glm::lookat, perspective clarification in OpenGL

Question

All yalls,

I set up my camera eye on the positive z axis (0, 0, 10), up pointing towards positive y (0, 1, 0), and center towards positive x (2, 0, 0). If, y is up, and the camera is staring down the negative z axis, then x points left in screen coordinates, in right-handed OpenGL world coordinates.

I also have an object centered at the world origin. As the camera looks more to the left (positive x direction), I would expect my origin-centered object to move right in the resulting screen projection. But I see the opposite is the case.

Am I lacking a fundamental understanding? If so, what? If not, can anyone explain how to properly use glm to generate view and projection matrices, in the default OpenGL right-handed world model, which are sent to shaders?

glm::vec3 _eye(0, 0, 10), _center(2, 0, 0), _up(0, 1, 0);
viewMatrix = glm::lookAt(_eye, _center, _up);
projectionMatrix = glm::perspective(glm::radians(45), 6./8., 0.1, 200.);

Another thing I find interesting is the red line in the image points in the positive x-direction. It literally is the [eye -> (forward + eye)] vector of another camera in the scene, which I extract from the inverse of the viewMatrix. What melts my brain about this is, when I use that camera's VP matrices, it points in the direction opposite to the same forward direction that was extracted from the inverse of the viewMatrix. I'd really appreciate any insight into this discrepancy as well.

Also worth noting: I built glm 0.9.9 via cmake. And I verified it uses the right-hand, [-1, 1] variants of lookat and perspective.

resulting image:

Rabbid76 Rabbid76 · Accepted Answer · 2018-07-12T05:10:14

I would expect my origin-centered object to move right in the resulting screen projection. But I see the opposite is the case.

glm::lookAt defines a view matrix. The parameters of glm::lookAt are in world space and the center parameter of glm::lookAt defines the position where you look at.

The view space is the local system which is defined by the point of view onto the scene. The position of the view, the line of sight and the upwards direction of the view, define a coordinate system relative to the world coordinate system. The view matrix transforms from the wolrd space to the view (eye) space.

If the coordiante system of the view space is a Right-handed system, then the X-axis points to the left, the Y-axis up and the Z-axis out of the view (Note in a right hand system the Z-Axis is the cross product of the X-Axis and the Y-Axis).

The line of sight is the vector form the eye position to the center positon:

eye    = (0, 0, 10)
center = (2, 0, 0)
up     = (0, 1, 0)

los = center - eye = (2, 0, -10)

In this case, if the center of the object is at (0, 0, 0) and you look at (0, 0, 2), the you look at a position at the right of the object, this means that the object is shifted to the left.
This will change, if you change the point of view e.g. (0, 0, -10) or flip the up vector e.g. (0, -1, 0).

glm::lookat, perspective clarification in OpenGL

1 Answers