Week 12 Solution
Question 1
This question was contributed by Daniel Lambert, age 8.
How can you tell that the image on the cover of the text book is not a photograph of a real scene?
If it was a photo, the camera would be visible in the reflection on the mirrored sphere like this.
Question 2
The teapot is actually modelled mostly using Bezier patches. The base and lid can be modelled as surfaces of revolution, and it’s also possible to model the spout using extrusion, but this is tricky. It’s actually done using Bezier patches. The surface is usually a metallic or at least a smooth surface. Hence we would probably model it as a highly specular (i.e. with a very high phong exponent (maybe, say, 40). The colour depends on the type of teapot, but if it’s the traditional gold, then that’s fine.
A barrel can be modelled easily as a surface of revolution. Another way it’s possible to imagine making a barrel shape is to use an ellipsoid from its implicit surface definition, then using constructive solid geometry, remove the top and bottom of the ellipsoid. A barrel has wood panelling. Wood is generally a matte texture, so not very much specularity — unless, of course, it’s polished or lacquered. Barrels are neither of these. To model the texture, we could use a normal texture map, but it might also be worth considering procedural texture mapping for more realistic effect. However, since it is likely to be used in a real-time game, probably we will go for a texture map. We could also use bump mapping to model the “grooves” between the wooden pieces which are assembled into barrels.
A shot barrel is the above surface of revolution, but you can use CSG to mark the hole in it as a cylinder. If the cylinder that shows where the weapon went through is C, and the barrel is B, then the CSG expression for the shape would be: B - C The surface is not much different from an unshot barrel. The only thing to be concerned about is visibility of internal textures and the texture at the point where it goes through the wood. It would look fake if we used the same texture for both the surface and the bit of the hole, but it probably doesn’t make too much difference.
Exhaust pipe can be accomplished in many ways, but the easiest is to use extrusion and have a cross section of the tube and have different scale factors at the beginning and end. Again, exhaust pipes are usually shiny chrome, so highly specular surfaces are good. If we really want to get that “chrome” look right, then we need to use an environment map so we get realistic reflections off the pipe. If we really wanted to model bits of dirt and stuff as well, we could apply a partially transparent texture.
Marble pillars? This is just straight combination. The cylinder in the middle is done either as a SOR or extrusion. The base is modelled using polygons. Marble is a situation where you really stand to gain from the use of solid textures to get that really nice marble effect. Plus, it would look very natural at the point where the base and the column meet: the textures would match so it would look pretty natural. Polished marble is pretty specular, so you may again need a Phong exponent, maybe not as high as for metallic surfaces, maybe 10.
Carapace is best modelled using Bezier patches, or their big brothers, NURBS patches. The carapace is probably partially specular but not too specular. It could have different colour patches on it, especially in the case of an alien.
Question 3
I am not providing a sample answer to this question. I do not want students repeating it verbatim as an answer to vaguely similar questions on the exam.