How can I get the depth of a jpg file?

Question

I want to retrieve the bit depth for a jpeg file using Python.

Using the Python Imaging Library:

import Image
data = Image.open('file.jpg')
print data.depth

However, this gives me a depth of 8 for an obviously 24-bit image. Am I doing something wrong? Is there some way to do it with pure Python code?

Thanks in advance.

Edit: It's data.bits not data.depth.

Answer 1

I don't see the depth attribute documented anywhere in the Python Imaging Library handbook. However, it looks like only a limited number of modes are supported. You could use something like this:

mode_to_bpp = {'1':1, 'L':8, 'P':8, 'RGB':24, 'RGBA':32, 'CMYK':32, 'YCbCr':24, 'I':32, 'F':32}

data = Image.open('file.jpg')
bpp = mode_to_bpp[data.mode]

Answer 2

Jpeg files don't have bit depth in the same manner as GIF or PNG files. The transform used to create the Jpeg data renders a continuous color spectrum on decompression.

Answer 3

PIL is reporting bit depth per "band". I don't actually see depth as a documented property in the PIL docs, however, I think you want this:

data.depth * len(data.getbands())

Or better yet:

data.mode

See here for more info.

Answer 4

I was going to say that JPG images are 24 bit by definition. They normally consist of three 8 bit colour channels, one for each of red, green and blue making 24 bits per pixel. However, I've just found this page which states:

If you use a more modern version of Photoshop, you'll notice it will also let you work in 16-bits per channel, which gives you 48 bits per pixel.

But I can't find a reference for how you'd tell the two apart.