CS 457/557 -- Winter Quarter 2018

Project #6

Geometry Shaders for Quantizing 3D Geometry: The Lego™ Project

100 Points

Due: March 12

This page was last updated: March 1, 2018

Introduction:

So many of the shader experiments I run are inspired by something I saw in a movie. In this case, I was intriqued by some of the things I saw in the Lego movies. If you haven't seen any of them, try to see the Batman Lego movie -- I think that's the funniest. Example:

Barbara Gordon, squeezing into the Batmobile: "Batman, why didn't you build more than one seat?"
Batman: "Because, last I looked, I only have one butt."

Sometimes childish humor is the best...

Requirements:

1. Your input should be something with triangles, e.g., the teapot, the torus, an OBJ file, etc.

2. Have a uLevel slider that controls the number of levels of triangle subdivision.

3. Have a uQuantize variable that controls the quantization equation (see below).

4. Have a boolean checkbox, uModelCoordinates so that you can switch between using model coordinates and using eye coordinates.

5. Do some sort of lighting. The quick-and-dirty-diffuse is fine.

Hints

• Your GLIB file could look something like this:

  ##OpenGL GLIB
  Perspective 70
  LookAt  0 0 3     0 0 0    0 1 0
  
  Vertex   lego.vert
  Geometry lego.geom
  Fragment lego.frag
  Program Lego  				\
  	uModelCoords <true>		\
  	uLevel <0 3 3>			\
  	uQuantize <2. 50. 50.>		\
  	uColor { 1.00 0.65 0.40 }
  
  Teapot
  #Obj giraffe.obj
  #Obj cow.obj
  #Obj dino.obj
  

• The geometry shader layouts will look like this:

  
  layout( triangles )  in;
  layout( triangle_strip, max_vertices=204 )  out;
  

• Pass a vNormal in from the vertex shader to the geometry shader. Pass a gLightIntensity and a built-in gl_Position from the geometry shader to the rasterizer.
  

  
  in vec3		vNormal[3];
  out float	gLightIntensity;
  

• Use uLevel to subdivide the triangle into smaller triangles. Use the s and t interpolation scheme:

  
  vec3 v = V0 + s*V01 + t*V02;
  

  that we looked at before to create new (x,y,z)'s to form new triangle strips. Use the same equation to also create new (nx,ny,nz)'s:

  
  vec3 n = N0 + s*N01 + t*N02;
  

  You can use the code from the Sphere Subdivision geometry shader.

• In the geometry shader, when emitting the output triangles, quantize each xyz vertex, like this:

  
  vec3
  QuantizeVec3( vec3 v )
  {
  	vec3 vv;
  	vv.x = Quantize( v.x );
  	vv.y = Quantize( v.y );
  	vv.z = Quantize( v.z );
  	return vv;
  }
  

• You can quantize a single number like this:

  
  float
  Quantize( float f )
  {
  	f *= uQuantize;
  	f += .5;		// round-off
  	int fi = int( f );
  	f = float( fi ) / uQuantize;
  	return f;
  }
  

• Don't do any matrix multiplications in the vertex shader, just pass gl_Vertex out as-is.

• In the geometry shader, the overall flow should be:

  
  V0, V1, and V2 are the corner points of the original triangle.
  V01 = V1 - V0.
  V02 = V2 - V0.
  
  N0, N1, and N2 are the corner normals of the original triangle.
  N01 = N1 - N0.
  N02 = N2 - N0.
  
  for each new triangle created by the triangle subdivision:
  {
  	for each (s,t) corner point in that new triangle:
  	{
  		Turn that (s,t) into an (nx,ny,nz)
  		Transform and normalize that (nx,ny,nz)
  		Use the (nx,ny,nz) to produce gLightIntensity
  
  		Turn that same (s,t) into an (x,y,z)
  		If you are working in ????? coordinates, multiply the (x,y,z) by the ModelView matrix
  		Quantize that (x,y,z)
  		If you are working in ????? coordinates, multiply the (x,y,z) by the ModelView matrix
  
  		Multiply that (x,y,z) by the Projection matrix to produce gl_Position
  		EmitVertex( );
  	}
  }
  

• Don't forget that you still need to manage the beginning and end of the linestrips.

• When you quantize coordinates like this, many elements of the geometry end up in the same place. Z-fighting is to be expected.

The Turn-In Process:

1. Your turnin will be done at http://engr.oregonstate.edu/teach and will consist of:
   
   1. All source files (.glib, .vert, .geom, .frag). You can zip this all together if you want.
   2. A PDF report with a title, your name, your email address, nice screen shots from your program, and the link to the Kaltura video demonstrating that your project does what the requirements ask for. Narrate your video so that you can tell us what it is doing.
   3. Be sure your video's protection is set to unlisted.
   4. Do not put your PDF into your zip file. Leave it out separately so my collect-all-the-PDFs script can find it.

Be sure that your PDF file is turned-in separately (not part of a .zip file).

Grading: