An unsigned 16-bit integer value indicating, in decidots/inch, the assumed resolution of the display device when the book content was laid out.

Comments

It is somewhat unfortunate that dpi usually means dots per inch, when it means decidots/inch in this context. A name like assumed resolution, or even ddpi, would be less ambiguous.

An unspoken assumption is that the device has the same resolution in the horizontal as in the vertical direction. This is not true of many inkjet devices, but it seems to be universally true for (current) E-ink devices. Note that the current header formats have 2-bytes of padding after this value. A reverse-engineer might conclude that the header format allows the specification of different resolutions in the X and Y dimensions. Since zero is an absurd value, it may indicate "resolution identical to other value". If the layout is consistent with other dimensional specifications, we may assume that the horizontal value precedes the vertical value. TODO: conduct tests to see if filling the padding bytes with various values has an effect on the display of text.