Yenya's World

Wed, 24 Feb 2010

Image Scaling

It is not very often when there is a software bug which is present in nearly all different implementations which do not even have common ancestor in terms of source code.

The image scaling bug is one of these exceptions. I wonder how many programs simply assume that the luminosity of the pixel created as a combination of the two pixels with luminosities of 0 and 255 (e.g. by downscaling the image) is somewhere around 128.

There are definitely several programs written by yours truly, which are built around this assumption. Altough I remember reading the NetPBM source code and seeing those odd calculations using a lookup table and wondering why they did not simply use the arithmetic mean.

I even think (but my memory is fading, so no strong statement here) that we used the arithmetic mean even in the computer graphics course during my studies.

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Thu, 11 Feb 2010

Web Comics Dying?

Is it just an unfortunate coincidence, or are several web comics sites slowly dying?

For example, Order of the Stick is updated only every two weeks or so, User Friendly runs repeated strips for about a year now, and even BOFH has not been updated since last December, and had only 17 stories last year.

Did the authors manage to get a real life finally? Or have they been swallowed by the Internet black hole which is Facebook?

Section: /world (RSS feed) | Permanent link | 2 writebacks

Wed, 10 Feb 2010

Playing with 6to4

We have finally got some time to work on native IPv6 inside a faculty network (which includes rewriting the iptables configuration to be protocol-neutral). In order to test it, I have enabled 6to4 at home.

So now I have a native IPv6 in my home network, and I can even directly SSH to devices in my home network from the university network, even though the home network is hidden behind a single IPv4 address. Apparently my traffic is routed symmetrically, as both directions use the same 6to4 relay in in Nuernberg.

As for the network parameters, I have a direct ping 13.2 ms, while the ping6 is 27.1ms. The transfer rate, on the other hand, is purely limited by my ISP (measured by SCPing a large file), and it is the same for both protocols - slightly above 500 KB/s. Now if only I had a nearer 6to4 relay (maybe in NIX.CZ?).

The setup in Fedora is relatively straightforward, except when the outgoing interface has an IPv4 address assigned from DHCP. So I had to add the IPv6 configuration manually, and will have to change it whenever I get a new IPv4 address (which is usually once per year or two).

Update - Wed, 10 Feb 2010: Fedora problems fixed

My previous statement about problems in Fedora was not true. I must have made a mistake somewhere, but after recheking my setup and restarting the network the 6to4 tunnel works as expected.

Section: /computers (RSS feed) | Permanent link | 4 writebacks

Sat, 06 Feb 2010


Tell me again why should anybody bother to buy iPhone, when Apple is actively hostile to the application developers, and the device cannot do Bluetooth, does not have audio tracks in a portable format, accessible as a mass storage, and the manufacturer does not want you to customize the device?

Section: /computers (RSS feed) | Permanent link | 4 writebacks

Fri, 05 Feb 2010

DHCP Relay

With our new WiFi access points, we run multiple SSIDs, each on its own VLAN. Unfortunately, we are observing an interesting problem: even though the DHCP server can see the WiFi client requesting the address and replies back, the reply sometimes does not reach the client. It vaguely depends on the number of clients on the particular WiFi AP. Today I think I have found what causes this problem.

In an unrelated event, our new 10GbE switches have arrived, and I have been configuring them. When browsing the manuals, I have noticed they have a feature called "DHCP relay", which causes DHCP requests to be magically switched to the primary VLAN of the switch, even when they are obtained from another VLAN. Well, one might think that almost nobody needs this obscure and unintuitive feature, so it is expected to be switched off by default.

Apparently the HP engineers do not share this point of view. Not only the DHCP relay is on by default on all newer ProCurve switches, but as a default setting, it is not mentioned in the "show config" command output, except when explicitly disabled. So nobody knows a new "feature" has been added, except when re-reading the manuals for every new ProCurve model.

I have found the DHCP relay feature being enabled even on other recently purchased switches. I have promptly disabled it, and we will see what happens with the above problem of missing DHCP replies.

Section: /computers (RSS feed) | Permanent link | 0 writebacks


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