OpenGL depth buffer inverted/flipper over y-axis - c++

Question

I'm trying to get the depthmap of a shape using the depth buffer (and ortographic projection), but somehow the y-axis is flipped.
I use an OpenCV Mat to display the depth map.
Here is my code:
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
int main(void)
{
GLFWwindow *window;
int screenWidth = 450, screenHeight = 350;
GLfloat color[] =
{
255, 0, 0,
0, 255, 0,
0, 0, 255
};
GLfloat vertices[] =
{
-200.0f, -50.0f, -100.0f,
250.0f, 150.0f, 0.0f,
100.0f, 300.0f, 0.0f
};
if (!glfwInit())
{
return -1;
}
// Create windows
window = glfwCreateWindow(screenWidth, screenHeight, "Test", NULL, NULL);
cv::namedWindow("Depthmap", cv::WindowFlags::WINDOW_AUTOSIZE);
if (!window)
{
glfwTerminate();
return -1;
}
glfwMakeContextCurrent(window);
glEnable(GL_DEPTH_TEST);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-200.0f, 250.0f, -50.0f, 300.0f, 0, 100.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
while (true)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glPushMatrix();
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, vertices);
glColorPointer(3, GL_FLOAT, 0, color);
glDrawArrays(GL_TRIANGLES, 0, 3);
glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);
glPopMatrix();
//To opencv mat
cv::Mat depth = cv::Mat_<float>(screenHeight, screenWidth);
glReadPixels(0, 0, depth.cols, depth.rows, GL_DEPTH_COMPONENT, GL_FLOAT, depth.data);
cv::imshow("Depthmap", depth);
glfwSwapBuffers(window);
glfwPollEvents();
}
return 0;
}
And here is the result:
The actual depth seems to be correct (the red point is 1.0 and the green and blue points are 0.0) but the y-axis is inverted. Why is this and how can I fix it properly?