Retinex: Correct your low-light images today!

I was processing some images for someone recently, and I ended up encountering issues with colour balance. The images looked okay on my monitor, but as soon as I printed them out, they took on a slight red-orange tint. Very interesting. I suspect that the root cause lies in some complex colourspace or device colour profile issue (which will take me ages to debug and track down), but I stumbled upon a filter in GIMP called Retinex, which provided a very useful workaround.

According to the GIMP documentation, retinex is an algorithm that improves the appearance of images that were taken in sub-optimal lighting conditions. It's probably best illustrated with an example:

(Above: An example of the retinex filter in action. Image source: The official GIMP documentation.)

As you can see, the things on the desk are much easier to pick out in the right image as compared to the left one. Apparently, the algorithm was invented at NASA's Langley Research Centre in 2004 to automatically enhance astronomical photographs - and has a full name of Multi-Scale Retinex with Color Restoration (MSRCR) - which is a bit of mouthful!

During my own testing, I've found it be most effective on outdoor pictures, or pictures with poor lighting. I've also found it to be rather prone to introducing noise into the image - so if a simple automatic white balance correction will suffice, then that's probably a better filter to apply than this one.

It's one of those things that's really useful to know about - because it might just solve your problem one day! To that end, I wanted to blog about it so that I don't forget :P

Sources and Further Reading

• Official GIMP Documentation on thee Retinex Filter
• Retinex Image Processing at NASA Langley Research Center
• The research paper released on the algorithm.

I've got some business cards!

I've been to several events of various natures now (like the Hardware Meetup), and at each one I've found that a 'business card' or two would be really handy to give to people so that they can remember the address of this website.

After fiddling with the design over about a week I (and Mythdael!) came up with the design you see above. Personally, I'm really pleased with them, so I decided to post here to show them off :-)

I'm finding that it's a rather good idea to promote and build your brand at these kind of events by showing people the cool things that you've created and learnt, and business cards seem to be just the thing that helps you do it.

Learning Prolog: Lab Session #6 - More Looping

Before I get into today's post, I wanted to include a picture of some firework that I managed to photograph a few days ago on my phone. A heard them outside and when I took a look out of the window, this is what I saw!

Anyway, Today's Prolog lab session was about more loops. These loops are different to the failure driven loops that we did last time though in that they are recursive instead. Here's an example:

loopa :-
    write('Enter \'end.\' to exit.'), nl,
    read(Input), (Input = end ; loopa).

The above is a simple rule that keeps asking for user input until the user enters 'end'.

?- loopa.
Enter 'end.' to exit.
|: hello.
Enter 'end.' to exit.
|: cheese.
Enter 'end.' to exit.
|: end.
true.

This is done through the semi colon, which acts as an or operator. Although this works, it isn't very readable. Let's try something else.

% Stopping condition
loopb(end).
% Main loop
loopb(Input) :-
    write('Enter \'end.\' to exit.'),
    read(Input2),
    loopb(Input2).

% Starter rule
go :-
    loopb(something).

Much better. The above is divided up into the stopping condition (i.e. when the user types end), the main loop, and a rule that kicks the rest of the program off. When go is called, we call loopb with something as a parameter immediately. Since this doesn't match the fact loob(end), Prolog looks at the rule we defined, loopb(Input). Then it asks the user for input, and starts the process over again until the user inputs end.

Here's an example of the above in action:

?- go.
Enter 'end.' to exit.gdsfgdrt.
Enter 'end.' to exit.sdfgsdgf.
Enter 'end.' to exit.sdfgsdfgertr.
Enter 'end.' to exit.cheese.
Enter 'end.' to exit.end.
true.

Having learnt this, one of the challenges was to write a rule in Prolog. A factorial is where you multiply an integer by all the integers greater than 0 but less than the integer. For example, 4! (4 factorial) is 4x3x2x1, which is 24). Here's what I came up with:

factorial(1, 1).
factorial(N, Factorial) :-
    N2 is N - 1,
    factorial(N2, Ans),
    Factorial is N * Ans.

And here's an example of the above in action:

?- factorial(6, Answer).
Answer = 720 .

That concludes this post on the 6th Prolog lab session. If you don't understand something (or I've made a mistake!) please post a comment below and I'll do my best to help you. Also, if you have any, post your firework pictures below. I'd be interested to see them!