Multiple Vertex Buffers OpenGL c++ - c++

Question

I want to draw 2 Vertex Buffers but it only draws the second one. I am using OpenGL 4.6 and the COMPAT profile. The code:
float buffer[] =
{
0.0f,0.0f,
1.0f,0.0f,
1.0f,1.0f
};
float buffera[] =
{
0.0f,0.0f,
1.0f,0.0f,
1.0f,-1.0f
};
unsigned int id;
glGenBuffers(1, &id);
glBindBuffer(GL_ARRAY_BUFFER, id);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), buffer, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
glBindBuffer(GL_ARRAY_BUFFER, 0);
unsigned int ida;
glGenBuffers(1, &ida);
glBindBuffer(GL_ARRAY_BUFFER, ida);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), buffera, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
glBindBuffer(GL_ARRAY_BUFFER, 0);
unsigned int indices[]
{
0,1,2
};
And then the draw calls:
glBindBuffer(GL_ARRAY_BUFFER, id);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);
glBindBuffer(GL_ARRAY_BUFFER, ida);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);

Answer 1

When glVertexAttribPointer is called, the the buffer which is currently bound to the target ARRAY_BUFFER is associated to the attribute. It is not sufficient to bind the buffer object. You have to do the vertex specification before drawing the object:
glBindBuffer(GL_ARRAY_BUFFER, id);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);
glBindBuffer(GL_ARRAY_BUFFER, ida);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);
I recommend to use a Vertex Array Object. The VAO is used to store the vertex specification.
unsigned int vao1;
glGenVertexArrays(1, &vao1);
glBindVertexArray(vao1);
unsigned int id;
glGenBuffers(1, &id);
glBindBuffer(GL_ARRAY_BUFFER, id);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), buffer, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
unsigned int vao2;
glGenVertexArrays(1, &vao2);
glBindVertexArray(vao2);
unsigned int ida;
glGenBuffers(1, &ida);
glBindBuffer(GL_ARRAY_BUFFER, ida);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), buffera, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
glBindVertexArray(vao1);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);
glBindVertexArray(vao2);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);

Answer 2

Since you are not using a VAO, only binding the buffer is not enough. The command which establishes from which buffer a draw command reads is glVertexAttribPointer. You have to move this command into to the draw commands (and since they aren't doing anthing, remove them from the initialization.
The code should be something like:
Init:
unsigned int id;
glGenBuffers(1, &id);
glBindBuffer(GL_ARRAY_BUFFER, id);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), buffer, GL_STATIC_DRAW);
unsigned int ida;
glGenBuffers(1, &ida);
glBindBuffer(GL_ARRAY_BUFFER, ida);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), buffera, GL_STATIC_DRAW);
Drawing:
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, id);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);
glBindBuffer(GL_ARRAY_BUFFER, ida);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (const void*)8);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);