CG Class 16, Thurs 2018-10-18

1   Chapter 7 slides ctd

  1. 7_3 Meshes.

    Big ideas:

    1. Meshes are common for representing complicated shapes.
    2. We can plot a 2 1/2 D mesh back to front w/o needing a depth buffer.
    3. Unstructured point clouds with millions of points from laser scanners are common. We can turn them into a mesh by using various heuristics about the expected object.
    4. Drawing the mesh edges a little closer than the faces is a hack that probably works most of the time if the scene isn't too complex. I.e., go ahead and use it, but don't be surprised.

  2. 7_4 Shadows.

    Big idea:

    1. If there is one light source, we can compute visibility with it as the viewer.
    2. Somehow mark the visible (i.e., lit) and hidden (i.e., shadowed) portions of the objects.
    3. Then recompute visibility from the real viewer and shade the visible objects depending on whether they are lit or shadowed.
    4. This works for a small number of lights.
    5. This capability is in OpenGL but not in WebGL, so this topic is enrichment only (= will not be examined).

  3. 7_5 Lighting and shading I.

Big big topic.

2   Lighting etc

  1. This Woman sees 100 times more colors than the average person.
  2. Phong lighting model: The total light at a pixel is the sum of
    1. Incoming ambient light times ambient reflectivity of the material at the pixel,
    2. Incoming diffuse light times diffuse reflectivity times a factor for the light source being low on the horizon,
    3. Incoming specular light times specular reflectivity times a factor for the eye not being aligned to the reflection vector, with an exponent for the material shininess,
    4. Light emitted by the material.
    5. That is not intended to be completely physical, but to give the programmer lots of parameters to tweak.
  3. In OpenGL you can do several possible levels of shading. Pick one of the following choices. Going down the list makes the shading better but costlier.
    1. Shade the whole polygon to be the color that you specified for one of the vertices.
    2. Bilinearly shade the polygon, triangle by triangle, from the colors you specified for its vertices.
    3. Use the Phong lighting model to compute the color of each vertex from that vertex's normal. Bilinearly interpolate that color over the polygon. That is called Gouraud shading.
    4. Bilinearly interpolate a surface normal at each pixel from normals that you specified at each vertex. Then normalize the length of each interpolated normal vector. Evaluate the Phong lighting model at each pixel from the interpolated normal. That is called Phong shading.
  4. Maureen Stone: Representing Colors as 3 Numbers (enrichment)
  5. Why do primary schools teach that the primary colors are Red Blue Yellow?

3   Possible spring courses

  1. ECSE-4740 Applied Parallel Computing for Engineers. Its instructor is excellent. There is also a 6000-level version.
  2. An independent Computer Graphics reading course directed by me.

4   RPI's long history in Computer Graphics

  1. Curtis Priem.
  2. Mike Wozny, Head of ECSE, was one of the founders of IEEE Computer Graphics and Applications, which printed that article by Maureen Stone.
  3. William Lorensen, co-inventor of marching cubes, earned his BS and MS here at RPI.