CODEWITHSUNDEEP
openglglm-math
Maxim Blinov
Maxim Blinov

Reputation: 946

OpenGL: Must any vertex Z coordinate be negative or not?

I understand that the camera in OpenGL is defined to be looking in the negative Z direction. So in a simple example, I imagine that for my vertices to be rendered, they must be defined similar to the following:

rawverts = {
    0.0f, 0.0f, -1.0f,
    0.0f, 0.5f, -1.0f,
    0.5f, 0.0f, -1.0f,
};

However, absolutely no guide will tell me the answer. Everywhere I look, the "Hello triangle" example is made with the z coordinate left at 0, and whenever a more complex mesh is defined the coordinates are not even shown. I still have no idea regarding the possible values of the coordinates for them to be drawn onto the screen. Take for example, glm::perspective:

glm::mat4 projectionMatrix = glm::perspective(
    FoV,         // The horizontal Field of View, in degrees : the amount of "zoom". Think "camera lens". Usually between 90° (extra wide) and 30° (quite zoomed in)
    4.0f / 3.0f, // Aspect Ratio. Depends on the size of your window. Notice that 4/3 == 800/600 == 1280/960, sounds familiar ?
    0.1f,        // Near clipping plane. Keep as big as possible, or you'll get precision issues.
    100.0f       // Far clipping plane. Keep as little as possible.
);

But how can the clipping planes be defined with any positive values? The camera faces the -Z direction! Furthermore, if I create near/far clipping planes at, say, -1 and -4, does this now invalidate any Z coordinate that is more than -1 or less than -4? Or are the raw z coordinates only ever valid between 0 and -1 (again, surely z coordinates categorically cannot be positive?)..?

But let's assume that what actually happens, is that OpenGL (or glm) takes the clipping plane values and secretly negates them. So, my -1 to -4 becomes 1 to 4. Does this now invalidate any Z coordinate that is less than 1 and more than 4, being the reason why 0.0f, 0.0f, -1.0f wont be drawn on the screen?

At this stage, I would treat treat an answer as simply a pointer to a book or some material that has information on this matter.

Upvotes: 4

Views: 1573

Answers (2)

Andreas
Andreas

Reputation: 5301

Your problem is that you don't understand the coordinate systems and transformations.

First of there is the window coordinates. It is the pixel grid in your window, pure an simple. There is no z-axis.

Next is NDC. Google it. It is a cube from -1 to 1 in xyz axes. If you load both modelview and projection matrices with identity this is the space you render in. By specifying viewport you transform from NDC to window coordinates. Pixels from vertices outside the cube is clipped.

What you do with projection and modelview matrix is that you create a transformation on the NDC cube, making it cover you objects. When moving the camera, you alter the transform. The transform can translate a vertex from any location to the NDC cube, including negative z-coords.

That is the short version of how things work. The long version is too long to enter here. For more information please ask specific questions or better yet read some litterature on the subject.

Upvotes: 0

54 69 6D
54 69 6D

Reputation: 694

No, points/vertices can have a positive z coordinate, but you won't see them unless the camera is moved back.

This article talks about that about a third of the way through.

Upvotes: 4

Related Questions