---
url: /glimpse/ecosystem/ruler.md
---
# Ruler

The ruler freezes the screen and measures distances between the points you click, in pixels. Every
option is in the [command reference](/config/cli/glimpse-lens).

## Measure a distance

```sh
glimpse-lens --tool ruler
```

Click where the measurement starts, then click where it ends. The pill under the lens shows the
horizontal and vertical offset, the length and angle of the segment, and the total so far.
Enter or Space places a point too, and the arrow keys move one pixel at a
time, so you can land exactly on an edge.

Hold Shift to keep the line straight across or straight up and down. Scroll to zoom in
on the point you are placing, and hold Shift while scrolling to resize the lens.

## Trace a path

After the first point, every click measures one segment and starts the next one from the same
point, so you can follow an outline or add up several gaps. A right click clears the path and starts over. Esc
finishes.

A path stays on one screen. Moving to another screen drops the point you were about to place
rather than drawing a line across both.

Distances are in the screen's own pixels. With display scaling at 2, a gap that looks 100 pixels
wide measures 200.

## Use it in scripts

When you finish, the ruler prints one line of JSON per segment, in the order you measured them:

```json
{ "angle": 0.0, "distance": 340.0, "dx": 340, "dy": 0, "from_x": 620, "from_y": 420, "to_x": 960, "to_y": 420 }
```

`distance` is the length, `dx` and `dy` the offsets, `angle` the slope from horizontal in degrees
(0 to 90), and the `from_` and `to_` fields the end points. To get the total length of a path:

```sh
glimpse-lens --tool ruler | jq -s 'map(.distance) | add'
```

The exit code is 0 when at least one segment was measured, 4 when nothing was (you pressed
Esc straight away, or a screen went away), and 1 for any other failure.

## Use it with AI agents

An agent checking a layout can ask you to measure it. It runs the ruler, you click across the gap
or the element in question, and the agent gets exact pixel numbers to compare with the design:

```sh
glimpse-lens --tool ruler
```

Each line is one measurement, so an agent can ask for several in one go, for example "measure the
left margin, then the right one".