| Re: texture mapping: arbitrary orientation impossible? [message #30535] |
Fri, 03 May 2002 16:16 |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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"Harald von der Osten-Woldenburg" <hvdosten@lb.netic.de> wrote
> it seems that it is only possible to combine pictures with 3-D-surfaces
> if they are orientated parallel to the coordinate axes. Is this correct?
> Or does perphaps anyone know a tricky method combining pictures of
> arbitrary orientation with surfaces? For example a picture rotated by 30
> degrees...?
You can map a texture onto a surface pretty much any way you want. The key
is getting the correct texture coordinates (and understanding what OpenGL
will do with them.) The texcoords map a normalized texel value to a vertex
on your polygon (in your case the 3d surface). If you want to rotate the
texture on the surface you change the texture coords accordingly.
I would personally work with the image data and rotate that. But if you
need to dynamically rotate your texture then you could transform the texture
coords. I haven't played with this a lot but I did take David Fanning's
Texture Surface example (www.dfanning.com) and arbitrarily rotated the
texture around. Assume you have a 2xN array of texture coordinates that map
your image onto your surface. Call the array "texcoords".
;first we need to translate our texcoords so they
;are centered about our local origin. This ensures
;the texture is rotated about the center.
texcoords = texcoords - 0.5
;get the transform matrix - use IDLgrModel because
;it does the math for us. You could easily build this
;by hand. We'll rotate the model 30 degs about the z
;axis.
model = obj_new('idlgrmodel')
model -> rotate, [0,0,1], 30.
ctm = model -> getctm()
;since we're dealing with a 2d transform, get rid of
;the extra dimensions on our 4x4 transform matrix
ctm = ctm[0:1,0:1]
;transform the texcoords so they are rotated 30 degs
;about 0,0
for n=0, (size(texcoords, /dimensions))[1] do $
texcoords[*,n] = texcoords[*,n] # ctm
;now we translate our texcoords back
texcoords = texcoords + 0.5
Now texcoords will map your image onto your surface rotated 30 degs.
Good Luck!
-Rick Towler
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