Read position data from Vertex Buffer in openGL - ios

Question

Say we have an object and we want to create multiple objects and move them independently based some algorithm.
Here is what is the process I am using:
Create a structure with the geometry of the object
Create an array of vertice buffers using the geometry of the object
Now in the rendering routine, I need to go through each one of those objects and alter their position based on a specific algorithm.
To accomplish this I need to get the current location of the object to compute the new position.
How can I get the current location of a vertice buffer? Clearly, I do not want to store outside the program all locations of the object since they are inside the vertice buffer.
EDIT: This is the code I am using to store and retrieve data from the model matrix of each object
// Set up Code
- (void)setupGL
{
[EAGLContext setCurrentContext:self.context];
[self loadShaders];
glEnable(GL_DEPTH_TEST);
for( int i; i<num_objects; i++) {
glGenVertexArraysOES(1, &_objectArray[i]);
glBindVertexArrayOES(_objectArray[i]);
glGenBuffers(1, &_objectBuffer[i]);
glBindBuffer(GL_ARRAY_BUFFER, _objectBuffer[i]);
glBufferData(GL_ARRAY_BUFFER, sizeof(objectData), objectData, GL_STATIC_DRAW);
glEnableVertexAttribArray(....);
glVertexAttribPointer(......, 3, GL_FLOAT, GL_FALSE, 24, BUFFER_OFFSET(0));
}
glBindVertexArrayOES(0);
}
//********************************************************
// Rendering Code
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
glUseProgram(_program);
for(int i=0; i<num_objects; i++) {
glBindVertexArrayOES(_objectArray[i]);
// Get Previous data
GLint uMatrix = glGetUniformLocation(_program, "modelMatrix");
glGetUniformfv(_program, uMatrix, dataOfCurrentObject);
// Get Previous data
... transform dataOfCurrentObject based on an algorithm and create newDataOfCurrentObject
// Update object with new data and draw
glUniformMatrix4fv(uMatrix, 1, 0, newDataOfCurrentObject);
glDrawArrays(GL_TRIANGLES, 0, 36);
}
}
The problem I have now is that the dataOfCurrentObject for object 'i' is identical to the newDataOfCurrentObject for object 'i-1'. In other words it appears that the code keeps track of only one model matrix for all objects, or it does not read correctly the model matrix of a specific object. Any ideas?

Answer 1

The simplest method that gets used is to set the object's position in the Model Matrix, and upload that object's appropriate Model Matrix as a Uniform each time you draw a new object. The [pseudo-]code would look like this:
for(game_object object : game_object_list) {
glUniformMatrix4f(modelMatrixUniformLocation, 1, false, object->model_matrix);
object->draw();
}
And when you need to update the object's position:
for(game_object object : game_object_list) {
object->model_matrix = Matrix.identity();
/*Any transformations that need to take place here*/
object->model_matrix = object->model_matrix.transpose(/*x*/, /*y*/);
/*Any other transformations that need to take place*/
}
Exactly what you put in there will vary depending on your needs. If you're programming a 2D game, you probably don't need a 4x4 model matrix. But the basic logic should be identical to what you eventually use.

Answer 2

You don't have to read anything back from your vertex buffers. You just need to keep in your application the tranform you need and pass it as uniform for every object.
For example if you need to translate the object, keep the modelViewMatrix in and every frame apply trasformation to the previous matrix :
GLKMatrix4 modelViewMatrix;
...
modelViewMatrix = GLKMatrix4Identity;
...
-(void) render {
modelViewMatrix = GLKMatrix4Translate(modelViewMatrix, deltaX, deltaY, deltaZ);
glUniformMatrix4fv(_modelViewUniform, 1,0, modelViewMatrix.m);
}
So you just need to keep reference to your objects modelView matrices and apply your transformations accordingly.
If you want to read data back from your vertex buffers (gpu) to your application you have to use Transform Feedback which is a bit more advanced technique and it is used when you modify your vertexes in the vertex shader and you need the results back. (It is not needed for your case).