gistfile1.cpp

        Camera camera = m_Scene.m_Camera;
        Vector cameraPosition = camera.GetPosition();

        // Camera Coordinates directions
        Vector negZ = (camera.GetTarget() - cameraPosition).Normalize();
        Vector posY = camera.GetUp().Normalize();
        Vector posX = negZ.Cross(posY).Normalize();

        float FOV = camera.GetFOV();
        float nearZ = camera.GetNearClip();
        float farZ = camera.GetFarClip();

        float aspectRatio = ((float) WINDOW_WIDTH / (float) WINDOW_HEIGHT);
        float halfScreenHeight = nearZ * tan(FOV * PI / 180);
        float halfScreenWidth = aspectRatio * halfScreenHeight;

        // Position of pixel -- Screen pixels go from X = [-halfScreenWidth , halfScreenWidth], Y = [-halfScreenHeight, halfScreenHeight]
        Vector relativePosition = posX * (screenX - halfScreenWidth) + posY * (screenY - halfScreenHeight) + negZ * nearZ;
        Vector pixelPosition = cameraPosition + relativePosition;

        Vector rayDirection = (pixelPosition - cameraPosition).Normalize();