The grid units are responsive to the existing grid scale, subdivisions, and overall blender UI scale settings. Further options are added to the viewport overlay menu configuring a scale multiplier for the unit labels, as well as their tint and opacity. The addon preferences define defaults for new scenes and take effect on next startup. Font/unit texture configs take effect immediately.

The unit cards use lookups on a regular png texture; you can copy/modify this atlas to contain any glyphs/representation you want, provided the size remains at 512x512 and glyphs are divided into a 5x5 grid. The addon prefs allow you to set a path to this custom texture.

The unit labels have an algorithm that maintains visibility and avoids crowding based on window/viewport pixel size and viewport camera properties.

Units are drawn in batch, with limiting of how many cards are drawn based on view frustum intersections with the perspective/ortho grid, to maintain high performance.

All versions are available in the product files, just be sure to grab the latest one unless you actually need to go back for some reason.

Changelog

v9
* Implemented units that stay on the edge of orthographic viewports even when the main axes are offscreen.
* Fixes for bad unit flipping in certain ortho views, e.g. top 90 degrees; using more robust algorithm.
* Consistent side of axis for units in ortho views.
* Taking into account viewport header menu visibility when drawing offscreen edge units.
* Improved unit sizing algorithm for tiny screens all the way to huge screens.

v8
* Fixed Quad View (CTRL-ALT-Q) unit rendering by correcting the way region data is fetched from the context.

v7
* Using theme axis colors for unit rendering.
* Implemented better formula for axis tint.

v6
* Unit packing, with addon prefs and Scene Units menu. This is useful for modular part workflows, common in game dev. E.g. where 1 unit is 512 cm, and you want the full units for your scene, but a grid that shows whole numbers for every 512 cm.

v5
* Ensuring labels reflect the scene's unit length scale setting.
* Improved rounding method, towards nearest grid line spacing, if the jumps between major units at the current viewing distance are at least that size.

v4
* Allowing the overall viewport show/hide toggle to control the grid unit settings, keeping it more in line with how the other viewport overlay controls work.

v3
* Adding support for length preference to be reflected in the labels used, e.g. centimeters, inches, meters, feet.
* Fixing labels for imperial unit system.
* Evened out ortho vs. perspective scale bump.
* Added a hack for *imperial units* to mimic the sort of major grid line jumps the Blender grid is doing at various scales; it's a confusing set of grid lines to begin with, but this addon's labels need to follow them.
* Fixing a draw issue with +Z/-Z axis orthographic at very small view distances.

v2
* Mac OS X support, better cross-platform shader code.

v1
* Initial release