# Inspecting pixels

Four things in atlas report numbers rather than pictures: the hover readout,
the statistics panel, the FITS header panel, and the histogram window. All of
them read the **raw array**, not the rendered image, so none of them move when
you change the [display scale](frames.md#scales).

(hover-readout)=
## Hover readout

Resting the cursor on a pixel puts its index and its count on the right of the
status bar:

```{eval-rst}
.. container:: sample-readout

   ::

      (341, 169)  27,922
```

The indices are 0-based, `x` across and `y` down, so the pair reads directly as
`data[y, x]` in whatever you are inspecting the frame with. This is numpy's
convention rather than DS9's 1-based one, and `y` counts down because atlas
draws the first row of the array at the top of the tile.

The count comes from the raw array, so it is a detector count and does not move
when the display scale changes. Blank pixels (NaN or inf) read as a dash rather
than as a number, and colour frames report one sample per channel.

A frame is usually shown smaller than it is, in which case several data pixels
share one screen pixel and the readout names one of them. It always names the
pixel whose count it shows.

The readout is always available: it needs no configuration, adds no panel, and
does no work at all until the cursor is over a frame. It sits beside the status
messages rather than replacing them, so neither overwrites the other.

:::{tip}
While a [live stream](live.md) is running, the readout re-reads the hovered
pixel as each frame arrives, so resting the cursor on one pixel shows its
counts changing. When tiling, it reports whichever tile the cursor is over,
prefixed with that frame's name, which need not be the current frame.
:::

(statistics)=
## Statistics

The **statistics** tool adds a dock panel summarising the current frame's pixel
values: mean, median, standard deviation, min and max, plus the pixel count.

```sh
atlas image.fits --enable statistics
```

```yaml
tools:
  statistics: true
```

The figures come from the raw array, not the rendered image, so they describe
detector counts and do not move when the display scale changes. Blank pixels
(NaN or inf) are excluded and reported separately, since a single NaN would
otherwise make every statistic NaN.

### Why the panel has an update rate

Recomputing is not free. The median alone costs about ten times the other
statistics put together, roughly 30 ms on a 2048x2048 frame. So while a live
stream is running the panel refreshes at `update_hz` (2 Hz by default) rather
than on every displayed frame:

```yaml
tools:
  statistics:
    enabled: true
    update_hz: 2.0      # between 0.1 and 60
```

Switching frames by hand still recomputes immediately, and a hidden panel does
no work at all, so closing the panel is a real way to give the display back its
time budget.

## Header panel

The **header** tool is the one tool that is on by default. It docks on the
right and shows the FITS header of whichever frame is current, following your
selection as you move between frames.

Hide and show it from the **View** menu, or switch it off entirely:

```sh
atlas image.fits --disable header
```

(histograms)=
## Histograms

The **histogram** tool adds **Tools → Histogram…**, which opens a window
plotting the pixel-value distribution of every frame currently on screen. In
tile mode that means all the tiles, one curve each, which is the quickest way
to see that two frames really do share a bias level.

```sh
atlas image.fits --enable histogram
```

The histogram window has its own **Log count axis** checkbox, independent of
the frame's [scale](frames.md#scales). A pixel histogram is usually dominated
by a single sky or bias peak, and a log count axis is what makes the faint tail
visible. Changing it affects only the plot, never the image.