Binding a second vertex buffer seems to spoil my first vertex buffer, OpenGL OES ios 5.1

Question

I'm creating two different vertex buffers, that use two different shaders to render them. As soon as I bind the second vertex buffer, the data I parked in the first vertex buffer seems to be corrupted or lost.

If I generate and draw one vertex buffer only, like so:

glGenBuffers( 1, &vb1 ) ;
glBindBuffer( GL_ARRAY_BUFFER, vb1 ) ;

// fill it..
glBufferData( .. )

Then, in the draw() loop,

glUseProgram( shader1 ) ;
glBindBuffer( vb1 ) ; // make sure it is bound
glDrawArrays( ... )   // draw it

Then it works fine, no problems, no errors (I am glGettingLastError() after every gl* call, so it seems this code is perfectly fine)

Now if I so much as generate and bind a second vertex buffer, any time after the first one was generated and bound,

// ran in init() -- then previously working code in draw() is broken
glGenBuffers( 1, &vb2 ) ;              // ok.. no problems with this line
glBindBuffer( GL_ARRAY_BUFFER, vb2 ) ; // spoils data in vb1?

As soon as I call glBindBuffer with this newly generated vb2 buffer, it seems the data in vb1 is completely dumped or lost. On attempts to draw vb1 (not vb2!), i get this crash:

enter image description here

I even filled the array with GL_STATIC_DRAW.

I don't understand, I thought these vertex buffers were supposed to retain data, even if another vertex buffer is created and initialized? .. what am I doing wrong?

what am I doing wrong? It's hard to say, considering that you've provided so little code, and the code you have put up is non-functional (binding a buffer has no effect on DrawArrays, only setting pointers controls what data is drawn). Can you show what you're actually doing? — Tim
Admittedly, this isn't a very good question. I'll delete it. — bobobobo
We're locked out of Meta at the moment, but I undeleted this question for you, so you're good to edit it now. — Brad Larson

bobobobo bobobobo · Accepted Answer · 2012-09-15T23:21:35

I found the answer to this question to be very subtle and difficult to find.

I thought I didn't need the vertex array declarations that the standard example uses, and you can see I left them out in the question. But it turns out using vertex array objects is vital to correctly binding indices.

So for example,

// Make vertex array OBJECT, _this does not store any data!_
// A "vertex array object" is a bit like a display list in
// that it will just remember and repeat the function calls you make,
// with the values you make them with.
glGenVertexArraysOES( 1, &va ) ; // make vertex array OBJECT. NOT used
// to store data..

// Bind the vertex array
glBindVertexArrayOES( va ) ; // "start recording the calls I make.."

// Make a buffer for the vertex array (place to store the data)
glGenBuffers( 1, &vb ) ;     // make the vertex BUFFER (used to
// store the actual data)

// Make the vertex array buffer active
glBindBuffer( GL_ARRAY_BUFFER, vb ) ;

glEnableVertexAttribArray( ... ) ;  // REMEMBERED IN VAO (vertex array object)
glVertexAttribPointer( ... ) ;      // REMEMBERED IN VAO

glBufferData( .. ) ; // send data down to gpu, not remembered by vao

// "Stop remembering" calls to glBindBuffer( .. ) and glEnableVertexAttribArray( .. )
glBindVertexArrayOES( 0 ) ; // "stop recording"

At draw time, simply:

glBindVertexArrayOES( va ) ; // runs ALL your glEnableVertexAttribArray() calls,
// as well as your glBindBuffer( .. ) call,

// now all that's left to do is draw
glDrawArrays( glDrawingStyle, 0, vertexCount );

Binding a second vertex buffer seems to spoil my first vertex buffer, OpenGL OES ios 5.1

1 Answers