Shallow Thoughts : : Jan

Snow day!

We're having a series of snow days here. On Friday, they closed the lab and all the schools; the ski hill people are rejoicing at getting some real snow at last.

It's so beautiful out there. Dave and I had been worried about this business of living in snow, being wimpy Californians. But how cool (literally!) is it to wake up, look out your window and see a wintry landscape with snow-fog curling up from the Rio Grande in White Rock Canyon?

The first time we saw it, we wondered how fog can exist when the temperature is below freezing. (Though just barely below -- as I write this the nearest LANL weather station is reporting 30.9°F. But we've seen this in temperatures as low as 12°F.) I tweeted the question, and Mike Alexander found a reference that explains that freezing fog consists of supercooled droplets -- they haven't encountered a surface to freeze upon yet. Another phenomenon, ice fog, consists of floating ice crystals and only occurs below 14°F.

It's also fun to watch the snow off the roof.

It doesn't just sit there until it gets warm enough to melt and run off as water. Instead, the whole mass of snow moves together, gradually, down the metal roof, like a glacier.

When it gets to the edge, it still doesn't fall; it somehow stays intact, curling over and inward, until the mass is too great and it loses cohesion and a clump falls with a Clunk!

When we do go outside, the snow has wonderful collections of tracks to try to identify. This might be a coyote who trotted past our house on the way over to the neighbors.

We see lots of rabbit tracks and a fair amount of raccoon, coyote and deer, but some are hard to identify: a tiny carnivore-type pad that might be a weasel; some straight lines that might be some kind of bird; a tail-dragging swish that could be anything. It's all new to us, and it'll be great fun learning about all these tracks as we live here longer.

Another stick figure in peril

One of my favorite categories of funny sign: "Stick figures in peril". This one was on one of those automated gates, where you type in a code and it rolls aside, and on the way out it automatically senses your car.

Accessing image metadata: storing tags inside the image file

A recent Slashdot discussion on image tagging and organization a while back got me thinking about putting image tags inside each image, in its metadata.

Currently, I use my MetaPho image tagger to update a file named Tags in the same directory as the images I'm tagging. Then I have a script called fotogr that searches for combinations of tags in these Tags files.

That works fine. But I have occasionally wondered if I should also be saving tags inside the images themselves, in case I ever want compatibility with other programs. I decided I should at least figure out how that would work, in case I want to add it to MetaPho.

I thought it would be simple -- add some sort of key in the images's EXIF tags. But no -- EXIF has no provision for tags or keywords. But JPEG (and some other formats) supports lots of tags besides EXIF. Was it one of the XMP tags?

Web searching only increased my confusion; it seems that there is no standard for this, but there have been lots of pseudo-standards over the years. It's not clear what tag most programs read, but my impression is that the most common is the "Keywords" IPTC tag.

Okay. So how would I read or change that from a Python program?

Lots of Python libraries can read EXIF tags, including Python's own PIL library -- I even wrote a few years ago about reading EXIF from PIL. But writing it is another story.

Nearly everybody points to pyexiv2, a fairly mature library that even has a well-written pyexiv2 tutorial. Great! The only problem with it is that the pyexiv2 front page has a big red Deprecation warning saying that it's being replaced by GExiv2. With a link that goes to a nonexistent page; and Debian doesn't seem to have a package for GExiv2, nor could I find a tutorial on it anywhere.

Sigh. I have to say that pyexiv2 sounds like a much better bet for now even if it is supposedly deprecated.

Following the tutorial, I was able to whip up a little proof of concept that can look for an IPTC Keywords tag in an existing image, print out its value, add new tags to it and write it back to the file.

import sys
import pyexiv2

if len(sys.argv) < 2:
    print "Usage:", sys.argv[0], "imagename.jpg [tag ...]"
    sys.exit(1)

metadata = pyexiv2.ImageMetadata(sys.argv[1])
metadata.read()

newkeywords = sys.argv[2:]

keyword_tag = 'Iptc.Application2.Keywords'
if keyword_tag in metadata.iptc_keys:
    tag = metadata[keyword_tag]
    oldkeywords = tag.value
    print "Existing keywords:", oldkeywords
    if not newkeywords:
        sys.exit(0)
    for newkey in newkeywords:
        oldkeywords.append(newkey)
    tag.value = oldkeywords
else:
    print "No IPTC keywords set yet"
    if not newkeywords:
        sys.exit(0)
    metadata[keyword_tag] = pyexiv2.IptcTag(keyword_tag, newkeywords)

tag = metadata[keyword_tag]
print "New keywords:", tag.value

metadata.write()

Does that mean I'm immediately adding it to MetaPho? No. To be honest, I'm not sure I care very much, since I don't have any other software that uses that IPTC field and no other MetaPho user has ever asked for it. But it's nice to know that if I ever have a reason to add it, I can.