# Volume Transformation

**QUESTION:** I have two volume data sets and I am trying to rotate
and translate them to match each other. It would be very nice if I could use
something like "XVOLUME_ROTATE, /T3D" or "XVOLUME_ROTATE, MATRIX=!P.T." (Of course this isn't
possible). What is the best way to accomplish this in the absence of such
a program?

** ANSWER:** There have been numerous discussions on volume rotations and
transformations on the
IDL newsgroup over the years. Here is a volume
transformation program that has been recommended recently. It was written by Martin Downing in the
UK. The transformation can be performed in "chunks", which improves memory efficiency.

The short program is printed here (without it's accompaning documentation). You can download the entire program by clicking the link.

FUNCTION Transform_Volume, volume, Rotation=rotation, $ Scale=scale, Translate=translate, Centre_Rotation=centre_rotation, $ T3Dmat=t3dmat, Inplace=inplace, Buffer_Size=buffer_size ; Error handling. Catch, theError IF theError NE 0 THEN BEGIN Catch, /Cancel ok = Dialog_Message(!Error.State.Msg) IF N_Elements(volume) EQ 0 THEN volume = Temporary(vol_t) RETURN, -1 ENDIF ; Find the dimensions of the volume. s = Size(volume) sx=s[1] & sy=s[2] & sz=s[3] st = sx*sy*sz IF Keyword_Set(inplace) THEN vol_t = Temporary(volume) ELSE vol_t = volume ; Create a transform matrix, if one is not provided. IF N_Elements(t3dmat) EQ 0 THEN begin IF N_Elements(rotation) EQ 0 THEN rotation =[0,0,0] IF N_Elements(centre_rotation) EQ 0 THEN centre_rotation=[(sx-1)/2.0,(sy-1)/2.0,(sz-1)/2.0] IF N_Elements(translate) EQ 0 THEN translate =[0,0,0] IF N_Elements(scale) EQ 0 THEN scale =[1,1,1] T3D, /Reset, Translate = -centre_rotation T3D, Rotate=rotation T3D, Translate= centre_rotation + translate, Scale=scale t3dmat = !P.T ENDIF ; Check buffer size. The size 128 is optimim on my system, You may ; want to try other values. IF N_Elements(buffer_size) EQ 0 THEN buffer_size = 128 IF buffer_size LE 0 THEN buffer_size = st ; Perform the transformations. FOR j=0L,(st-1),buffer_size DO BEGIN ; Account for possible odd last chunk. bufsize = buffer_size < (st-j) ; Generate volume coordinates by interpolating temporary array of volume indices. i = j + Lindgen(bufsize) coords = [ [(i MOD sx)],[((i / sx) MOD (sy))], [(i / (sx * sy))], [Replicate(1b, bufsize)]] coords = Temporary(coords) # t3dmat vol_t[j:j+bufsize-1] = Interpolate(volume, coords[*,0], coords[*,1], coords[*,2], Missing=0) ENDFOR ; Return the transformed volume. RETURN, vol_t END

Copyright © 1997-2001 David W. Fanning

Last Updated 24 November 2001