> Message: 12 > Date: Sun, 15 Feb 2004 20:13:44 +0100 (CET) > Subject: Re: [Jmol-users] Re: RE: rasmol axes orientation > From: "Miguel Howard" <[EMAIL PROTECTED]> > To: <[EMAIL PROTECTED]> > Reply-To: [EMAIL PROTECTED] > > I am going to try to state the right-hand axes orientation ... as I > understand it. > > +X goes to the right > +Y goes up > +Z comes out of the screen and towards the user > > Using the thumb, index finger, mid finger method ... > palm is towards you > thumb points to the right -> +X > index finger points up -> +Y > middle finger points back at you -> +Z
This is correct. For what it's worth, it's also the axis definition used by OpenGL, which seems a good standard to follow, rather than (for example) defining +X to the right, +Y down and +Z into the screen, which would also be a right-handed system. The usual convention for rotations is that a positive rotation about any axis is counter-clockwise about that axis, viewed down the axis towards the origin. Or equivalently, clockwise looking out from the origin, or in the direction of rotation of a corkscrew advancing along the axis away from the origin. However, there is a catch: there are two ways of interpreting a rotation. One can regard the rotations as being applied to the _object_, in this case the molecule being viewed, with the axis system staying in the same place. This is the _active_ convention. Alternatively, one can regard the rotations as being applied to the _axes_, in which case the molecule stays in the same place and you have to imagine walking round the object, carrying the axes with you. This is the _passive_ convention. Both conventions have their uses. OpenGL distinguishes between 'modelling transformations', which rotate (or reflect or scale) the object, and are active transformations, and 'viewing transformations' which are changes to the position and orientation of the viewpoint, i.e. passive transformations. My own view is that for JMol the active convention would be easier to handle and understand, but others may disagree. In any case there can only be one viewing transformation, or possibly two for stereo pairs, while a complicated object may be constructed by assembling many components, each with its own 'modelling transformation'. -- Anthony Stone http://www-stone.ch.cam.ac.uk/ University Chemical Laboratory, Email: [EMAIL PROTECTED] Lensfield Road, Phone: +44 1223 336375 Cambridge CB2 1EW Fax: +44 1223 336362 ------------------------------------------------------- SF.Net is sponsored by: Speed Start Your Linux Apps Now. Build and deploy apps & Web services for Linux with a free DVD software kit from IBM. Click Now! http://ads.osdn.com/?ad_id=1356&alloc_id=3438&op=click _______________________________________________ Jmol-users mailing list [EMAIL PROTECTED] https://lists.sourceforge.net/lists/listinfo/jmol-users
