Every visual Mac storage tool draws a treemap, but not every treemap is laid out the same way underneath, and the layout algorithm has a real effect on how quickly you can read the result. This is a look at the two dominant approaches — squarified and slice-and-dice — plus the secondary choices of colouring, nesting, and zoom that shape a map's usability.
What a treemap layout algorithm actually decides
Given a set of items with known sizes that need to fit inside a rectangle, proportional to their share of the total, there's more than one geometrically valid way to divide that rectangle up. A layout algorithm is the rule that decides where each item's boundary goes. Two items of the same size will always get the same area under any correct algorithm — the difference is in the aspect ratio (wide and short versus roughly square) of the resulting shapes, and that difference matters a lot for readability.
Slice-and-dice: simple, but prone to slivers
The earliest and simplest approach, slice-and-dice, divides the rectangle with parallel cuts — alternating horizontal and vertical as you descend into nested levels — sized proportionally along one axis at a time. It's straightforward to implement and to reason about.
Its practical weakness shows up with a folder that has many small items alongside a few large ones: slice-and-dice can produce very long, thin slivers for the small items, which are hard to read (a label doesn't fit inside a sliver a few pixels wide) and hard to click precisely. The effect compounds at deeper nesting levels, where slivers within slivers become close to unreadable.
Squarified: optimised for near-square shapes
The squarified algorithm, introduced in visualisation research in the early 2000s, works differently: instead of committing to a single cut direction, it groups items and chooses cut placement specifically to keep the resulting rectangles as close to square as geometrically possible, given the proportions involved. A near-square rectangle is easier to label, easier to click accurately, and easier to compare by eye against its neighbours than a sliver is.
The tradeoff is that squarified layouts don't preserve a strict left-to-right or top-to-bottom order matching, say, alphabetical or size-rank order the way slice-and-dice more naturally can — items get placed based on what produces good aspect ratios, not necessarily in a simple scan order. For a tool whose main job is "help me see the biggest thing fast," this tradeoff clearly favours squarified; for one where preserving a specific reading order matters more, slice-and-dice's simplicity has its own value.
Most storage tools built or substantially updated in the last fifteen years use a squarified or squarified-like approach for exactly this readability reason, though the specific algorithm any individual tool uses isn't always documented publicly.
Flat versus nested representation
A separate design choice is whether the map shows only one level of the hierarchy at a time (flat, with drill-down navigation replacing the view as you click deeper) or attempts to show multiple nested levels simultaneously (nested, with smaller rectangles-within-rectangles visible at once).
Flat-with-drill-down keeps each screen simpler and each rectangle larger and more readable, at the cost of losing context about where you are in the overall hierarchy — you have to rely on a breadcrumb or back button to know how deep you've gone. Nested keeps context visible at all times but sacrifices readability at each level, since available pixels are now split across more visual information simultaneously. Most tools lean flat-with-drill-down for the top few levels and offer nested detail only within a single reasonably-sized region, as a middle ground.
Colouring choices and what they communicate
Colour in a treemap typically encodes one of a few things: file type or extension (so all images are one colour family, all archives another), depth in the hierarchy, or in more specialised tools, a recognised category (a colour reserved specifically for identified developer caches, say). Colour-by-type is the most common default because it lets you spot, for instance, that a folder's bulk is video files versus documents at a glance, without reading labels.
The failure mode to watch for is a palette with too many similar hues, or one that doesn't hold up for colour-blind users — red/green distinctions specifically are a common accessibility miss in data visualisation generally, treemaps included. A well-designed colour scheme uses a palette that stays distinguishable under the most common forms of colour blindness, not just under typical vision.
Zoom and drill-down interaction
The interaction pattern for entering a folder — click to zoom in, with the clicked rectangle expanding to fill the view — is close to universal across modern tools, differing mainly in transition smoothness and whether a breadcrumb trail is shown to support navigating back up multiple levels at once rather than one click at a time.
Labelling: the detail that determines whether any of this works
None of the layout or colour choices above matter much if the resulting rectangles can't be labelled clearly enough to read at a glance. A squarified layout that produces near-square shapes still needs a labelling strategy that decides, for each rectangle, whether there's room for a full name, an abbreviated one, or no text at all — falling back to colour and relative size alone once a rectangle gets too small for any label to fit legibly.
The failure mode to watch for is a tool that either crams unreadable, truncated text into every rectangle regardless of available space, or drops labels inconsistently in a way that makes it hard to tell which rectangle you're actually looking at without hovering or clicking. A well-designed map treats labelling as a first-class layout concern, not an afterthought applied uniformly regardless of each rectangle's actual size.
Performance at scale
A treemap's layout algorithm also has to run fast enough to feel instant when you click to drill into a large folder, ideally recalculating and redrawing within a fraction of a second even for folders containing thousands of items. Squarified layout, while more visually effective than slice-and-dice, is also somewhat more computationally involved to calculate, since it has to evaluate multiple candidate groupings to find near-square results rather than applying a single fixed cut rule. In practice this difference is small enough on modern hardware that it rarely matters, but it's part of why some very old or resource-constrained tools historically favoured the simpler slice-and-dice approach.
Comparing the choices
| Choice | Option A | Option B | Practical effect |
|---|---|---|---|
| Layout algorithm | Slice-and-dice | Squarified | Squarified reads faster with many differently-sized items |
| Depth shown | Flat + drill-down | Nested | Flat is more readable per level; nested keeps more context |
| Colour basis | File type | Depth or category | Type-based colouring shows content mix at a glance |
Where VolumeLens fits
VolumeLens uses a squarified-style layout specifically because it keeps rectangles closer to readable, clickable proportions even in folders with a wide spread of item sizes, with flat drill-down navigation so each level stays uncluttered. The goal throughout is the same one this whole series keeps coming back to: a map you can read correctly in seconds, not one that just looks impressive. You can see it against your own disk on the features page or by downloading VolumeLens directly.