Hey everyone!

EDIT: Oh, excuse me, after panically posting something here I managed it out, like always, sorry again. I simply forgot to glBindBufferBase before glGetBufferSubData, now it's perfect!( I think, lol ). I hope this will be useful for someone. Sorry and Thanks again!

I am using terrain displacement method in GLSL, so I want to get the data from it to game core and use it. ( Don't ask me why I am using displacement in GLSL, it's just easier and I can't get that good results in main code, lol ).

So, I got vertex array/buffer objects. Here's what I do:

    // Initialization code

    glGenVertexArrays( ...
    glBindVertexArray( .... ...
    // Bind generated data here. ( X, 0.0f, Z )
    ......

    glGenBuffers( 1, &m_transformFeedback );
    glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, m_transformFeedback);
    glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER, m_map_size * sizeof( ncVec3 ), NULL, GL_DYNAMIC_COPY);
    glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, 0);
    glGenQueries(1, &m_transformFeedbackQuery); // Temp, eats a lot of performance.

So this works okay, no errors. And here's how I render this:

    // Bind vertex array with vertex buffers ( vertices, normals, uvs ).
    ......
 
    // Drawing now.
    glEnable(GL_RASTERIZER_DISCARD);
    
    glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, m_transformFeedback);
    
    glBeginQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN, m_transformFeedbackQuery); // temp
    glBeginTransformFeedback(GL_TRIANGLES);
    
    glDrawArrays( GL_TRIANGLE_STRIP, 0, m_map_size );
    
    glEndTransformFeedback();
    glEndQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN);

    glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, 0);
    glDisable(GL_RASTERIZER_DISCARD);
    ......
     GLuint primitives;
        // THIS RETURNS RIGHT NUMBER OF PRIMITIVES! 
    glGetQueryObjectuiv(m_transformFeedbackQuery, GL_QUERY_RESULT, &primitives);
    g_Core->Print( LOG_INFO, "%i primitives written.\n", primitives );
 
// HERE I GOT NOTHING! It returns 1.0, 1.0, 1.0 or just 0.0, 0.0, 0.0
// tmapData is allocated or just static array, no difference, same result.
    glGetBufferSubData( GL_TRANSFORM_FEEDBACK_BUFFER, 0, sizeof(tmapData), tmapData );
// Random numbers.
    g_Core->Print( LOG_INFO, "%f %f %f\n", tmapData[53].x, tmapData[353].y, tmapData[3534].z );

Shader code:

        
        glAttachProgram( vert and fragments )
       
        const char * names[] = { "VertexOut" };
        // GL_INTERLEAVED_ATTRIBS or GL_SEPARATED_ATTRIBS, not working yet :(
        glTransformFeedbackVaryings( program, 1, names, GL_INTERLEAVED_ATTRIBS );
        glLinkProgram( ...

// Vertex shader:
out vec3 vertexOut;        

         vec4 map_data = texture( gSampler[4], hmap_coord );
	float displacement = u_map_scale * map_data.r;
	 vertex.y += displacement;
	vertexOut = vertex; // Even tried to vertexOut = vec3(10.0, 10.0, 10.0);

What could be problem? I am only using vertex and fragment shader, no geometry ( don't want to )

Thanks in advance!