I wonder what value you could add to this with a notebook printed with digitally referenced paper? By printing an unobtrusive sort of barcode on each page, you could determine which part of which page of which notebook each scanned pixel came from, and what lighting conditions it was photographed under. What could you do with that?

Well, the simplest and most greyface application is forms; you can define particular areas of each page as being particular form fields. If you're blogging, you might have a field for a "slug" that appears in the URL, for example, or a field for tags, or checkboxes for some tags (plus a special page to declare the meanings of the checkboxes). Or, if you're tracking expenses, you could have a checkbox for each expense category and columns for the date and the amount. For recipes, you might have a section for listing ingredients, with a column for unit of measure, a column for quantity, and a column for the ingredient name. Etc.

For me, the special feature of paper notebooks that cellphones and other computers suck at is drawing. If I want to draw a diagram or illustration, it just works much better on paper: my pencil point occludes much less drawing area than my finger does, there's no tracking error where the ink appears 2 mm to the side of the point, it has much lower latency, and I can draw finer lines. But scanning those drawings into a computer is a pain, because I have to illuminate them evenly and hold them flat while I photograph them, which still probably involves some perspective distortion. Barcodes on the paper, together with reference lines and reference color swatches, could solve that problem, as well as providing information about which parts of the paper are occluded, if any.

For a few special applications like numismatics and entomology, the paper could provide a precise physical measurement reference for specimens.

Combining drawing with filling out forms, you can make a font from your handwriting; this is enormously easier if you can correct the various distortions. In http://canonical.org/~kragen/oilpencil/ I spent about 24 hours fiddling with various graphics programs, but there was a website I found somewhere where you can print out a form, draw the font on it, upload the scan, and download your TrueType font. This kind of thing might help with training OCR, too, especially if you don't have access to GPT-3. (Or if OpenAI decides to peremptorily destroy everything you've built because one of your users uses your service to write about their dead fiancee: https://towardsdatascience.com/openai-opens-gpt-3-for-everyo...)

Other ways to combine drawing and filling out forms include sketching orthographic projections to build 3-D models; coloring a coloring book; drawing maps for Minetest and similar grid-cell games (especially 2-D ones); drawing heightfields; and sketching different keyframes of an animation to automatically morph between. You could even draw a 2-D continuum of keyframes, thus providing an animation character that's continuously variable along two different axes; you might put time on the theta axis and some sort of emotion along the radius axis.

(You can also apply these ideas with drawings that are input via other media, such as touchscreens, Wacom tablets, and mice, not only scanning paper. When you're scanning paper it's hard to get feedback as you're drawing, although you could maybe glance at your cellphone screen periodically, or use a projector like DynamicLand, or have a continuously updated monitor using a webcam feed. It could even use the occlusion information from the barcode to patch in remembered images wherever your hands were occluding the paper.)

What should the barcodes look like?

In 02001 Anoto announced their "Digital Paper" approach: https://www.wired.com/2001/04/anoto/. As explained in https://en.wikipedia.org/wiki/Digital_paper this uses an unobtrusive 2-D barcode scanned by a camera in a "digital pen" (later called the "Fly Pen", 02005) to locate the pen in an enormous global "virtual desktop"; I think the NeoLAB "Neo smartpen" works the same way. This was all before cameraphones went mainstream and high resolution. They got 300 patents but fortunately everything they filed in 02001 expires this year. Anoto's barcodes use a grid of slightly displaced grid dots.

The Fly Pen provided a sort of graphical user interface on the paper, using audio for output. It was sort of aimed at kids doing schoolwork and playing games. It failed in 02009. The founder started a new company called Livescribe focusing on notetaking; the Livescribe smartpen allows you to spatially organize and annotate a continuous audio recording. It has been more commercially successful: https://en.wikipedia.org/wiki/Livescribe

Tiny unobtrusive dots might not reproduce reliably on a cellphone camera, though having been published in WIRED in 02001 means the technique is in the public domain (or will be next year). A better idea might be to use thin horizontal and vertical grid lines whose thickness varies slightly, perhaps in a pastel subtractive primary like cyan, magenta, or yellow; then you can optionally remove them in software after scanning. Scanning a whole page at a time, instead of a tiny area around a pen point like the "digital pens" described above, gives you a great deal more space for redundant page ID data in the barcode; probably 48 bits or so is sufficient.