Storing Data on the GPU

• I am looking at HW5
• Start with Program.js
  • This is the "main" for this example.
    • Look at the points defined by CP1... CP8
    • These are then turned into a series of lines when defining cube.
    • But notice that this cube is just a series of lines...
    • Draw this by hand.
  • This data is stored on the CPU in CUBE
  • We then construct a "widget"
    • Widget is a generic term for a thing.
    • You could probably call this something better but ...
    • It is responsible for maintaining a connection to data over on the GPU.
      • It will initially store the data
      • It will make the data active when it needs to.
      • It will draw the object when requested.
    • The call to SetupVBO in the constructor.
      • We first create a buffer.
        • gl.createBuffer()
        • This is equivalent to declaring a pointer on the GPU.
        • It returns an index to a buffer.
        • In GLES this is called GenBuffers
      • Next we need to bind to the buffer.
        • gl.BindBuffer(GLenum target, GLuint buffer);
        • The target describes what we plan on storing in the buffer.
          • gl.ARRAY_BUFFER are vertex attributes.
          • gl.ELEMENT_ARRAY_BUFFER are index
          • There are many others.
        • The target is the buffer we just generated.
        • If this is the first time we "bind" to the buffer, it allocates the right type of space based on the target.
        • This makes the buffer "active"
          • Ie it is "the" buffer.
          • All buffer actions occur to this buffer until another is bound.
      • We finally need to move the data over
        • void gl.bufferData(target, ArrayBufferView srcData, usage);
        • The array type.
        • The actual data.
        • How we plan to use this.
          • This is not hard and fast.
          • But it gives the GPU a clue how to organize the data for best use.
          • If the use is STATIC_DRAW
            • We don't plan to change this data.
          • DYNAMIC_DRAW - we plan to change the data frequently.
          • There are many others here as well.
    • Later we will need to connect the VBO to a variable in a program
      • GLint gl.GetAttribLocation( GLuint program, const GLchar *name);
      • Takes a compiled program (more later)
      • And the name of an attribute in that program.
      • It returns the "location" of the attribute in the program.
    • Display
      • To display an object we need to make it active by making a call to gl.bindBuffer with it's buffer.
      • We need to map this data to the position in the shader.
      • void gl.vertexAttribPointer(index, size, type, normalized, stride, offset);
        • This takes the index of the variable in the shader program.
        • The number of components at each point (ie x,y,z = 3).
        • The type of the points we want to draw.
        • If we want the data to be normalized.
          • This is ignored for float.
        • The stride
        • The starting point.
      • We can store data in several ways.
        • In this case we stored data as x0,y0,z0,x1,y1,z1...xn,yn.zn
          • So the stride is 0
          • And the offset is 0.
        • We could have stored the data as x0,y0,z0,r0,g0,b0,x1,y1,z1,r1 ...
          • The stride would have been 6*sizeof(float).
          • We would have specified another attribute for color
          • And the offset would have been 3*sizeof(float)
        • If we only wanted to draw the back face, we would have skipped the first few items.
          • The offset would have been n*sizeof(float).
        • See HW5B
        • See HW5C