CODEWITHSUNDEEP
c++openglglm-math
Gary Allen
Gary Allen

Reputation: 1370

Why values [2][0] and [2][1] of glm::frustumLH_ZO are negated?

Let's say I am coming from a left-handed world space. My positive Z is into the screen, and my Y is up, the same as OpenGL's default in clip space.

It is my understanding that, when coming from a right-handed world space (which is somewhat required when using older functions such as gluLookAt which maps the forward vector to the -Z axis) the Z co-ordinated is flipped before computing the projection matrix (orthographic/perspective). This is due to the fact that a right-handed system would have negative Z into the screen, which needs to be flipped before being passed to OpenGL.

I have confirmed that, for a function such as orthoLH_ZO, no flipping takes place. In fact, the matrix is a "standard" orthographic projection that also works in e.g. Vulkan i.e. it keeps the handedness (except for the fact that Y would be down in Vulkan so bottom and top would switch).

I would therefore expect glm::frustumLH_ZO to also be such a "standard" matrix since no flip needs to take place. However, when looking at its source code, elements [2][0] and [2][1] are negated compared to e.g. Vulkan's frustum perspective projection matrix (discussed here at 11:49).

These elements are:

Result[2][0] = (right + left) / (right - left);
Result[2][1] = (top + bottom) / (top - bottom);

instead of my expected:

Result[2][0] = -(right + left) / (right - left);
Result[2][1] = -(top + bottom) / (top - bottom);

I don't believe this is due to a Z-flip, as that should also cause e.g. element [2][3] to be -1 and not +1.

Why are these entries in the matrix flipped?

Upvotes: 2

Views: 330

Answers (1)

derhass
derhass

Reputation: 45322

Why are these entries in the matrix flipped?

You are right, they shouldn't be, but they are in the most recent glm code base to date. Looks like you found a bug in glm. It did probably go unnoticed so long because a) most people use glm with default GL conventions, and b) these two elements are typically 0 in your standard symmetrical frustum (left = -right and top = -bottom).

Upvotes: 2

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