I'm trying to understand projection matrices and I'm messing around with a ortho matrix in OpenGL and it seems like the Y Axis is inverted.

I have a simple square on the screen made using this vertex data:

float size = 32.0f;
float zIndex = -1.0f;
vertices[0] = 0.0f;
vertices[1] = 0.0f;
vertices[2] = zIndex;

vertices[3] = size;
vertices[4] = 0.0f;
vertices[5] = zIndex;

vertices[6] = 0.0f;
vertices[7] = size;
vertices[8] = zIndex;

vertices[9] = size;
vertices[10] = size;
vertices[11] = zIndex;

indices[0] = 0;
indices[1] = 1;
indices[2] = 2;

indices[3] = 1;
indices[4] = 3;
indices[5] = 2;

I'm using a Right Hand Coord system. X+ goes to the right, Y+ goes up, and Z+ goes towards myself (the direction out of the screen). So I would expect the square to be drawn like this

But when I run and render everything it looks like:

Even when using GLM's ortho matrix it is rendered like this

//GLM ortho setup for 0, 0 center 
glm::mat4 projection = glm::orthoRH(-(800.0f / 2.0f), 800.0f / 2.0f, 600.0f / 2.0f, -(600.0f 
glm::mat4 mvp = projection;
glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &mvp[0][0]);

Also if I use a model matrix and translate it using a positive Y value the red square moves down when I would expect it to move up. I know this is related to the above issue in some way

static float y = 0.0f;
y += 0.1;
	
glm::mat4 model(1.0f);
model = glm::translate(model, glm::vec3(0.0f, y, 0.0f));

glm::mat4 view(1.0f);

glm::mat4 projection = glm::orthoRH(-(800.0f / 2.0f), 800.0f / 2.0f, 600.0f / 2.0f, -(600.0f / 2.0f), 1.0f, 100.0f);

glm::mat4 mvp = projection * view * model;
glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &mvp[0][0]);

Is the Y axis being inverted normal? I must not be understanding something right?

This might come from the framework you use for displaying your rendered image.

When working in screen coordinated, openGL put the origin to the bottom left corner whereas image files put the origin in the top left.

You could try scale your projection matrix to do the flipping for you.