Saturday, December 13, 2008

Perigee: The Big Full Moon

I took these two pictures outside the Googleplex this evening, thanks to the hint given by a colleague. As NASA explains, the full Moon of Dec 12 was the biggest and brightest because it's actually closest to Earth compared to all the other Moons we've seen and will see in 2008.

I like the Moon, especially when it is this shiny. However, Earth is my favorite planet because it's bigger and has a more interesting surface, and it's still superior at supporting life. Nevertheless, traveling to the moon might be entertaining: In addition to playing around in its low gravity I'd like to point my camera back at Earth to take a shot like this, but with a lot more blue and white.

Wednesday, December 10, 2008

Fun with ZDDs

I had a chance to attend Don Knuth's 14th annual Christmas Tree Lecture at nearby Stanford.

Great talk. Like most computer scientists, I was familiar with BDDs before, but I had never even heard of ZDDs - and yes, I didn't even know what it was prior to attending the talk. It turns out that ZDDs are a compact way to represent families of sets - for some problems more compact than any other technique, and there is a nice algebra with operations over them that can be implemented efficiently. The talk was easy to follow, full of examples, explanations, and even some good humor - but the best part is just how much Knuth himself is fascinated by the subject, which might very well be contagious in a good sense: he's made me a fan of ZDDs. When it becomes available online, I recommend watching this, and I'm planning on attending the lecture next year (and the following years). For now, most of the material covered in the talk is available as a preprint in Fascicle 1b of the Art of Computer Programming: Binary Decision Diagrams under the section "Zero-suppressed BDDs: A combinatorial alternative". Knuth said he will update this section soon and it will also appear in print.

Monday, December 8, 2008

Mammoth Mountain and Emerald Bay

On Thanksgiving weekend Stefan and I stayed in South Lake Tahoe for the first night, and then went on to Mammoth early in the morning. It was a lengthy drive, but the snow in Mammoth was worth it. Coming down the Kingsbury Grade into Carson Valley was especially scenic this time - houses were hiding under a blanket of fog.

Mammoth Mountain is a big resort, and it has interesting terrain. The snow earlier in the week mostly skipped Tahoe and went here, so they had a variety of terrain open, including a few double diamonds. This picture however is from a trail that wasn't open. There wasn't a "closed" sign either (I swear), but the lift (lift 14) sure was not running. Now that I had sailed into this, I did my best to enjoy it; but which path to take - left or right? Thinking that the only visible track in the snow must have been a local who knew what he/she was doing I went right. The first few turns were great (fresh snow!!!), but then I had to unstrap: not enough coverage - oops! - my run ended in a field of boulders...

The wind blew hard at the top of Mammoth Mountain, but the views made up for this. The picture below shows the sunny backside - which is not for skiing but just for looking. The upper, steeper section of the mountain is on the opposite side, mostly in shade, and as we dropped over the edge we were greeted by a lengthy sheet of ice. After this it was decent snow though, and the "Face Lift" which doesn't go all the way to the top made enough of this terrain accessible without the inconvenience of wind and ice.

The next day we went to Emerald Bay at Lake Tahoe, so I had to take a shot of the island.

We hiked up the Eagle Lake trail and returned via the Bayview Trail. We did not find much snow on the trail - it was certainly easy to hike in regular hiking boots.

The sun set and we came by Granite Lake, which I really liked.

A bit later, Emerald Bay put on its post-sunset glow as well.