OpenGL draws weird lines on top of polygons - c++

Question

Let me introduce you to Fishtank:
It's an aquarium simulator I am doing on OpenGL to learn before going into Vulkan.
I have drawn many fish like these:
Aquarium
Now I added the grid functionnality which goes like this:
Grid
But when I let it turn for some time, these lines appear:
Weird Lines
I've seen somewhere to clear the Depth Buffer, which I did, but that doesn't resolve the problem.
Here's the code of the function:
void Game::drawGrid()
{
std::vector<glm::vec2> gridVertices;
for (unsigned int x = 1; x < mGameMap.mColCount; x += 1) //Include the last one as the drawed line is the left of the column
{
gridVertices.push_back(glm::vec2(transformToNDC(mWindow, x*mGameMap.mCellSize, mGameMap.mCellSize)));
gridVertices.push_back(glm::vec2(transformToNDC(mWindow, x*mGameMap.mCellSize, (mGameMap.mRowCount-1)*mGameMap.mCellSize)));
}
for (unsigned int y = 1; y < mGameMap.mRowCount; y += 1) //Same here but special info needed:
// Normally, the origin is at the top-left corner and the y-axis points to the bottom. However, OpenGL's y-axis is reversed.
// That's why taking into account the mRowCount-1 actually draws the very first line.
{
gridVertices.push_back(glm::vec2(transformToNDC(mWindow, mGameMap.mCellSize, y*mGameMap.mCellSize)));
gridVertices.push_back(glm::vec2(transformToNDC(mWindow, (mGameMap.mColCount - 1)*mGameMap.mCellSize, y*mGameMap.mCellSize)));
}
mShader.setVec3("color", glm::vec3(1.0f));
glBufferData(GL_ARRAY_BUFFER, gridVertices.size()*sizeof(glm::vec2), gridVertices.data(), GL_STATIC_DRAW);
glVertexAttribPointer(0, 2, GL_FLOAT_VEC2, GL_FALSE, sizeof(glm::vec2), (void*)0);
glEnableVertexAttribArray(0);
glDrawArrays(GL_LINES, 0, gridVertices.size()*sizeof(glm::vec2));
glClear(GL_DEPTH_BUFFER_BIT);
}
I'd like to erase those lines and understand why OpenGL does this (or maybe it's me but I don't see where).

Answer 1

This is the problematic line:
glDrawArrays(GL_LINES, 0, gridVertices.size()*sizeof(glm::vec2));
If you look at the documentation for this function, you will find
void glDrawArrays( GLenum mode, GLint first, GLsizei count);
count: Specifies the number of indices to be rendered
But you are passing the byte size. Hence, you are asking OpenGL to draw more vertices than there are in your vertex buffer. The specific OpenGL implementation you are using is probably reading past the end of the grid vertex buffer and finds vertices from the fish vertex buffer to draw (but this behavior is undefined).
So, just change it to
glDrawArrays(GL_LINES, 0, gridVertices.size());
A general comment: Do not create vertex buffers every time you want to draw the same thing. Create them at the beginning of the application and re-use them. You can also change their content if needed, but be careful with that since it comes with a performance price. Creating vertex buffers is even more costlier.