Add item OpenCV in User Guide (#2796)

OpenCV tools: Intelligent scissors documentation

Co-authored-by: Nikita Manovich <nikita.manovich@intel.com>

cvat/apps/documentation/user_guide.md

@@ -26,6 +26,7 @@
   - [Track mode (advanced)](#track-mode-advanced)
   - [Attribute annotation mode (advanced)](#attribute-annotation-mode-advanced)
   - [AI Tools](#ai-tools)
+  - [OpenCV Tools](#opencv-tools)
   - [Annotation with rectangle by 4 points](#annotation-with-rectangle-by-4-points)
   - [Annotation with polygons](#annotation-with-polygons)
   - [Annotation with polylines](#annotation-with-polylines)
@@ -858,6 +859,7 @@ Switching between user interface modes.
 |Icon |Description |Links to section |
 |-- |-- |-- |
 |![](static/documentation/images/image189.jpg)|`AI Tools` |[AI Tools](#ai-tools)|
+|![](static/documentation/images/image201.jpg)|`OpenCV` |[OpenCV](#opencv)|
 |![](static/documentation/images/image167.jpg)|`Rectangle`|[Shape mode](#shape-mode-basics); [Track mode](#track-mode-basics);<br/> [Drawing by 4 points](#annotation-with-rectangle-by-4-points)|
 |![](static/documentation/images/image168.jpg)|`Polygon` |[Annotation with polygons](#annotation-with-polygons); [Track mode with polygons](#track-mode-with-polygons) |
 |![](static/documentation/images/image169.jpg)|`Polyline` |[Annotation with polylines](#annotation-with-polylines)|
@@ -1138,6 +1140,40 @@ Detectors are used to automatically annotate one frame. Supported DL models are
 - This action will automatically annotates one frame.
   In the [Automatic annotation](#automatic-annotation) section you can read how to make automatic annotation of all frames.
 
+## OpenCV tools
+
+The tool based on [Open CV](https://opencv.org/) Computer Vision library which is an open-source product that includes many CV algorithms. Some of these algorithms can be used to simplify the annotation process.
+
+First step to work with OpenCV is to load it into CVAT. Click on the toolbar icon, then click `Load OpenCV`.
+
+  ![](static/documentation/images/image198.jpg)
+
+Once it is loaded, the tool's functionality will be available.
+
+### Intelligent scissors
+
+Intelligent scissors is an CV method of creating a polygon by placing points with automatic drawing of a line between them.
+The distance between the adjacent points is limited by the threshold of action,
+displayed as a red square which is tied to the cursor.
+
+- First, select the label and then click on the `intelligent scissors` button.
+
+  ![](static/documentation/images/image199.jpg)
+
+- Create the first point on the boundary of the allocated object.
+You will see a line repeating the outline of the object.
+- Place the second point, so that the previous point is within the restrictive threshold.
+After that a line repeating the object boundary will be automatically created between the points.
+
+  ![](static/documentation/images/image200_detrac.jpg) 
+
+  To increase or lower the action threshold, hold `Ctrl` and scroll the mouse wheel.
+  Increasing action threshold will affect the performance.
+  During the drawing process you can remove the last point by clicking on it with the left mouse button.
+
+- Once all the points are placed, you can complete the creation of the object by clicking on the icon or clicking `N`.
+As a result, a polygon will be created (read more about the polygons in the [annoation with polygons](#annotation-with-polygons)).
+
 ## Annotation with rectangle by 4 points
 
 It is an efficient method of bounding box annotation, proposed
@@ -1618,8 +1654,8 @@ Many UI elements have shortcut hints. Put your pointer to a required element to
 | Shortcut                   | Common                                                                                                   |
 | -------------------------- | -------------------------------------------------------------------------------------------------------- |
 |                            | _Main functions_                                                                                         |
-| `F2`                       | Open/hide the list of available shortcuts                                                                |
-| `F3`                       | Go to the settings page or go back                                                                       |
+| `F1`                       | Open/hide the list of available shortcuts                                                                |
+| `F2`                       | Go to the settings page or go back                                                                       |
 | `Ctrl+S`                   | Go to the settings page or go back                                                                       |
 | `Ctrl+Z`                   | Cancel the latest action related with objects                                                            |
 | `Ctrl+Shift+Z` or `Ctrl+Y` | Cancel undo action                                                                                       |