Machine Check Exception

Update: This bug has been fixed in GEANT 4.9.1, released on December 14, 2007. Go get it!

Apologies to people reading this (all 3 of you) who don’t care about the GEANT4 simulation library.

I’m pitching this out there in the hopes that Google will index it and help some confused physicist:

G4TessellatedSolid, and therefore the subclass G4ExtrudedSolid, are broken in GEANT 4.9, and most likely earlier releases as well. Geometry optimization (which is enabled in G4LogicalVolume by default) requires the voxelizer to divide the detector geometry into rectangular regions in order to speed up tracking. The voxel generator needs to be able to determine which volumes in your detector overlap with each voxel region. To do this, it calls the G4Tessellated::CalculateExtent() (or G4ExtrudedSolid::CalculateExtent()) passing in an affine transformation describing the solid’s current orientation in the global coordinate system.

The implementation of CalculateExtent() first checks for the easy case, which is a transformation that is pure translation, and behaves correctly in that case. However, if the current transformation has any rotation component at all, then the if statement takes the other branch which is empty! The method obviously does not behave correctly in that case, and the voxelizer will give you nutty results, including mysteriously appearing, disappearing, or clipped volumes.

This problem is now being tracked as bug #983 in the GEANT4 bug database. The workaround is to either disable optimization in the parent G4LogicalVolume of your tessellated or extruded solids, or to patch the G4TessellatedSolid::CalculateExtent() method as described in the bug report.

Always remember to visualize your GEANT4 detector volumes with the raytracer, or you might not catch problems like this!pornstar karina
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After spending 4 days with the new 10.5 release of Mac OS X, I’ve been pretty impressed. Visually, things have improved, except for the obvious problems with the dock and the menu bar. I had the same initial negative reaction to the translucent 3D dock that most other people did, but it has grown on me slowly. The translucent menu bar, however, is simply atrocious if you do not have a very color-neutral (black, white or grey) background. The most common way to correct this is to add a white stripe to the top of your background image. OpaqueMenuBar does this for you automatically whenever your background changes.

Looks aside, I think Leopard’s biggest advance is the amount of attention Apple has shown to developers. (Clearly, OS X is stealing the love from the iPhone…)

X11.app 2.0 is generally a huge improvement. Now based on the X.org 7.2 code base, it draws on the XDarwin code base for X/OS X integration, rather than the source of the old Panther/Tiger X11.app, which was based on XFree86. As a result, there have been a number of regressions, but it sounds like the future of X11.app will be much better. In particular, the source is now being hosted in the X.org git repository, and the main developer is committed to engaging with the user community.

There are a number of glitches, though. Using a fullscreen X desktop (which I suspect is not terribly common) is broken, as is dragging an X window to another display if you have two monitors on your computer. Most annoying, the patch to fix the yellow cursor bug was dropped on the floor, and didn’t make it into X11.app 2.0. The author has since fixed this in an alpha release on the XDarwin wiki page. The Xquartz binary he posts there works great for me, so I’m happy for now.

The launchd program, which is like init/rc/cron/at/inetd all rolled together, is used to pull off two neat tricks in Leopard. First, the $DISPLAY variable is set to a socket that launchd monitors, so the X server now starts automatically on demand. This means you can start up Terminal, do some work, and as soon as you start an X application, you’ll see X11.app appear. The second trick is now an ssh-agent is started on demand when you use SSH. Apple’s ssh-agent can fetch passphrases for your keys from the OS X Keychain as well. You don’t need to use SSHKeychain any more (which is good since it had a major memory leak on my system). The only downside to the ssh-agent Keychain support is there is no obvious way to expire the ssh keys in the agent when the Keychain locks. Once those keys are decrypted into the ssh-agent memory, they stay valid even after you lock your Keychain.

Python has been updated to 2.5.1, which is great because it solves a linker problem I had with the Python bindings for ROOT. The Leopard install of Python includes easy_install, numpy, twisted, and some other handy stuff. In addition, there are new Objective-C/Cocoa bindings, and it comes along with py2app, for generating proper-looking Mac applications entirely written in Python!

The Cocoa programmers are probably excited about Objective-C 2.0, which adds garbage collection and some other improvements, like a compact syntax for looping over an iterator. I’ve been reading up on Objective-C, and the message passing style of object-orientation reminds me greatly of Python’s duck-typing. I find the syntax unspeakably ugly looking, but that’s really just a matter of taste. You can get used to anything, really.