Switch axis Y and Z in PyOpenGL

Question

I would like to switch the Y and Z axis orientation in PyOpenGL. I have tried using matrix transformations but I have not been able to do it.

Code:

glMatrixMode(GL_PROJECTION)
glLoadIdentity()
glOrtho(self.zoom, -self.zoom, -self.zoom, self.zoom, -5000, 5000)
glMatrixMode(GL_MODELVIEW)
glClearColor(1, 1, 1, 0)
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
glLoadMatrixf(self.m)

Where:

self.zoom = 150
self.m = [[1, 0, 0, 0],
          [0, 0, 1, 0],
          [0, 1, 0, 0],
          [0, 0, 0, 1]]

Wrong result:

Using identity matrix:

Expected:

Edit: This works:

        glMatrixMode(GL_PROJECTION)
        glLoadIdentity()
        glOrtho(self.zoom, -self.zoom, -self.zoom, self.zoom, -5000, 5000)
        up = 1
        if self.theta == 360:
            up = -1
        gluLookAt(self.x, self.y, self.z, 0, 0, 0, 0, up, 0)
        glMatrixMode(GL_MODELVIEW)
        glClearColor(1, 1, 1, 0)
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
        glLoadMatrixf(self.m)

*"[...] but I have not been able to do it."* - You've to be more specific. What exactly is the issue? The operations seems to be syntactically and semantically correct. — Rabbid76, Dec 16 '19 at 10:36
@Rabbid76 I have added pictures of the result I get. The third one is edited using GIMP, it is not a real pic, it is what I would like to achieve — Jaime02, Dec 16 '19 at 13:00
What you mean is that the call to `glLoadMatrixf(self.m)` works, but it doesn't do what you expect it to do? — Rabbid76, Dec 16 '19 at 13:04
@Rabbid76 Yes, the elements in the scene are not rendered properly as you can see in the first pic — Jaime02, Dec 16 '19 at 13:05

Rabbid76 · Accepted Answer · 2019-12-17T10:44:48.687

1

A 2 dimensional vector can be rotated by 90°, by swapping the components and inverting one of the components:

Rotate (x, y) left is (-y, x)
Rotate (x, y) right is (y, -x)

What you actually do is to turn the right handed matrix to a left handed matrix. It is a concatenation of a rotation by 90° and mirroring.

Change the matrix:

Either

self.m = [[1, 0,  0, 0],
          [0, 0, -1, 0],
          [0, 1,  0, 0],
          [0, 0,  0, 1]]

or

self.m = [[1,  0, 0, 0],
          [0,  0, 1, 0],
          [0, -1, 0, 0],
          [0,  0, 0, 1]]

Note, the same can be achieved by a rotation around the x-axis. e.g:

glLoadIdentity()
glRotatef(90, 1, 0, 0)

If you've a view matrix and a model matrix, then you've to multiply the modle matrix to the view matrix by glMultMatrix:

glMatrixMode(GL_MODELVIEW)
glLoadIdentity()
gluLookAt(self.x, self.y, self.z, 0, 0, 0, 0, up, 0)
glMultMatrixf(self.m)

edited Dec 17 '19 at 10:44

answered Dec 16 '19 at 13:11

Rabbid76

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Using the glRotate option I get the same result as picture 1, the perspective is wrong – Jaime02 Dec 16 '19 at 14:17
I mean, Why do i get the 1st picture which is wrong instead of the thrid one? – Jaime02 Dec 16 '19 at 14:24
No it is not. The perspective should remain unchanged – Jaime02 Dec 16 '19 at 15:13
Why the glRotate(90, 1, 0, 0) is not equal to an axis rotation of 90 degrees around X axis? – Jaime02 Dec 16 '19 at 15:22
the gluLookAt function was at the modelview matrix, it should be in the projection matrix, right? – Jaime02 Dec 17 '19 at 10:08
@PepeElMago33 No, never! "look at" sets the view matrix!. So it is the 1st component of `GL_MODELVIEW`. But all the model transformation have to be done after "look at" – Rabbid76 Dec 17 '19 at 10:10
check edit, that works – Jaime02 Dec 17 '19 at 10:15
Take into account that that lines get executed each frame, the matrix multiplication causes a weird rotational effect – Jaime02 Dec 17 '19 at 20:02
Just found this: https://www.opengl.org/archives/resources/faq/technical/viewing.htm – Jaime02 Dec 17 '19 at 20:08
No it does not "cause a weird rotation effect" because of `glLoadIdentity()` – Rabbid76 Dec 18 '19 at 10:28

Switch axis Y and Z in PyOpenGL

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