Graphics Computer CS175 Assignment 9: Fur
Due: Monday December 1st
The starter code for this part of the assignment is in the directory starter. It contains the code (the main program is asst9.cpp), the necessary textures for the shells and ns, as well as all the complete shaders needed. There are comments in asst9.cpp indicating the places in the code where you can render the ns and shells to draw the fur. There is an idle function activated by default, which you will need in order to simulate the dynamics. And there is a registered function for special keys (special keyboard), which you will need to handle the keyboards arrow keys.
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Straight Fur
In this assignment, a furry surface is drawn in three stages. All the drawing code is in the function draw in asst9.cpp, you can follow the explanation below in the code. First the surface itself is drawn using the shader pair shader. Second, the ns are drawn. To draw them, we rst enable alpha-blending, and disable Z-buer writes. We use the shader fins shader-pair to draw ns. Last, shells are drawn using the shader shells shader-pair. To draw them we use alpha-blending (which was enabled before), and we turn Z-buer writing back on. The code should compile and run as is. The places where your code for drawing shells and ns will be inserted are in the draw function, and are marked with a comment. All the code for sending modelview matrices, light-source position, . . . to the shaders is already in place. There are only a few variables in the shaders that your code will have to set in order to render the fur. These are: When drawing both shells and ns, you must send in both vertex positions (using glVertex3f), as well as normals (using glNormal3f).
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When drawing the ns, for every vertex sent to the vertex shader, you will set two attribute-variables: vtexCoord fins, and vTangent fins. Handles for these are h vtexCoord fins, and h vTangent fins. Because a n has two triangles and four vertices, you will assign texture coordinates (vtexCoord fins) to map the entire n texture to each n. vTangent fins will hold the (tangent) direction of the fur, to be used for lighting calculations downstream. The direction of the fur at a point is the normal of the surface since the fur is always pointing outwards from the surface When drawing the shells, before drawing each of the layers, you must rst set the uniform-variable valpha exponent shells using the formula described in the comment in the code. When drawing the shells, for every vertex sent to the vertex shader, you will set the attribute-variable: vtexCoord shells. These are the texture coordinates used to lookup the shell texture. Each triangle in the current shell will access the same portion of the texture, so it will have the same texture coordinates. The code already loads and renders the bunny. You can add the n and then shell drawing progressively, and always see the result as you add them. The only places that need code to be inserted in are the marked comments in the draw function. The interface for this program is simply an arcball to rotate the camera with the left mouse-button, and two keys: pressing s will toggle shell-rendering on/o, and f will toggle n rendering on/o. This interface is already coded for you. You will use the GLUT UP KEY and GLUT DOWN KEY to change the density of the fur. You will implement this by changing the texture coordinates for the triangle. Because we set the textures to wrap, you can use coordinates outside of the 0..1 range. You will use the GLUT LEFT KEY and GLUT RIGHT KEY to change the length of the fur
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Combing the Fur
Imagine a single straight hair starting at a vertex p, and made up of m straight segments going in the direction of the normal n, ending at the straight tip s, as in gure 1. We draw a shell vertex at the top of each of the segments and pass n for shading in this shell. To comb the hair, will store a desired tip location for each hair t. We will now think of the hair as a polyline with m segments that starts at p, and ends at t. The rst segment will be in the original normal direction, but for each subsequent segment, n will be updated using an appropriate n. Given p, t m and n, you will gure out the n. Shells will be drawn at the ends of each of these m segments. Each n quad, will now be broken up into a bunch of small quads with its vertical edges made up of the m segments.
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Figure 1: Combing hair: hair-tip at rest is s, but we will bend the hair from the root (p) to get to the current hair tip (t). Note that n is not the unit height normal, just proportional to it (n = f ur m N , where N is the surface normal at p). Note also that the hair length at rest is m n = m n , which is a free (adjustable) parameter of your model. You choose m, the number of segments that the hair is split into (a value of 15-30 is usually sucient). Given a unit normal at p, and a value for m, you can compute n such that m n is the desired length of the hair at rest. Then given p, s, t, m, and n, there is a single n that makes the hair bend to get to the desired hair tip t.
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Dynamics
Store a velocity vector v with each tip. To animate the fur do the following at each idle call Make sure the combed tip t is within a small ball of straigt tip s. Compute forces to act on the tip. There will be some gravity force (0, g, 0), where g is some constant, and a spring force. The spring force will be in the direction of the normalized s t. The springs magnitude will be a stiness constant times t s 2 . Update the tip positoin: t = t + v for some . Update the velocity: v = v + f . Damp the velocity: v = v dampingfactor. The damping factor will be some number slightly less than 1.0. Project t on to the plane at s orthogonal to n The +,- keys will control the stiness, and the x,z keys will control the damping. When the bunny is moved, it does not move the combed tips. The tips will be spring back using the dynamics.
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