Sunday, August 30, 2009

Stitching Together Scanned Images

Recently, I was scanning a lot of legal sized documents, but my scanner could only scan A4 sized paper. So I had to scan each document in parts and then stitch them together afterwards.

There is lots of software available for stitching together photos into a panorama, but surprisingly, I had trouble finding software for stitching together flat images (or for creating a "linear panorama" as some people call it) even though this task would supposedly be a little bit easier than making panoramas. Of course, it's possible to manually load up the images into a paint program and manually align them and blend them, but I had to stitch together a lot of documents, so this was too time-consuming for me.

In the past, I've used software like Hugin, Panotools, or Autostitch for stitching together photos, but I could never quite get the various tutorials on how to handle linear panoramas to work quite right.

I own a copy of Photoshop Elements 6.0, which does contain algorithms for this sort of stitching (under File...New...Photomerge Panorama). Initially, I was quite pleased with the results, but after inspecting the final stitched images more carefully, I began to realize that the algorithm had lots of problems rotating images in order to get good alignment between them. If you only superficially inspect the final result, everything seems fine because Adobe seems to use some sort of fancy blending algorithm (something like Enblend) that hides a lot of these imperfections. Unfortunately, the blending algorithm can't hide the fact that because of poor alignment, lines that should be straight aren't. Even when you manually rotate the images to help the alignment algorithm, Photoshop Elements doesn't help you try to fine-tune the rotation or positioning afterwards, so it's hard to get things aligned quite right (I was hoping that if I could get things close to alignment, the algorithm would "snap" things to the best position). Adobe is always tweaking their algorithms, so it's possible that later versions of Photoshop Elements give better results though.

In the end, I actually found a research tool from Microsoft that provided excellent automated results. Even better, Microsoft's Image Composition Editor was very fast and very easy to use, so I highly recommend this program. Eventually, I hope to figure out what I was doing wrong with Hugin because it's always useful to have an alternative that supports a lot of manual tweaking, but so far the results of the Image Composition Editor have been so good that I've only needed to resort to alternate tools (i.e. Adobe Photoshop Elements) only once, which occurred when I was stitching four images together that had only a small amount of overlap.

Saturday, June 06, 2009

Vista Service Pack Installation Problems

I recently installed Windows Vista SP2, and like Vista SP1, I seemed to have problems getting it installed properly. It would spend a huge amount of time installing itself, reboot, and then fail during its third stage, and then spend a lot of time reverting the install. Windows Update would show Code 80004005 in the error details field for why the update failed.

Fortunately, after browsing the web, I found a forum post (I no longer remember where it is), where people mentioned that the problem comes up with dual-boot machines. My machine has both Linux and Windows installed, and Vista apparently fails its TPM security checks or something like that if the GRUB boot manager starts up in-between the computer starting and Vista starting. Fortunately, I installed GRUB on my Linux partition instead of my MBR, so I could simply change my active/boot partition to boot directly into Windows. Afterwards, the service pack could apply itself, and then I could boot on a Linux live CD to reset the active/boot partition back to the Linux partition.

Personally, I find this sort of annoying. It would be nice if Windows wouldn't die on dual-boot machines or give users a prompt to allow then to ignore TPM problems during the boot process or, at the very least, give more useful error messages.

Saturday, January 24, 2009

MTU Problems When Using Vista Internet Connection Sharing (ICS) with an XBox 360

So like other people, I don't really like the idea of paying $100 for an XBox 360 wifi adapter, so I'm using Internet Connection Sharing through my laptop to get my XBox connected to a wireless access point (yes, there are easier ways of doing this, but like usual, I'm doing something strange).

But when I did this, the XBox kept complaining that my MTU was set too low since it needed an MTU of 1364. This is a little silly since a proper IP stack should automatically fragment its packets to accomodate the MTU, but whatever.

I ran the command netsh interface ipv4 show interfaces to find out what the MTUs on my Vista laptop were, and they were all set to 1300. This seemed a little strange because 1300 is oddly low and also a nice round number that a human must have decided on. But I certainly hadn't set my MTU to 1300, so I'm not sure why it was set to that.

I spent a while browsing around, but finally I figured it out. It was that annoying Cisco VPN client on my laptop. I have no idea why they set the MTU so low (what sort of intermediate router would use 200 bytes of overhead per packet? It's silly to reserve so much), but the VPN Client comes with a Set MTU program that I was able to use to set everything back to normal, and then my XBox stopped complaining.

Friday, January 16, 2009

Running TPC-H Queries on MySQL

Getting the TPC-H queries to work on MySQL isn't too hard, but it isn't always clear what to do. I took the instructions from here and converted them to work with MySQL.

Once TPC-H is started, you can create a database and tpch account:

mysql -u root -p
mysql> CREATE USER 'tpch'@'%' IDENTIFIED BY 'password';
mysql> CREATE DATABASE tpch;
mysql> GRANT ALL ON tpch.* to 'tpch'@'%';
mysql> USE tpch;
mysql> \. tpch/gen/dss.ddl

Then, in the gen directories, you can modify the query generator to generate queries that are as close as possible to what you need:

cp makefile.suite makefile
#Modify makefile to use
#In tpcd.h, SQLSERVER section:
# change #define SET_DBASE "use %s;\n"
# change #define SET_ROWCOUNT "limit %d;\n\n"
# change #define START_TRAN "BEGIN WORK;"
# change #define END_TRAN "COMMIT WORK;"

Then you can start generating database data at the right scale factor

./dbgen -s 1

And also modify the constraints/indices file so that it's compatible with mysql:

# Modify dss.ri
# use a search and replace in order to remove "CONNECT TO TPCD", remove references to "TPCD." and remove the lines "COMMIT WORK;"

Then you can load all the data into the databases with

mysql -u tpch -p
mysql> use tpch;
# Same with orders, lineitem, nation, partsupp, part, region, supplier
mysql> \. dss.ri

You then need to change the case of the table names because the queries use lower-case table names I think whereas the dss.ddl uses upper-case names for the tables:

mysql> alter table NATION rename nation;
# Ditto for supplier, region, partsupp, part, orders, lineitem, customer

Finally, you can test some of the queries

cp dists.dss queries
cd queries
../qgen -c tpch -s 1 1

I think the queries still need to be modified a bit to be compatible. I think queries with a limit may need the semi-colon moved around, the precision indicator during date arithmetic in query 1 may need to be removed, and the method for naming columns in query 13 might need changing.