PositionEntity and the object's matrix

BlitzMax Forums/MiniB3D Module/PositionEntity and the object's matrix

LT	(Posted 2007) [#1]

I've noticed that if I use PositionEntity on a TMesh and then check it's matrix translation values, the z coordinate(grid[3,2]) has the opposite sign. Is that a bug or is there a good reason for it?

marksibly

(Posted 2007) [#2]

Hi,

Probably due to the fact that GL uses a right handed coord system while b3d uses a left handed one (or vice versa...).

In other words, in GL z's <0 are in front of eye and >0 are behind - the opposite of b3d.

Looks like Si has dealt with this by negating z somewhere along the way, but another approach is to use a D3D-like projection matrix instead of the standard glFrustum/gluPerspective ones. Here's what I use:

Type TMat4

	Field ix#,iy#,iz#,iw#
	Field jx#,jy#,jz#,jw#
	Field kx#,ky#,kz#,kw#
	Field tx#,ty#,tz#,tw#

	Method glLoadMatrix()
		glLoadMatrixf Varptr ix
	End Method
	
	Function FromFrustum:TMat4( near_left#,near_right#,near_bottom#,near_top#,near#,far# )
		Local t:TMat4=New TMat4
		Local near2#=near*2
		Local w#=near_right-near_left
		Local h#=near_top-near_bottom
		Local d#=far-near
		t.ix=near2/w
		t.jy=near2/h
		t.kx=(near_right+near_left)/w
		t.ky=(near_top+near_bottom)/h
		t.kz=(far+near)/d
		t.kw=1
		t.tz=-(far*near2)/d
		Return t
	End Function
End Type

This allows you to use b3d style 'z's everywhere without having to negate anything. Only slightly tricky bit is glFrontFace CW/CCW test flags get reversed.

The above is actually the same as the standard glFrustum matrix except for a '1' instead of a '-1'.

LT	(Posted 2007) [#3]

That makes sense, but I guess I just expected that functions like PositionEntity would be adapted to whatever coordinate system was being used for matrices.

Anyway, I have managed to adapt by negating the z at appropriate times. Exporting animations from 3dsMax has proven tricky, but doable. I'm thrilled to have BMax and MiniB3D and Klepto's revisions available.

Oh, and I appreciate you taking the time to post. Thanks! :)