Pages, Never Photos: Designing the Fullness Slider

Every album tool has to answer one question: how much should go on a page? Most answer with a preset, three vague words you pick before seeing anything. Pressbook 1.2.2 answers with an instrument. This is a short post about its design, because the constraints that shaped it say a lot about how we build.

One rule, never broken

The fullness slider runs from airy to packed. Drag it and the album re-plans live: more pages with room to breathe, or fewer pages, every page full. The rule is that the slider changes pages, never photos. Whatever you set, every photo you chose stays in the album. Density is a layout decision, and layout decisions should never quietly cost you a picture of your kid.

Photos do get set aside during a build, near-duplicates and frames below the quality floor, but that is a separate judgment with a separate home: a drawer, grouped by reason, one tap to restore. The slider cannot reach it. Two different decisions, two different controls, and neither one deletes anything.

The graph is the contract

Above the slider sits a small bar graph: how many pages of each size the plan will produce, one bar per photos-per-page count, with the peak marked. Under it, a readout in plain words, about 34 pages, 96 photos. This is not a preview illustration. It is the actual plan the engine will execute, recomputed as you drag.

Drawing that graph honestly forced a harder question: what do you show while the engine is still computing? The answer we shipped is nothing dressed up as something. While the numbers are being counted, the bars go hollow and breathe, the readout says counting pages, and the slider waits. No skeleton pretending to be data, no spinner pretending to be progress. When the bars fill in, they are true.

Takes are the undo you can hold

The slider pairs with takes. Rebuild the album at a new fullness and the old version is not gone, it is take one, and you can flip back to it whole. Changing your mind stops being destructive, which changes how willing you are to explore. It is the same reason photographers shoot contact sheets: comparison beats commitment.

Why an instrument and not a setting

A setting asks you to predict. An instrument lets you feel. The slider, the graph, the readout, and takes together make density something you adjust with your eyes open, the way you would adjust anything physical. And because the whole computation runs on the device, the feedback loop is immediate, there is no round trip to hide behind a progress bar.

The rebuilt engine that makes this possible, chapters, burst stacks, balanced people, is covered in how Pressbook reads a camera roll, and the rest of the release in the 1.2.2 notes.

Try the slider on your own roll, free on the App Store. iPhone and iPad, iOS 18 and up.