Dynamic VBO read/write in GLSL? - opengl

Question

Right now it seems to me that my interleaved VBO is strictly ' read-only ' but I want to update it every frame (preferrably from GLSL).
I have a planet that moves around in an orbit, the code below is for rendering points of the orbit.
Problem outline:
I want each point on that orbit to have its own "lifetime", logic:
when the planet passes each consecutive point? update lifetime to 1.0 and reduce with time!
This will be used to create a fading orbitpath of each moving object. Right now Im just looking for ways to manipulate the vbo...
How can I read AND write within GLSL to and from a VBO ? Can anyone post example please?
Update: I modified the code above to work with transform feedback (suggested by user Andon M. Coleman) but I think I might be doing something wrong (I get glError):
Setup:
// Initialize and upload to graphics card
glGenVertexArrays(1, &_vaoID);
glGenBuffers(1, &_vBufferID);
glGenBuffers(1, &_iBufferID);
glGenBuffers(1, &_tboID);
// First VAO setup
glBindVertexArray(_vaoID);
glBindBuffer(GL_ARRAY_BUFFER, _vBufferID);
glBufferData(GL_ARRAY_BUFFER, _vsize * sizeof(Vertex), _varray, GL_DYNAMIC_DRAW);
// TRANSFORM FEEDBACK
glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, _tboID); // Specify buffer
// Allocate space without specifying data
glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER,
_vsize*sizeof(Vertex), NULL, GL_DYNAMIC_COPY);
// Tell OGL which object to store the results of transform feedback
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, _vBufferID); //correct?
glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);
glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE,
sizeof(Vertex), reinterpret_cast<const GLvoid*>(offsetof(Vertex, location)));
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE,
sizeof(Vertex), reinterpret_cast<const GLvoid*>(offsetof(Vertex, velocity)));
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _iBufferID);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, _isize * sizeof(int), _iarray, GL_STREAM_DRAW);
render method():
//disable fragment, so that we do a first run with feedback
glEnable(GL_RASTERIZER_DISCARD);
glBindVertexArray(_vaoID);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _iBufferID);
glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, _tboID);
glBeginTransformFeedback(_mode);
glDrawElements(_mode, _isize, GL_UNSIGNED_INT, 0);
glEndTransformFeedback();
glBindTransformFeedback(GL_TRANSFORM_FEEDBACK, 0);
glBindVertexArray(0);
glDisable(GL_RASTERIZER_DISCARD);
// then i attempt to do the actual draw
glBindVertexArray(_vaoID);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _iBufferID);
glDrawElements(_mode, _isize, GL_UNSIGNED_INT, 0);
glBindVertexArray(0);
And - right before linking:
const GLchar* feedbackVaryings[] = { "point_position" };
glTransformFeedbackVaryings(_ephemerisProgram->getProgramID(), 1, feedbackVaryings, GL_INTERLEAVED_ATTRIBS);

Answer 1

you can not change the content of your VBO from the rendering pipline of Opengl, but you can use tricks to update them depending on the time, also if you are using Opengl 4.4 you can use ComputeShaders but it's a little bit complicated to explain it in here, hust google for it, good luck.

Answer 2

How can I read AND write within GLSL to and from a VBO?
You can't. VBOs are strictly readonly from the normal rendering shaders. Modification is not possible at all (because that would open an infathomable deep barrel of worms) but using transformation feedback the results of the shader stages can be written into a buffer.
Or you use compute shaders.
Problem outline: I want each point on that orbit to have its own "lifetime", logic: When the planet passes each consecutive point? update lifetime to 1.0 and reduce with time!
Sound like a task for a compute shader. But honestly I don't think there's much to gain from processing this on a GPU.