Drawing Two Objects in OpenGL4? - c++

Question

I am trying to draw 2 objects in OpenGL. The window/viewport is (0,0,950,1050). I am not sure this is the right way of doing it, but I thought so. I thought the idea was to create a VBO/VAO per object, bind it, set the data, and repeat that operation for each object.
Then when the objects are to be drawn:
set the shader
bind the data of the first object we want to draw using its vbo (say vbo1)
do the drawing call
bind the data of the next object we want to draw using its vbo (say vbo2)
do the drawing call
...
When I do this, I only get the points of the second object drawn on the screen, but with the color of the first object (it's blue instead of red).
My mistake must be obvious to any expert out there. What do I miss?
// BLUE -----------------------------------
GLuint vbo1, vao1;
glGenBuffers(1, &vbo1);
glBindBuffer(GL_ARRAY_BUFFER, vbo1);
float arr1[] = { 10, 10, 10, 110, 110, 110, 110, 10 };
glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 8, arr1, GL_STATIC_DRAW);
glGenVertexArrays(1, &vao1);
glBindVertexArray(vao1);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);
// RED -----------------------------------
GLuint vbo2, vao2;
glGenBuffers(1, &vbo2);
glBindBuffer(GL_ARRAY_BUFFER, vbo2);
float arr2[] = { 400, 400, 400, 800, 800, 800, 800, 400 };
glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 8, arr2, GL_STATIC_DRAW);
glGenVertexArrays(1, &vao2);
glBindVertexArray(vao2);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);
while (!glfwWindowShouldClose(window))
{
glClearColor(1, 1, 1, 0.1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glUseProgram(pointShader);
GLint loc;
loc = glGetUniformLocation(pointShader, "pointColor");
// it draws this one but with the color blue!
glBindBuffer(GL_ARRAY_BUFFER, vbo2);
float ptColor2[3] = { 1, 0, 0 };
glUniform3fv(loc, 1, ptColor2);
glDrawArrays(GL_POINTS, 0, 4);
// it doesn't draw this one???
glBindBuffer(GL_ARRAY_BUFFER, vbo1);
float ptColor1[3] = { 0, 0, 1 };
glUniform3fv(loc, 1, ptColor1);
glDrawArrays(GL_POINTS, 0, 4);
glfwSwapBuffers(window);
glfwWaitEvents();
}
EDIT 2 WORKING CODE
Thank you very much to both Reto Koradi and Datenwolf. Combining the answers, helped to come with the right answer. It's sad, these things are not explained properly in books. Hope this post will help other beginners (sorry if the result is a bit misleading, I swapped color between when I asked the question, and when I got the answer).
// RED -----------------------------------
GLuint vbo1, vao1;
glGenVertexArrays(1, &vao1);
glBindVertexArray(vao1);
glGenBuffers(1, &vbo1);
glBindBuffer(GL_ARRAY_BUFFER, vbo1);
float arr1[] = { 10, 10, 10, 110, 110, 110, 110, 10 };
glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 8, arr1, GL_STATIC_DRAW);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);
// BLUE -----------------------------------
GLuint vbo2, vao2;
glGenVertexArrays(1, &vao2);
glBindVertexArray(vao2);
glGenBuffers(1, &vbo2);
glBindBuffer(GL_ARRAY_BUFFER, vbo2);
float arr2[] = { 400, 400, 400, 800, 800, 800, 800, 400 };
glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 8, arr2, GL_STATIC_DRAW);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);
while (!glfwWindowShouldClose(window))
{
float ratio;
int width, height;
glfwGetFramebufferSize(window, &width, &height);
glClearColor(1, 1, 1, 0.1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glUseProgram(pointShader);
GLint loc;
loc = glGetUniformLocation(pointShader, "pointColor");
// red
glBindVertexArray(vao1);
float ptColor1[3] = { 1, 0, 0 };
glUniform3fv(loc, 1, ptColor1);
glDrawArrays(GL_POINTS, 0, 4);
// blue
glBindVertexArray(vao2);
float ptColor2[3] = { 0, 0, 1 };
glUniform3fv(loc, 1, ptColor2);
glDrawArrays(GL_POINTS, 0, 4);
glfwSwapBuffers(window);
glfwWaitEvents();
}
EDIT 3
Note though the order from the first code fragment for the VAO/VBO order declaration would also work. So the above version and the one below are both valid:
// RED -----------------------------------
GLuint vbo1, vao1;
glGenBuffers(1, &vbo1);
glBindBuffer(GL_ARRAY_BUFFER, vbo1);
float arr1[] = { 10, 10, 10, 110, 110, 110, 110, 10 };
glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 8, arr1, GL_STATIC_DRAW);
glGenVertexArrays(1, &vao1);
glBindVertexArray(vao1);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);
// BLUE -----------------------------------
GLuint vbo2, vao2;
glGenBuffers(1, &vbo2);
glBindBuffer(GL_ARRAY_BUFFER, vbo2);
float arr2[] = { 400, 400, 400, 800, 800, 800, 800, 400 };
glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 8, arr2, GL_STATIC_DRAW);
glGenVertexArrays(1, &vao2);
glBindVertexArray(vao2);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);
The only thing that was really missing in the code was glBindVertexArray.

Answer 1

The problem is in your draw loop, where you call:
glBindBuffer(GL_ARRAY_BUFFER, vbo2);
It does not matter which GL_ARRAY_BUFFER is currently bound when you make the draw call. The correct buffer needs to be bound when you call glVertexAttribPointer(), which you correctly did in your setup code.
The VAOs track all your vertex setup state. So before each draw call, you need to bind the corresponding VAO by calling glBindVertexArray(), instead of the glBindBuffer() calls you have in the posted code:
glBindVertexArray(vao2);
glUniform3fv(...);
glDrawArrays(...);
glBindVertexArray(vao1);
glUniform3fv(...);
glDrawArrays(...);

Answer 2

You must create and bind the Vertex Array Object before specifying the vertex attribute data locations (glVertexAttribPointer). The VAO kind of takes ownership of the data that belongs to the VBO currently bound when making those calls.
(EDIT accidently submitted while still typing)
Assuming you have a core profile context, when you attempt to create that first buffer object, there's no VAO to bind it to yet, so the whole creation of the BO fails. Hence when you try to draw it, nothing gets drawn at all. But what you see drawn is the BO you intended to bind to the second VAO, but because you got the order of operations wrong it ends up in the first VAO.