GSoC 2013: image processing of ND images (update #1)

My project proposal for image processing of ND images in GNU Octave was accepted for Google Summer of Code 2013. If all goes as planned, not only will it be a very nice improvement to the image package but also a chapter on my PhD thesis.

I decided to warm up by implementing montage() which basically displays all pages (or frames) of an ND image as a single 2D image. It is not part of the proposal but the coding period hasn't started anyway, and it will be really useful for the rest of the project. So useful indeed, I had wrote my own version of it last year, completely unaware of Matlab's implementation and of course, completely incompatible. So I had to do it all over again.

Following the trend of changing 20000 other small things first, my first contribution ended up being a fix for `ind2rgb() causing a complaint in just two hours. Never had I made a commit with such immediate repercussions. Still, I'm taking it positively because:

1. means I'm changing code that people use and care about;
2. my fix was actually correct :)

I guess the main problem users will have is understanding ND images. While images can have many dimensions, the standard is to have them as a 4D matrix, the multiple images concatenated into the 4th dimension, and the 3rd being reserved for the colorspace. Expect a new section for the Octave manual.

To implement montage(), the most difficult part was dealing with a cell array of filepaths for images. Handling all different possibilities wasn't trivial, trying to think of all possible combinations to create a correct image with all of them. In special, multipage indexed images are troublesome because I have never seen one. And because imread() seems to always return a single colormap, a possible Octave bug may exist when reading multipage indexed image where each page has different colormaps. Reading the TIFF6 file specifications such file could in theory exist. However, I don't think they actually do.

In the end, montage() is finished and added to the image package (csets 98903291ef63 and 307eee5730bf. In addition to Matlab's API, I included options to add margins between panels, and configure the colour for both margins and panel background. It's far from being the smartest piece of code I ever wrote, but smart code would use a lot of permute(), reshape(), and indexing trickery, turning it into a maintenance problem.

It took me 4 times longer than originally planned. I'm blaming the Irish weather.

Ireland being famous for its non stop rain, has had amazingly sunny weather for 2 weeks now. Galway has been the world procrastination central as everyone's out getting as much sun as possible.