Friday, April 17, 2009

Test if uploaded file is JPEG, PNG or TIFF

I've been looking at some of uploads that went wrong on YayArt lately, and it turns out that people sometimes submit images with the wrong extension, e.g. "someimage.png" when it's really a JPEG. This confuses the image backend we're using to process large images, VIPS, so it reports back an error.

I did a bit of googling, and it seems the easiest way out is to simply check the first few bytes of the file for magic numbers. So here's a bit of Python code for checking for whether the file data belongs to a JPEG, PNG or TIFF image:
def is_jpg(data):
return data[:2] == '\xff\xd8'

def is_png(data):
return data[:8] == '\x89PNG\x0d\x0a\x1a\x0a'

def is_tiff(data):
return data[:4] == 'MM\x00\x2a' or data[:4] == 'II\x2a\x00'
If the file is already on disk, you can grab the first few bytes with
f = open("somefile.jpg", 'r')
data =
if is_jpeg(data):
ext = ".jpg"
elif is_png(data):
ext = ".jpg"
Of course this won't test that the whole file is valid. But it's easier to do that afterwards with an image library once the extension is correct.

The magic numbers are documented in the specifications for the formats. You can also find some help for other formats in the source code of the file command on Unix systems.

Update: I'm liking this so much that I ended up putting it in a separate file and making a convenience function for getting an extension like '.jpg'. Grab the Python file here. I also added support for GIF. Here's another easy reference for magic file numbers.

Second update: I've updated the code, there was a bug detecting JPEGs from certain digital cameras that put Exif data in the first segment. Suffice to check the two first bytes of the JPEG, then the problem does not occur.

Wednesday, April 8, 2009

Good art and bad art

There's a couple of things I've learned in the process with YayArt that I'd like to share. First, the key question: what is good art?

If you're like me, you'll probably think that this is impossible to say, or at least so difficult to answer that you need to have studied art for many years, be part of the art elite, to answer in a qualified way.

That's right. But in my opinion also wrong.

One approach to definition of good art is the Aristotelian approach. We try to pinpoint the common traits of the subject. This is a slippery path, but we can still do some.

First, art is a result of craftmanship which is performed with the purpose of engaging you in some way, by moving you, touching your feelings. It must be more than just an everyday thing.

If you're part of the art elite, you would add to this that good new art must bring something fresh to the table every time. It must be innovative. The past is important, to the point that new works that in the past would have been considered good art seize to be interesting.

If you're not part of the art elite but like me, you're probably more interested in the looks than prior art. It must be pleasing to the eye, or at least pleasingly unpleasant. The small details that are hard to define, but nevertheless obvious, must be right.

Good or bad art?

There's a conflict between these opposing views, and I believe this conflict leads directly to museums and obscure works like the infamous diamond skull by Damien Hurst, which most people are likely to agree is pretty ugly.

Why do most people hardly ever go to museums for inspiration? Is it because they're not susceptible to art? They lack the gene that enables them to experience art? This seems like a dubious explanation. Witness the commercial success of photography and Hollywood. Pictures work. And good art is, by definition, capable of engaging people.

Good or bad art?

The explanation is probably rather that going to a museum is cumbersome, and when you finally get there you might only see art that is definitely different, but doesn't engage you. It doesn't speak to you. Art that doesn't engage you is bad art, by definition - for you.

With YayArt we're trying to make it less cumbersome to see the art. And we are trying to replace the elitist notion of good art as being innovative art by something else. It's not that having an art elite is bad. It's just that we think there's a large proportion of people who could enjoy and benefit from art if the current art market would serve them. Or a new art market emerged.

The definition of good art we're using is a platonic definition, i.e. it works by examples. For instance, to define a chair to native from the jungle we wouldn't talk about furniture and four legs but instead point at the six chairs around the dining table and say, now do you understand?

At YayArt we show you a piece of art and ask, is it good or bad?

Good or bad art?

This is not abstract, so it is easy for everyone, not just art pros, to answer. What we're implicitly asking are questions like, does it move you? Do you like it? Everybody can answer that.

And when you think about it, this is really what good art is about.