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this may not be what actually happens, but this is what I'd like:

x and z are floor.

y is up and down.

yr rotation 0 is facing east. because that's what rotation 0 is in cartesian coordinates.
yr is yaw
xr is pitch
zr is roll

the rotations need to be within the range: 0 <= r < 360 (so modulo 360 should work)

minimap: view from above
          +z?
           |
           |
           |
           |
-x --------+-------- +x
           |
           |
           |
           |
          -z?
the reason the minimap has positive x being down is so that camera view at rotation 0 is with
positive x to the right so 2D vector graphics don't need any dimensions flipped

camera view: facing rotation 0
          +y
           |
           |
           |
           |
-x --------+-------- +x
           |
           |
           |
           |
          -y

increasing yr causes rotation counter-clockwise on the minimap. (around y axis)


this is what minetest does. I guess I'll do this.

camera angle 0 is north.
north is +Z
south is -Z
west  is -X
east  is +X
up    is +Y
down  is -Y

there is no angle 0 actually. 0 < angle <= 360

I guess I should use yaw.