ithmbrdr

Reader for iPod ithmb files

Background

In an attempt to recover lost images from an iPod nano 3rd generation (the almost square, slightly fat ones) I stumbled across the ithmb file format. Apple saves thumbnails of pictures synchronised with an iPod (and apparently older iPhones as well) in these files in varying resolutions. The idea was that the screens were not capable of showing high resolutions anyways, so lower resolution images were sufficient. Some of the highest resolutions were also meant to be shown on a television.

Finding information about the ithmb format turned out to be rather tedious. There are some closed source programs, mostly for Windows, that seem to be to extract the image information. However, they usually added a watermark that could only be removed with some extra payment. The only free source of information I could find were some (old) internet forum threads, such as this one. The only open source project that attempts to read ithmb files that I could find is Keith's iPod Photo Reader, which was the most helpful resource by far.

Check out the README.txt that Keith wrote for a lot of information on the format. It seems that he figured most/all(?) of this out on his own, which is a great achievement.

For the iPod nano 3rd generation, that I had available here, it turns out that the highest resolution images (720 x 480 pixels) were in files called F1067_x.ithmb where x indicates the chunk, since these files are broken apart if there are too many images in one.

The images are stored consecutively in the ithmb files, without any header. Note that each image uses 720 * 480 * 2 bytes. The colors were encoded in YCbCr or YUV (not sure) with a 4:2:0 chroma sampling scheme. Note that the data is saved in the order below.

The first 720 * 480 bytes encode the luminance for each pixel. The next 360 * 240 bytes encode the Cb or U chrominance (essentially as a 360 x 240 pixel grayscale) and the following 360 * 240 bytes encode the Cr or V chrominance.

After some more exploration, it seems like the last 720 * 120 bytes contain a version of the lower half of the picture encoded in the first 3/4-th of the file. It is encoded in YCbCr or YUV in blocks of four bytes as Cb Y Cr Y. This is similar to a 4:2:2 chroma sampling scheme. However, the weird thing is that the luminance channel seems to miss half the rows, while the chroma information is fully available and is the same as the lower half of the respective chroma channels encoded above. It is possible that this chunk is just meant for padding, but it is unclear why half the image would be saved here, especially in this vertically squished fashion.

Example

Assuming you have the file F1067_3.ithmb in the current folder.

./ithmbrdr F1067_3.ithmb

creates a folder (if it does not exist) called F1067_3 and extracts all images from F1067_3.ithmb to this folder and saves them as PNG files.

./ithmbrdr -id "9" F1067_3.ithmb

extracts only the 10th image (0-based indexing) from F1067_3.ithmb.

./ithmbrdr -id "3-8" F1067_3.ithmb

extracts images 4 to 9 (0-based indexing). Ranges are inclusive on both ends.

TODOs

• Figure out how the Photo Database file can be read. This might give reveal extra information about the images
• See if I can find more test data to try other iPod/iPhone models
• What is the best way to convert the colors to RGB? There seem to be many similar conversion formulas.