Photoshop’s PSD file format

Photoshop’s native format, PSD, doesn’t get a lot of discussion. It’s Photoshop’s default format, and people use it for projects if only for that reason, so we really should know something about it. A lively place to start is “Fun Photoshop File Format Facts” on the Postlight blog. For serious investigation, look at Adobe’s specification. There’s also a short article on, with some information about the format’s history.

There are actually two related Photoshop native formats: PSD (which doesn’t seem to stand for anything) and PSB (which apparently stands for “Big PSD”). The Postlight article notes that “at 150 DPI, a [PSB] file can be 167 feet tall.” The spec is clearly designed to have generous upper limits, so it can be converted to and from all the other formats Photoshop supports. You can have 30,000 pixels in each dimension in PSD, 300,000 in PSD. (Why not 32,767?) You can have 56 channels, each with as many as 32 bits. There are nine different color modes. Macintosh resource forks, which went out when OS X came in around the turn of the century, are still supported. There are almost a hundred different resource types for holding metadata, color profiles, and so on.

The image data section seems simple, but “terse” is the better word. Image data for multiple channels is described in a different place, and I can’t immediately see how one relates to the other. Maybe I’m just missing something, not having studied the spec in detail, but I get the feeling it really doesn’t have enough information to understand how the pixels are supposed to be laid out in all the different cases.

Nobody’s going to use PSD as an archival format, but there are numerous PSD converters available, so people have figured all this out. It looks like a lot of work.

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