OpenGL buffers usage - c

Question

I am new to OpenGL and I am currently reading OpenGL Superbible. In the following code, it creates 3 buffer names, each one is bound to GL_TEXTURE_BUFFER_ARB, then some data is buffered.
My question is, when a new buffer is bound to GL_TEXTURE_BUFFER_ARB, what happens to the previous bufferde data? How is it used?
// Create 3 new buffer objects
glGenBuffers(3,texBO);
glGenTextures(1, &texBOTexture);
int count = 0;
float* fileData = 0;
// Load first texBO with a tangent-like curve, 1024 values
fileData = LoadFloatData(“LumTan.data”, &count);
if (count > 0)
{
glBindBuffer(GL_TEXTURE_BUFFER_ARB, texBO[0]);
glBufferData(GL_TEXTURE_BUFFER_ARB, sizeof(float)*count,
fileData, GL_STATIC_DRAW);
delete fileData;
}
// Load second texBO with a sine-like curve, 1024 values
fileData = LoadFloatData(“LumSin.data”, &count);
if (count > 0)
{
glBindBuffer(GL_TEXTURE_BUFFER_ARB, texBO[1]);
glBufferData(GL_TEXTURE_BUFFER_ARB, sizeof(float)*count,
fileData, GL_STATIC_DRAW);
delete fileData;
}
// Load third texBO with a linear curve, 1024 values
fileData = LoadFloatData(“LumLinear.data”, &count);
if (count > 0)
{
glBindBuffer(GL_TEXTURE_BUFFER_ARB, texBO[2]);
glBufferData(GL_TEXTURE_BUFFER_ARB, sizeof(float)*count,
fileData, GL_STATIC_DRAW);
delete fileData;
}
// Load the Tan ramp first
glBindBuffer(GL_TEXTURE_BUFFER_ARB, 0);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_BUFFER_ARB, texBOTexture);
glTexBuffer(GL_TEXTURE_BUFFER_ARB, GL_R32F, texBO[0]);
glActiveTexture(GL_TEXTURE0);
Actually I don't understand where the data in these buffers is actually used. To me, the code looks like:
int x = 3;
x = 7; //what about 3?
x = 5; //what about 7?
Moreover, after doing this (binding buffers, then buffering data into these buffers) to the three buffers, all the buffers are unbound from GL_TEXTURE_BUFFER_ARB by binding 0 to it.

Answer 1

All OpenGL object are represented by an unique id.
So if you create a Shader, Programm, Buffer, ... then you get an ID for these objects. These Objects live on the graphic card until you call delete them with glDelete[...] the commands you call for those objects are applied to the currently bound object glBind[...].
You can imagine it like a dialog between you and the gpu:
You tell the GPU with which object you currently want to work
The GPU activate this object for you
You then tell the GPU what you want to do with the the currently active object
The GPU applies this information to the active object.
With glGenBuffers(3,texBO) you create 3 Buffers are create and their ids are stored in texBO.
glBindBuffer(GL_TEXTURE_BUFFER_ARB, texBO[0]);:
Active the buffer with that is represented by the id stored in texBO[0]
glBufferData(GL_TEXTURE_BUFFER_ARB, sizeof(float)*count,
fileData, GL_STATIC_DRAW);:
Popolate the currently active Buffer (in this case texBO[0]) with the data stored in fileData.
glBindBuffer(GL_TEXTURE_BUFFER_ARB, texBO[1]);:
Now activates the buffer with that is represented by the id stored in texBO[1] the other buffer still exists but is not currently bound.