Histogram Overlay
=================

The histogram overlay provides real-time visualization of your image's tonal distribution directly on the image canvas, allowing you to monitor intensity levels while performing adjustments.

Activating the Histogram Overlay
---------------------------------

The histogram overlay can be toggled on and off using either of these methods:

* Click the **Histogram** button in the main toolbar
* Press :kbd:`Ctrl+H` (or :kbd:`Cmd+H` on macOS)

.. figure:: ../_images/GUI/histogram_overlay_example.png
   :alt: Histogram overlay displayed on an astronomical image
   :align: center
   :width: 100%

   The histogram overlay showing RGB channel distribution on an image

Features
--------

Channel Display
^^^^^^^^^^^^^^^

The histogram overlay displays the tonal distribution for each color channel:

* **Red channel** - Displayed in red
* **Green channel** - Displayed in green  
* **Blue channel** - Displayed in blue
* **Monochrome images** - Displayed in white

For color images, all three RGB channels are shown simultaneously with their respective colors, making it easy to identify color balance and clipping issues.

Logarithmic Scale
^^^^^^^^^^^^^^^^^

The histogram automatically adapts to display data in logarithmic scale when appropriate, revealing detail in both shadow and highlight regions that might otherwise be difficult to see in a linear scale. The default display is based on the settings defined in Siril’s preferences.

Statistics Display
^^^^^^^^^^^^^^^^^^

Below the histogram curves, you'll find statistical information about your image:

* **Minimum value (μ)** - The darkest pixel value in the image
* **Median value (Med)** - The middle value of the tonal distribution
* **Standard deviation (σ)** - A measure of the spread of values

These statistics are displayed with high precision and update in real-time as you make adjustments to your image.

Use Cases
---------

The histogram overlay is particularly useful when:

* **Stretching images** - Monitor clipping in real-time during histogram transformation
* **Color balancing** - Observe how color corrections affect each channel's distribution
* **Assessing exposure** - Quickly identify if an image is under- or over-exposed
* **Calibration review** - Verify that calibration frames are producing the expected tonal distribution

Tips
----

* The overlay remains visible across different image processing operations, allowing continuous monitoring of your adjustments
* The histogram automatically updates when you switch between images or modify the current image
* For best visibility, the overlay is rendered with semi-transparency to avoid obscuring important image details