Drawing multiple objects in OpenGL with different buffers - opengl

Question

With OpenGL shaders, I want to render two objects. Each is defined by a set of vertex positions and vertex indices for the triangles. When I make my buffers, I use the following code:
// Object 1 vertex positions
glBindBuffer(GL_ARRAY_BUFFER, object1_vertex_buffer);
glBufferData(GL_ARRAY_BUFFER, object1_vertices_size, object1_vertices, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
// Object 1 vertex indices
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object1_index_buffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, object1_indices_size, object1_indices, GL_STATIC_DRAW);
// Object 2 vertex positions
glBindBuffer(GL_ARRAY_BUFFER, object2_vertex_buffer);
glBufferData(GL_ARRAY_BUFFER, object2_vertices_size, object2_vertices, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
// Object 2 vertex indices
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object2_index_buffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, object2_indices_size, object2_indices, GL_STATIC_DRAW);
And then when I render my scene, I use the following code:
// Object 1
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object1_index_buffer);
glDrawElements(GL_TRIANGLES, object1_num_indices, GL_UNSIGNED_INT, (void*)0);
// Object 2
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object2_index_buffer);
glDrawElements(GL_TRIANGLES, object2_num_indices, GL_UNSIGNED_INT, (void*)0);
However, this results in only object 2 being drawn. What am I doing wrong?

Answer 1

If you have openGL 3.3+ you should use a VAO:
glBindVertexArray(vao1);
// Object 1 vertex positions
glBindBuffer(GL_ARRAY_BUFFER, object1_vertex_buffer);
glBufferData(GL_ARRAY_BUFFER, object1_vertices_size, object1_vertices, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
// Object 1 vertex indices
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object1_index_buffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, object1_indices_size, object1_indices, GL_STATIC_DRAW);
glBindVertexArray(vao2);
// Object 2 vertex positions
glBindBuffer(GL_ARRAY_BUFFER, object2_vertex_buffer);
glBufferData(GL_ARRAY_BUFFER, object2_vertices_size, object2_vertices, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
// Object 2 vertex indices
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object2_index_buffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, object2_indices_size, object2_indices, GL_STATIC_DRAW);
and then you only need to bind the right vao when drawing:
// Object 1
glBindVertexArray(vao1);
glDrawElements(GL_TRIANGLES, object1_num_indices, GL_UNSIGNED_INT, (void*)0);
// Object 2
glBindVertexArray(vao2);
glDrawElements(GL_TRIANGLES, object2_num_indices, GL_UNSIGNED_INT, (void*)0);
otherwise you will need to repeat the bind and glVertexAttribPointer calls between draws:
// Object 1
glBindBuffer(GL_ARRAY_BUFFER, object1_vertex_buffer);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object1_index_buffer);
glDrawElements(GL_TRIANGLES, object1_num_indices, GL_UNSIGNED_INT, (void*)0);
// Object 2
glBindBuffer(GL_ARRAY_BUFFER, object2_vertex_buffer);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object2_index_buffer);
glDrawElements(GL_TRIANGLES, object2_num_indices, GL_UNSIGNED_INT, (void*)0);

Answer 2

If this is all the code present, then there are several problems:
One has to enable vertex attributes by calling glEnableVertexAttribArray.
In the drawing code you are only rebinding the index buffer, but not the vertex buffer. Both, glBindBuffer(GL_ARRAY_BUFFER,... and glVertexAttribPointer change the current state, so you are overriding the first objects setting with the second ones. If you really want to work without VAOs, you'll have to bind the correct buffer before each draw call and update the glVertexAttribPointer
There is no VAO in use (VAOs are mandatory when working in Core profile but I would recommend using them also in compatibility mode)