Computer Graphics environment in Matlab

@(515.1 Computer Graphics)[matlab]

Recently, we are using matlab to do computer vision experiments. Thus this note introduces some function controlling the elementary graphics environment.

For Computer Graphics, the basic 3 components are

  • Object
    • position and shape
    • material (BRDF)
  • Light
  • Camera

And the rendering engine will take these components and give out a physical looking image.

Light and Camera

Basic property of light is the direction of light, and point source vs parallel light. light specify the light property.

For more complex control of light source see commands

  • lighting, flat render each facet the same color, gouraud interpolate the normal vector field on the curved surface.

view specify the camera center line of sight. view(az,el) or view(v3) will specify the camera direction. Normally it’s enough for plotting figures.

  • Note that view() can defaultly return the 4 by 4 projection matrix formed by getAxesTransformationMatrix method. see viewmtx for reference.

For more specific control of camera, it has the full set of commands

  • campos set the position of camera
  • camtarget set the target of line of sight
    • camlookat higher level function that target an object (using its handle)
  • camroll roll the camera view around the line of sight or any axis.
    • camup roll the camera so that the up direction in the view match the vector.
    • Note in auto mode [0,0,1] i.e. z direction usually means up. so you will not rotate across y axis in a full cycle.
  • camlight will fix the light relative to the view point.

3d Shape Description

patch, vertices, faces

  • In the most simple case, it’s a simplicial complex, or triangle mesh network. It’s simple, vertices is a (nv,3) float array encoding the 3d coordinates of every vertex. And faces is a (nf,3) int array encoding the row index of vertices it consists.
  • But a face can consists of any number of vertices

patch('Faces',F,'Vertices',V) uses the Face and Vertices data to create shape, i.e. it’s kind of the rendering engine in Matlab! Note:

  • patch can not only visualize shapes in the computer graphics sense, it can visualize color data defined on faces or vertices. For example, from doc 
    vertices = [0 0; 0 5; 5 0; 3 3; 3 6; 6 3];
faces = [1 2 3; 4 5 6];
C = [5; 1; 4; 3; 2; 6];
p = patch('Faces',faces,'Vertices',vertices,'FaceVertexCData',C);
p.FaceColor = 'interp';

vertices = [0 0; 0 3; 4 0; 0 4; 0 7; 4 4; 5 0; 5 3; 9 0];
faces = [1 2 3; 4 5 6; 7 8 9];
C = [0; 0.6667; 1];
p = patch('Faces',faces,'Vertices',vertices,'FaceVertexCData',C);
p.FaceColor = 'flat';

See the figure in patch Ch doc. en doc, patch colormap.

Triangulation

Note the 2d shape and 3d shapes could be represented as simplicial complex, i.e. triangulation. Using this representation, many properties of the shape like the normal vector at the vertices or facets could be calculated.

BRDF

BRDF is specified by the material command in matlab

Functions

isosurface isonormal isocaps patch