Using glDrawElements with multiple meshes - opengl

Question

I can succesfully draw a scene with glDrawArrays which looks like this:
This technique is a bit slow so i decided to make a indexbuffer and tried to use glDrawElements. The result of that looks like this:
As you probably can see, the squares top right are rendered incorrectly and square below the airplane dissappeared.
The code for generating the buffers
//create vertex and index buffer
glGenBuffers(1, &gVertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, gVertexBuffer);
glGenBuffers(1, &gIndexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, gIndexBuffer);
// Define the size of the buffers
GLuint floatAmount = 0;
GLuint GLuintAmount = 0;
for each (MeshObject* mesh in meshes)
{
floatAmount += mesh->GetFloatAmount();
GLuintAmount += mesh->GetGLuintAmount();
}
glBufferData(GL_ARRAY_BUFFER, floatAmount, 0, GL_STATIC_DRAW);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, GLuintAmount, 0, GL_STATIC_DRAW);
// Define size and offset of the different subdata in the buffers
GLuint offsetVer = 0;
GLuint offsetInd = 0;
for each (MeshObject* mesh in meshes)
{
// Set offset for mesh
mesh->SetOffset(offsetVer / sizeof(Point));
mesh->SetOffsetInd(offsetInd);
glBufferSubData(GL_ARRAY_BUFFER,
offsetVer,
mesh->GetFloatAmount(),
mesh->GetPoints().data());
glBufferSubData(GL_ELEMENT_ARRAY_BUFFER,
offsetInd,
mesh->GetGLuintAmount(),
mesh->GetIndicies().data());
offsetVer += mesh->GetFloatAmount();
offsetInd += mesh->GetGLuintAmount();
}
... and the code for the rendering
glBindBuffer(GL_ARRAY_BUFFER, gVertexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, gIndexBuffer);
mat4 vwMatrix = localCamera->GetPVMatrix() * mh->GetWorld();
glUniformMatrix4fv(projectionviewworldMatrixUniformLocation, 1, GL_FALSE, &(GLfloat)vwMatrix[0][0]);
//glDrawArrays(GL_TRIANGLES, mh->mesh->GetOffset(), mh->mesh->GetPoints().size());
glDrawElements(GL_TRIANGLES, mh->mesh->GetIndicies().size(), GL_UNSIGNED_INT, (void*)mh->mesh->GetOffsetInd());
//GLuint size = mh->mesh->GetIndicies().size();
//GLuint IndSize = mh->mesh->GetOffsetInd();
//glDrawElements(GL_TRIANGLES, size, GL_UNSIGNED_INT, (void*)IndSize);

Answer 1

You need to add offsetVer on each element of the index buffer. Here is an example:
Mesh A ( 1 triangle)
Vertices: v0 v1 v2
Indices: 0 1 2
Mesh B ( 1 triangle)
Vertices: v3 v4 v5
Indices: 0 1 2
That is how your combined buffer looks like:
Vertices: v0 v1 v2 v3 v4 v5
Indices: 0 1 2 0 1 2
That is how it should be:
Vertices: v0 v1 v2 v3 v4 v5
Indices: 0 1 2 3 4 5