Interaction with Shaders and Further Examples

• OpenGL has a generic system for allowing the user to pass data in a VBO along to the shader.
• This is the VertexAttribute
• This is related to data that is associated with each vertex.
• Attributes are specified in the shaders
  • In color.vert there are three attributes specified
    • vPosition, the vertex position
    • vColor, the vertex color
    • vScale, a per vertex scale factor
      • Dumb, I know but I wanted to illustrate a point.
  • Looking at the code in the shader, you can see how each is used
  • We will begin looking at shaders more soon, but this will do for now.
• In the program color.C
  • Attrib_IDs has changed to include the new attributes
    • The names do not have to match, just the positions.
  • The data that we stick in the VBO (vertices) has changed.
  • It includes the new data.
  • In the setup routine, we specify the two additional data values
    • glVertexAttribPointer
    • glEnableVertexAttribArray
    • Note, the location is in terms of bytes.
• According to this source
  • glVertexAttribPointer does two things
    • State where (in the VBO) the data comes from
    • Tells OpenGL how to interpret (the format of) that data.
  • Sizes are limited between 1 and 4 elements.
    • If the shader requires more components then the data supplies, they are increased by (0,0,0,1)
    • So (x,y) becomes (x,y,0,1) for vertex position)
    • This is not true for doubles
• Storing the data
  • They say we should use interleaved arrays whenever possible.
  • But colors2.C shows another option
    • A single struct with multiple arrays.
    • Not a big fan of this method
    • All three arrays are packed (in a contiguous manner) in the same struct.
    • I think this would be hard to build nicely.
    • It was a pain to initialize
    • offsetof(type, field) returns the offset, in bytes of the field from the beginning of the structure.
  • And here is with multiple VBOs
    • colors3.C
    • To do the initialization, I needed to compile with -std=c++11
    • I used his vec.h class for my data types.
      • He had sqrt and ceilf in the std library for this
      • I removed it.
      • But you do need to include <cmath>
      • I would start using this, it has a bunch of utilities we
        • Want to use
        • Don't want to write.
• In twoThings.C I create multiple VAOs and draw them.