I'm working on trying to save transform feedback output into a pair of uniform buffers and I keep getting
"The specified operation is invalid for the current OpenGL state"
after beginning the transform feedback.
Basically, I'm trying to convert the flocking example from superbible #5 to OpenGL ES 3.

Here is the setup:

Code :
    GLuint positionBlockIndex = glGetUniformBlockIndex(flockingUpdateProgram, "PositionBlock");
    GLuint velocityBlockIndex = glGetUniformBlockIndex(flockingUpdateProgram, "VelocityBlock");
    GLint positionBlockSize,velocityBlockSize;
    glGetActiveUniformBlockiv( flockingUpdateProgram, positionBlockIndex, GL_UNIFORM_BLOCK_DATA_SIZE, &positionBlockSize );
    glGetActiveUniformBlockiv( flockingUpdateProgram, velocityBlockIndex, GL_UNIFORM_BLOCK_DATA_SIZE, &velocityBlockSize );
    // Set up UBOs
    for (i = 0; i < 2; i++)
    {
        glUniformBlockBinding(flockingUpdateProgram, positionBlockIndex, 0);
        glBindBufferBase(GL_UNIFORM_BUFFER,0,position_ubo[i]);
        glBufferData(GL_UNIFORM_BUFFER, positionBlockSize, NULL, GL_DYNAMIC_DRAW);
        glUniformBlockBinding(flockingUpdateProgram, velocityBlockIndex, 1);
        glBindBufferBase(GL_UNIFORM_BUFFER,1,velocity_ubo[i]);
        glBufferData(GL_UNIFORM_BUFFER, velocityBlockSize, NULL, GL_DYNAMIC_DRAW);
    }


Here is the render call:

Code :
 // Depending on whether we're rendering an even or odd frame...
    if (frame_index & 1)
    {
        // Read from second set of buffers (c)
        glBindBufferBase(GL_UNIFORM_BUFFER,1, velocity_ubo[1]);
        glBindBufferBase(GL_UNIFORM_BUFFER,0, position_ubo[1]);
        glBindVertexArray(update_vao[1]);
 
        // Write to first
        glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, flock_position[0]);
        glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 1, flock_velocity[0]);
    }
    else
    {
        // Read from first set of buffers (a)
        glBindBufferBase(GL_UNIFORM_BUFFER,1, velocity_ubo[0]);
        glBindBufferBase(GL_UNIFORM_BUFFER,0, position_ubo[0]);
        glBindVertexArray(update_vao[0]);
 
        // Write to second
        glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, flock_position[1]);
        glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 1, flock_velocity[1]);
    }
 
    // Turn off rasterization for the simulation pass (no fragment shader)
    glEnable(GL_RASTERIZER_DISCARD);
 
    // Press record...
    glBeginTransformFeedback(GL_POINTS);
    // Draw
    glDrawArrays(GL_POINTS, 0, flock_size);// **ERROR HERE**
    // Press stop...
    glEndTransformFeedback();
 
    // Turn rasterization back on
    glDisable(GL_RASTERIZER_DISCARD);