how to use glDrawElements with glBindVertexArray properly - c++

Question

what am trying to do is using glDrawElements to draw without redundancy of vertices as follow:
Model ModelManager::CreateModel(std::vector<glm::vec3>&vertices, std::vector<uint16_t>&vertexIndeces)
{
//Vertecies
GLuint vertexArray;
glGenVertexArrays(1, &vertexArray);
glBindVertexArray(vertexArray);
GLuint vBufferId;
glGenBuffers(1, &vBufferId);
glBindBuffer(GL_ARRAY_BUFFER, vBufferId);
glBufferData(GL_ARRAY_BUFFER, vertices.size(), vertices.data(), GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
GLuint iBufferId;
glGenBuffers(1, &iBufferId);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, iBufferId);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, vertexIndeces.size(), vertexIndeces.data(), GL_STATIC_DRAW);
glBindVertexArray(0);
//
return Model(vertexArray, vBufferId, iBufferId, vertexIndeces.size());
}
and then when I draw :
void Model::Draw()
{
if (vertexArray)
{
isFinishedIniting = true;
glBindVertexArray(vertexArray);
glDrawElements(GL_TRIANGLES, elementCount, GL_UNSIGNED_SHORT, 0);
glBindVertexArray(0);
}
}
shaders:
#version 120
void main()
{
gl_Position= gl_ModelViewProjectionMatrix*gl_Vertex;
}
#version 120
void main()
{
gl_FragColor=vec4(1.0,0.0,0.0,0.0);
}
the obj file am trying to load is easy I hand made it :
v 0.0 0.0 0.0
v 1.0 1.0 0.0
v -1.0 1.0 0.0
v -1.0 -1.0 0.0
v 1.0 -1.0 0.0
f 1/1/1 2/1/1 3/1/1
f 1/1/1 4/1/1 5/1/1
so it should show two red Triangles ,but it's not drawing anything to the screen!

Answer 1

There are a couple of problems in this code:
The sizes passed to glBufferData() look wrong:
glBufferData(GL_ARRAY_BUFFER, vertices.size(), vertices.data(), GL_STATIC_DRAW);
...
glBufferData(GL_ELEMENT_ARRAY_BUFFER, vertexIndeces.size(), vertexIndeces.data(), GL_STATIC_DRAW);
Both vertices and vertexIndeces() are vectors. The .size() method on a vector gives the number of elements, while glBufferData() expects the size in bytes. To fix this, change the code to:
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(vertices[0]),
vertices.data(), GL_STATIC_DRAW);
...
glBufferData(GL_ELEMENT_ARRAY_BUFFER, vertexIndeces.size() * sizeof(vertexIndeces[0]),
vertexIndeces.data(), GL_STATIC_DRAW);
The API calls use a mix of fixed function attributes and generic vertex attributes. Based on the version in the shader code, and the shader code itself (particularly the use of gl_Vertex), you're using OpenGL 2.1 level shaders with fixed function attributes. Therefore, you need to use glEnableClientState() and glVertexPointer() instead of glEnableVertexAttribArray() and glVertexAttribPointer():
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, 0);
You can use generic vertex attributes, but then you need to declare a variable of type attribute in the vertex shader, instead of using gl_Vertex.