Why is the z coordinate flipped after multiplying with a matrix in GLSL - OpenGL

Question

I'm making a small game engine in which i want to draw stuff using OpenGL. I abstracted all the OpenGL objects into classes (Buffers, VertexArrays, Shaders, Programs...). Everything worked fine until i got to 3D rendering. I implemented my own matrices and vectors(i didn't use like glm), and when i multiply my vertex position in the shader with any matrix, the z coordinate flips (z = -z). I even tried with the identity matrix. Here is the vertex shader:

#version 330 core

layout(location = 0) in vec4 i_pos;
layout(location = 1) in vec4 i_color;

out vec4 p_color;

uniform mat4 u_MVP;
uniform vec4 u_pos;

void main()
{    
    gl_Position = u_MVP * (i_pos + u_pos);
    p_color = i_color;
}

I used the u_Pos uniform just for debugging reasons. And here i set the uniforms:

void Frame() override
    {       
        deltaTime = timer.Reset();

        if (Input::GetKey(Key::W).value == KeyDown) pos.z += deltaTime;     
        if (Input::GetKey(Key::S).value == KeyDown) pos.z -= deltaTime;             

        //mat4f(1.0f) creates a identity matrix
        shaderSelection.SetUniform("u_MVP", mat4f(1.0f));       
        shaderSelection.SetUniform("u_pos", vec4f(pos));

        ren.DrawTriangles(vertexArray, indexBuffer, shaderSelection);
    }

Although im sure there's nothing with the matrix struct, here it is:

template<typename T = float, int sizeX = 4, int sizeY = 4>
struct BLAZE_API mat
{
private:
    T v[sizeY][sizeX];
public:
    mat()
    {
        for (unsigned i = 0; i < sizeX * sizeY; i++)
            ((T*)v)[i] = 0;
    }
    mat(T* ptr, bool transpose = false)
    {
        if (transpose)
            for (unsigned i = 0; i < sizeX * sizeY; i++)
                ((T*)v)[i] = ptr[i];
        else
            for (unsigned i = 0; i < sizeX * sizeY; i++)
                ((T*)v)[i] = ptr[i % sizeY * sizeX + i / sizeY];        
    }
    mat(T n)
    {
        for (int x = 0; x < sizeX; x++)
            for (int y = 0; y < sizeY; y++)
                if (x == y)
                    operator[](x)[y] = n;
                else
                    operator[](x)[y] = 0;
    }   
    mat(const mat<T, sizeX, sizeY>& mat)
    {
        for (int x = 0; x < sizeX; x++)
            for (int y = 0; y < sizeY; y++)
                v[x][y] = mat[x][y];
    }

    inline T* operator[] (unsigned i) const { return (T*)(v[i]); }  
    inline void operator= (const mat<T, sizeX, sizeY>& mat)
    {
        for (int x = 0; x < sizeX; x++)
            for (int y = 0; y < sizeY; y++)
                v[x][y] = mat[x][y];
    }
};

And the SetUniform does this:

glUniformMatrix4fv( ... , 1, GL_FALSE, m[0]);

I made the matrix struct such that i don't have to use GL_TRUE for transpose parameter in glUniformMatrix4fv. I am pretty sure it isnt my matrix implementation that is inverting the z coordinate. It is like the camera is looking in the -Z direction, but when i move a object in the +X direction it moves also +X on the screen(also applies for Y direction), which it shouldn't if the camera is facing -Z. Is this supposed to happen, if so can i change it?

Answer 1

If you do not transform the vertex coordinates (or transform it by the Identity matrix), then you directly set the coordinates in normalized device space. The NDC is a unique cube, with the left, bottom, near of (-1, -1, -1) and the right, top, far of (1, 1, 1). That means the X-axis is to the right, the Y-axis is upwards and the Z-axis points into the view.

In common the OpenGL coordinate system is a Right-handed system. In view space the X-axis points to the right and the Y-axis points up.
Since the Z-axis is the Cross product of the X-axis and the Y-axis, it points out of the viewport and appears to be inverted.

To compensate the difference in the direction of the Z-axis in view space in compare to normalized device space the Z-axis has to be inverted.
A typical OpenGL projection matrix (e.g. glm::ortho, glm::perspective or glm::frustum) turns the right handed system to a left handed system and mirrors the Z-axis.

That means, if you use a (typical) projection matrix (and no other transformations), then the vertex coordinates are equal to the view space coordinates. The X-axis is to the right, the Y-axis is upwards and the Z-axis points out of the view.

In simplified words, in normalized device space the camera points in +Z. In view space (before the transformation by a typical projection matrix) the camera points in -Z.

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Note if you setup a Viewing frustum, then 0 < near and near < far. Both conditions have to be fulfilled. The geometry has to be between the near and the far plane, else it is clipped. In common a view matrix is used to look at the scene from a certain point of view. The near and far plane of the viewing frustum are chosen in that way, that the geometry is in between.
Since the depth is not linear (see How to render depth linearly in modern OpenGL with gl_FragCoord.z in fragment shader?), the near plane should be placed as close as possible to the geometry.