterals of
zero width, which can’t be seen.
[Card and Mitchell 2002]
The starter code provides a class EdgeMesh that stores the information needed to draw
these silhouette edges. It creates a quadrilateral (4 vertices and 2 triangles) for every edge in
the original mesh, as shown above. Each vertex stores its position, its displacement direction
direction (labeled nsmooth in the diagram above), and the normals of the adjacent faces
leftNormal and rightNormal. Implement a shader program that checks each vertex for
whether it lies on a silhouette edge, that is, whether nleft · v and nright · v have different signs. If
so, displace the vertex by thickness*direction when computing the output
gl_Position. The thickness should be passed into the shader from your main program as
a uniform variable.
Left: bunnyLowres.obj with only vertices on silhouette edges displaced.
Right: with vertices on all edges displaced.
Reference Images
I’ve added a new mesh called sphere.obj to the data zip file. It’s just a sphere with 24
slices and 12 stacks. The results of your program on this mesh should look like the following
as you progress through the assignment:
From left to right: the initial view before you implement anything; Phong shading with the
standard Blinn-Phong model; ramp shading with standardDiffuse.bmp and
standardSpecular.bmp (this is identical to the Blinn-Phong model); ramp shading with
toonDiffuse.bmp and toonSpecular.bmp; and the same with silhouette edges drawn.
If you fail to normalize the fragment normal when doing Phong shading,
you will get an incorrect result that looks like the image on the right. Note
that even if your vertex normals are normalized, the rasterizer will
interpolate them to fragments by averaging, and the average of two unit
vectors may not itself be a unit vector!
Above and Beyond
All the assignments in the course will include great opportunities for students to go beyond
the requirements of the assignment and do cool extra work. We don’t offer any extra credit
for this work — if you’re going beyond the assignment, then chances are you are already
kicking butt in the class. However, we do offer a chance to show off… While grading the
assignments the TAs will identify the best 4 or 5 examples of people doing cool stuff with
computer graphics. After each assignment, the selected students will get a chance to
demonstrate their programs to the class!
Once you get the hang of them, shaders can be really fun! Try out some different textures and
lighting effects. One interesting possible extension of our 1D ramp textures could be to use a
single 2D texture that you look up using both the diffuse and specular intensities. Other
things you can do with shaders is to add stripes, waves, random noise, or bumps to the
surface.
If your ideas for going beyond the requirements would make your code more difficult for the
TAs to grade, please help them out by submitting a standard version of your assignment first
through the normal website link, and then email the TAs the fancier version of your
assignment.
Support Code
The webpage where you downloaded this assignment description also has a download link
for support code to help you get started. The support code for this assignment is a simple
program using the SDL-based engine, similar to the ones we have used before. You should
also download separately the zip file containing the meshes and texture images.
The support code defines a program structure and everything you need to read the mesh
data. To make locating data files simpler, we have a header file called config.hpp that
contains absolute paths to your data files. You should edit this file with the full path
(e.g. “C:\Users\Turing\a5\data” or “/home/turing/a5/data”) where you’ve
placed the data files. We will modify this file appropriately when grading your assignment.
Handing It In
When you submit your assignment, you should include a README file. This file should
contain, at a minimum, your name and descriptions of design decisions you made while
working on this project. If you attempted any “above and beyond” work, you should note that
in this file and explain what you attempted.
When you have all your materials together, zip up the source files and the README, and
upload the zip file to the assignment hand-in link on our Moodle site. Any late work should be
handed in the same way, and points will be docked as described in our syllabus.
Further Reading
Note: You don’t need to read these articles to implement the assignment. They’re only
provided in case you’re curious and want to learn more about non-photorealistic rendering.
Mitchell, Francke, and Eng, “Illustrative Rendering in Team Fortress 2”, Non-Photorealistic and
Artistic Rendering 2007.
http://www.valvesoftware.com/publications/2007/NPAR07_IllustrativeRenderingInTeamFort
ress2.pdf
Gooch, Gooch, Shirley, and Cohen, “A Non-Photorealistic Lighting Model for Automatic
Technical Illustration”, SIGGRAPH 1998.
http://www.cs.northwestern.edu/~ago820/SIG98/abstract.html
Card and Mitchell, “Non-Photorealistic Rendering with Pixel and Vertex Shaders”, in ShaderX:
Vertex and Pixel Shaders Tips and Tricks, 2002.
http://developer.amd.com/wordpress/media/2012/10/ShaderX_NPR.pdf
Gooch, Hartner, and Beddes, “Silhouette Algorithms”.
https://www.cs.rutgers.edu/~decarlo/readings/gooch-sg03c.pdf
