Freehand

R2026b

Freehand region of interest

Description

A Freehand object specifies the shape and position of an axes-based hand-drawn region of interest (ROI). You can customize the appearance and interactive behavior of the ROI.

Axes-based ROIs enable you to draw and edit regions of interest on images displayed using imshow or in MATLAB^® axes. Axes-based ROIs offer maximum customization of appearance and behavior, including line width, marker style, and edge color. Use axes-based ROIs for fine-grained control over ROI appearance, integration with existing axes-based apps, or access to the assisted freehand, crosshair, or cuboid ROI types. For a comparison of axes-based ROIs with other ROIs, see Choose Approach to Annotate ROIs in Images.

For more information about using a freehand ROI, including keyboard shortcuts, see Tips.

Creation

There are two ways to create a Freehand object. For more information, see Create Axes-Based ROIs.

• Use the drawfreehand function. Use this function when you want to create the ROI and set the appearance in a single command. You can specify the shape and position of the ROI interactively by drawing the ROI over an image using the mouse.

• Use the images.roi.Freehand function described here. Use this function when you want to specify the appearance and behavior of the ROI before you specify the shape and position of the ROI. After creating the object, you can specify the shape and position interactively by using the draw function.

Syntax

roi = images.roi.Freehand

roi = images.roi.Freehand(ax)

roi = images.roi.Freehand(___,PropertyName=Value)

Description

roi = images.roi.Freehand creates a Freehand object with default properties.

roi = images.roi.Freehand(ax) creates the ROI in the axes specified by ax.

roi = images.roi.Freehand(___,PropertyName=Value) sets properties of the ROI using one or more name-value arguments.

Example: images.roi.Freehand(Color="y") creates a yellow colored Freehand object.

Input Arguments

ax — Parent of ROI

gca (default) | Axes object | UIAxes object

Parent of the ROI, specified as an Axes object or a UIAxes object.

Properties

Shape and Position

Closed — Close ROI

true or 1 (default) | false or 0

Close the ROI, specified as a numeric or logical 1 (true) or 0 (false). When true, the Freehand object closes the ROI by connecting the last point drawn to the first point drawn.

Position — Position of ROI

[] (default) | n-by-2 numeric matrix

Position of the ROI, specified as an n-by-2 numeric matrix where n is the number of vertices or points defining the ROI. Each row represents the [x y] coordinates of a vertex or point. The Freehand object generates these points as you draw the ROI shape interactively. To work with fewer points, use the reduce function.

Smoothing — Smoothing applied to edge of ROI

1 (default) | nonnegative number

Smoothing applied to the edge of ROI after interactive placement, specified as a nonnegative scalar. The Freehand object filters the x and y coordinates of the ROI using a Gaussian smoothing kernel with a default standard deviation of 1. The size of the Gaussian filter is 2*ceil(2*Smoothing)+1.

You can see the smoothing effect only after completing the drawing. Changing the value of Smoothing after completing the drawing has no effect on the ROI.

Waypoints — Position point is waypoint

[] (default) | logical vector

Position point is waypoint, specified as a logical vector of the same length as the Position property. Elements in Waypoints with the value true identify points in the Position matrix that are waypoints. By default, the Freehand object generates all the points that define the ROI and only makes points at locations of increased curvature into waypoints. You can turn all the points, or some subset of points, into waypoints by using code similar to roi.Waypoints(1:4:end) = true;.

Waypoints appear as circular shapes on the ROI edge. You can use waypoints to reshape the ROI by clicking and dragging the waypoint with the mouse. Moving waypoints modifies the freehand-drawn region between the waypoint that you clicked and the adjacent waypoints.

Color and Styling

Color — ROI color

[0 0.4470 0.7410] (default) | RGB triplet | hexadecimal color code | color name | short color name

ROI color, specified as an RGB triplet, a hexadecimal color code, a color name, or a short color name.

For a custom color, specify an RGB triplet or a hexadecimal color code.

• An RGB triplet is a three-element row vector whose elements specify the intensities of the red, green, and blue components of the color. The intensities must be in the range [0,1], for example, [0.4 0.6 0.7].

• A hexadecimal color code is a string scalar or character vector that starts with a hash symbol (#) followed by three or six hexadecimal digits, which can range from 0 to F. The values are not case sensitive. Therefore, the color codes "#FF8800", "#ff8800", "#F80", and "#f80" are equivalent.

Alternatively, you can specify some common colors by name. This table lists the named color options, the equivalent RGB triplets, and the hexadecimal color codes.

Color Name	Short Name	RGB Triplet	Hexadecimal Color Code	Appearance
"red"	"r"	[1 0 0]	"#FF0000"
"green"	"g"	[0 1 0]	"#00FF00"
"blue"	"b"	[0 0 1]	"#0000FF"
"cyan"	"c"	[0 1 1]	"#00FFFF"
"magenta"	"m"	[1 0 1]	"#FF00FF"
"yellow"	"y"	[1 1 0]	"#FFFF00"
"black"	"k"	[0 0 0]	"#000000"
"white"	"w"	[1 1 1]	"#FFFFFF"

This table lists the default color palettes for plots in the light and dark themes.

Palette	Palette Colors
"gem" — Light theme default Before R2025a: Most plots use these colors by default.
"glow" — Dark theme default

You can get the RGB triplets and hexadecimal color codes for these palettes using the orderedcolors and rgb2hex functions. For example, get the RGB triplets for the "gem" palette and convert them to hexadecimal color codes.

RGB = orderedcolors("gem");
H = rgb2hex(RGB);

Example: Color="r"

Example: Color="green"

Example: Color=[0 0.4470 0.7410]

Example: Color="#00FFFF"

FaceAlpha — Transparency of ROI face

0.2 (default) | number in the range [0, 1]

Transparency of the ROI face, specified as a number in the range [0, 1]. When the value is 1, the ROI face is completely opaque. When the value is 0, the ROI face is completely transparent.

LineWidth — Width of ROI border

positive number

Width of the ROI border, specified as a positive number in points. The default value is three times the number of points per screen pixel, such that the border is three pixels wide.

MarkerSize — Marker size

positive number

Marker size, specified as a positive number in points. The default value is eight times the number of points per screen pixel, such that markers are eight pixels in size.

SelectedColor — Color of ROI when selected

"none" (default) | RGB triplet | hexadecimal color code | color name | short color name

Color of the ROI when selected, specified as an RGB triplet, a hexadecimal color code, a color name, a short color name, or "none". If you specify "none", then the value of Color defines the color of the ROI for all states, selected or not.

For a custom color, specify an RGB triplet or a hexadecimal color code.

• An RGB triplet is a three-element row vector whose elements specify the intensities of the red, green, and blue components of the color. The intensities must be in the range [0,1], for example, [0.4 0.6 0.7].

• A hexadecimal color code is a string scalar or character vector that starts with a hash symbol (#) followed by three or six hexadecimal digits, which can range from 0 to F. The values are not case sensitive. Therefore, the color codes "#FF8800", "#ff8800", "#F80", and "#f80" are equivalent.

Alternatively, you can specify some common colors by name. This table lists the named color options, the equivalent RGB triplets, and the hexadecimal color codes.

Color Name	Short Name	RGB Triplet	Hexadecimal Color Code	Appearance
"red"	"r"	[1 0 0]	"#FF0000"
"green"	"g"	[0 1 0]	"#00FF00"
"blue"	"b"	[0 0 1]	"#0000FF"
"cyan"	"c"	[0 1 1]	"#00FFFF"
"magenta"	"m"	[1 0 1]	"#FF00FF"
"yellow"	"y"	[1 1 0]	"#FFFF00"
"black"	"k"	[0 0 0]	"#000000"
"white"	"w"	[1 1 1]	"#FFFFFF"

This table lists the default color palettes for plots in the light and dark themes.

Palette	Palette Colors
"gem" — Light theme default Before R2025a: Most plots use these colors by default.
"glow" — Dark theme default

You can get the RGB triplets and hexadecimal color codes for these palettes using the orderedcolors and rgb2hex functions. For example, get the RGB triplets for the "gem" palette and convert them to hexadecimal color codes.

RGB = orderedcolors("gem");
H = rgb2hex(RGB);

Example: SelectedColor="r"

Example: SelectedColor="green"

Example: SelectedColor=[0 0.4470 0.7410]

Example: SelectedColor="#00FFFF"

StripeColor — Color of ROI stripe

"none" (default) | RGB triplet | hexadecimal color code | color name | short color name

Color of the ROI stripe, specified as an RGB triplet, a hexadecimal color code, a color name, a short color name, or "none". If you specify "none", then the ROI edge is a solid color specified by Color. Otherwise, the edge of the ROI is striped, with colors alternating between the colors specified by Color and StripeColor.

For a custom color, specify an RGB triplet or a hexadecimal color code.

• An RGB triplet is a three-element row vector whose elements specify the intensities of the red, green, and blue components of the color. The intensities must be in the range [0,1], for example, [0.4 0.6 0.7].

• A hexadecimal color code is a string scalar or character vector that starts with a hash symbol (#) followed by three or six hexadecimal digits, which can range from 0 to F. The values are not case sensitive. Therefore, the color codes "#FF8800", "#ff8800", "#F80", and "#f80" are equivalent.

Alternatively, you can specify some common colors by name. This table lists the named color options, the equivalent RGB triplets, and the hexadecimal color codes.

Color Name	Short Name	RGB Triplet	Hexadecimal Color Code	Appearance
"red"	"r"	[1 0 0]	"#FF0000"
"green"	"g"	[0 1 0]	"#00FF00"
"blue"	"b"	[0 0 1]	"#0000FF"
"cyan"	"c"	[0 1 1]	"#00FFFF"
"magenta"	"m"	[1 0 1]	"#FF00FF"
"yellow"	"y"	[1 1 0]	"#FFFF00"
"black"	"k"	[0 0 0]	"#000000"
"white"	"w"	[1 1 1]	"#FFFFFF"

This table lists the default color palettes for plots in the light and dark themes.

Palette	Palette Colors
"gem" — Light theme default Before R2025a: Most plots use these colors by default.
"glow" — Dark theme default

You can get the RGB triplets and hexadecimal color codes for these palettes using the orderedcolors and rgb2hex functions. For example, get the RGB triplets for the "gem" palette and convert them to hexadecimal color codes.

RGB = orderedcolors("gem");
H = rgb2hex(RGB);

Example: StripeColor="r"

Example: StripeColor="green"

Example: StripeColor=[0 0.4470 0.7410]

Example: StripeColor="#00FFFF"

Visible — ROI visibility

"on" (default) | "off" | on/off logical value

ROI visibility, specified as "on" or "off", or as a numeric or logical 0 (false) or 1 (true). A value of "on" is equivalent to true, and "off" is equivalent to false. The value is stored as an on/off logical value of type OnOffSwitchState.

Value	Description
"on"	Display the ROI.
"off"	Hide the ROI without deleting it. You can still access the properties of an invisible ROI.

Labels

Label — ROI label

'' (default) | character vector | string scalar

ROI label, specified as a character vector or string scalar. By default, the ROI has no label ('').

LabelAlpha — Transparency of text background

1 (default) | number in the range [0, 1]

Transparency of the text background, specified as a number in the range [0, 1]. When set to 1, the text background is completely opaque. When set to 0, the text background is completely transparent.

LabelTextColor — Label text color

"black" (default) | RGB triplet | hexadecimal color code | color name | short color name

Label text color, specified as an RGB triplet, a hexadecimal color code, a color name, or a short color name.

For a custom color, specify an RGB triplet or a hexadecimal color code.

• An RGB triplet is a three-element row vector whose elements specify the intensities of the red, green, and blue components of the color. The intensities must be in the range [0,1], for example, [0.4 0.6 0.7].

• A hexadecimal color code is a string scalar or character vector that starts with a hash symbol (#) followed by three or six hexadecimal digits, which can range from 0 to F. The values are not case sensitive. Therefore, the color codes "#FF8800", "#ff8800", "#F80", and "#f80" are equivalent.

Alternatively, you can specify some common colors by name. This table lists the named color options, the equivalent RGB triplets, and the hexadecimal color codes.

Color Name	Short Name	RGB Triplet	Hexadecimal Color Code	Appearance
"red"	"r"	[1 0 0]	"#FF0000"
"green"	"g"	[0 1 0]	"#00FF00"
"blue"	"b"	[0 0 1]	"#0000FF"
"cyan"	"c"	[0 1 1]	"#00FFFF"
"magenta"	"m"	[1 0 1]	"#FF00FF"
"yellow"	"y"	[1 1 0]	"#FFFF00"
"black"	"k"	[0 0 0]	"#000000"
"white"	"w"	[1 1 1]	"#FFFFFF"

This table lists the default color palettes for plots in the light and dark themes.

Palette	Palette Colors
"gem" — Light theme default Before R2025a: Most plots use these colors by default.
"glow" — Dark theme default

You can get the RGB triplets and hexadecimal color codes for these palettes using the orderedcolors and rgb2hex functions. For example, get the RGB triplets for the "gem" palette and convert them to hexadecimal color codes.

RGB = orderedcolors("gem");
H = rgb2hex(RGB);

Example: LabelTextColor="r"

Example: LabelTextColor="green"

Example: LabelTextColor=[0 0.4470 0.7410]

Example: LabelTextColor="#00FFFF"

LabelVisible — Label visibility

"on" (default) | "off" | "hover"

Label visibility, specified as one of these values.

Value	Description
"on"	Label is visible when the ROI is visible.
"hover"	Label is visible only when the mouse is hovering over the ROI.
"off"	Label is not visible.

Interactivity

ContextMenu — Context menu

ContextMenu object

Context menu that displays when you right-click the ROI, specified as a ContextMenu object. You can create a custom context menu by using the uicontextmenu function and then configuring context menu properties.

Deletable — Context menu provides option to delete the ROI

true or 1 (default) | false or 0

Context menu provides an option to delete the ROI, specified as a numeric or logical 1 (true) or 0 (false). When the value is true, you can delete the ROI interactively using the context menu. When the value is false, the context menu option to delete the ROI is disabled.

In both cases, you can delete the ROI outside of the context menu by using the delete function.

DrawingArea — Area of axes in which you can interactively place ROI

"auto" (default) | "unlimited" | [x,y,w,h]

Area of the axes in which you can interactively place the ROI, specified as one of the values in this table.

Value	Description
"auto"	The drawing area is the current axes limits (default).
"unlimited"	The drawing area has no boundary and ROIs can be drawn or dragged to extend beyond the axes limits.
[x,y,w,h]	The drawing area is restricted to a rectangular region beginning at (x,y), and extending to width w and height h.

FaceSelectable — ROI face can capture clicks

true or 1 (default) | false or 0

ROI face can capture clicks, specified as a numeric or logical 1 (true) or 0 (false). When true, the ROI face captures mouse clicks. When false, the ROI face does not capture mouse clicks.

InteractionsAllowed — Interactivity of ROI

"all" (default) | "none" | "reshape" | "translate"

Interactivity of the ROI, specified as one of the values in this table.

Value	Description
"all"	You can fully interact with the ROI. Reshape the ROI by changing the position of individual vertices, or move (translate) the entire ROI.
"none"	You cannot interact with the ROI or access the context menu. No drag points are visible.
"translate"	You can move the entire ROI within the drawing area. The relative position of vertices is fixed and you cannot reshape the ROI.
"reshape"	You can reshape the ROI by changing the position of individual vertices. You cannot move the ROI.

You can access the context menu for all values of InteractionsAllowed except "none".

Multiclick — Control freehand drawing style during interactive placement

false or 0 (default) | true or 1

Control the freehand drawing style during interactive placement, specified as a numeric or logical 0 (false) or 1 (true). When the value is false, a single click and drag gesture completes the freehand ROI. When the value is true, multiple click and drag gestures can be combined with straight edges to make a more complex freehand ROI shape.

Selected — Selection state of ROI

false, 0, or "off" (default) | true, 1, or "on"

Selection state of the ROI, specified as "off" or "on", or as a numeric or logical 0 (false) or 1 (true). A value of "off" is equivalent to false, and "on" is equivalent to true.

You can also set this property interactively. For example, clicking on the ROI selects the ROI and sets this property to true. Similarly, pressing the Ctrl key and clicking the ROI deselects the ROI and sets the value of this property to false.

Parent/Child

HandleVisibility — Visibility of ROI handle in Children property of parent

"on" (default) | "off" | "callback"

Visibility of the ROI handle in the Children property of the parent, specified as one of the values in this table.

Value	Description
"on"	The object handle is always visible (default).
"off"	The object handle is hidden at all times.
"callback"	The object handle is visible from within callbacks or functions invoked by callbacks, but not from within functions invoked from the command line.

Parent — ROI parent

Axes object | UIAxes object

ROI parent, specified as an Axes or UIAxes object.

Identifiers

Tag — Tag to associate with ROI

'' (default) | character vector | string scalar

Tag to associate with the ROI, specified as a character vector or string scalar. Use the tag value to find the ROI object in a hierarchy of objects using the findobj function.

UserData — Data to associate with ROI

any MATLAB data

Data to associate with the ROI, specified as any MATLAB data. For example, you can specify a scalar, vector, matrix, cell array, string, character array, table, or structure. The Freehand object does not use this data.

Object Functions

addlistener	Create event listener bound to event source
beginDrawingFromPoint	Begin drawing ROI from specified point
bringToFront	Bring ROI to front of Axes stacking order
createMask	Create binary mask image from ROI
draw	Begin drawing ROI interactively
inROI	Query if points are located in ROI
reduce	Reduce density of points in ROI
wait	Block MATLAB command line until ROI operation is finished

Examples

Create Freehand ROI Non-interactively

Read an image into the workspace and display it.

I = imread("baby.jpg");
figure
imshow(I)

Create a freehand ROI on the image, using the Position property to specify the vertices of the ROI. For programmatically created ROIs, if you want the ROI drawn in a specific axes, you must specify that axes as an input argument. Otherwise, an instance of the images.roi.Freehand class is created but not displayed. In this example, specify the current axes (gca) to draw the ROI on the image in that axes.

h = images.roi.Freehand(gca,Position=[100 150;200 250;300 350;150 450]);

Set Up Listeners for Freehand ROI Events

Read an image into the workspace.

I = imread('cameraman.tif');

Display the image.

imshow(I);

Draw a freehand ROI on the image. Because this example specifies the size and location of the ROI, you do not have to call the draw method to enable interactive drawing.

roi = images.roi.Freehand(gca,'Position',[10 15;100 50;100 150;15 145]);

Set up listeners for ROI moving events. When you move it, the ROI sends notifications of these events and executes the callback function you specify.

addlistener(roi,'MovingROI',@allevents);
addlistener(roi,'ROIMoved',@allevents);

The allevents callback function displays at the command line the previous position and the current position of the Freehand ROI.

function allevents(src,evt)
evname = evt.EventName;
    switch(evname)
        case{'MovingROI'}
            disp(['ROI moving Previous Position: ' mat2str(evt.PreviousPosition)]);
            disp(['ROI moving Current Position: ' mat2str(evt.CurrentPosition)]);
        case{'ROIMoved'}
            disp(['ROI moved Previous Position: ' mat2str(evt.PreviousPosition)]);
            disp(['ROI moved Current Position: ' mat2str(evt.CurrentPosition)]);
    end

end

More About

Events

To receive notification from the ROI when certain events happen, set up listeners for these events. You can specify a callback function that executes when one of these events occurs. When the ROI notifies your application through the listener, it returns data specific to the event. Look at the event class for the specific event to see what is returned.

For an example, see Set Up Listeners for Freehand ROI Events.

Event Name	Trigger	Event Data	Event Attributes
DeletingROI	ROI is about to be interactively deleted.	event.EventData	NotifyAccess: private ListenAccess: public
DrawingStarted	ROI is about to be interactively drawn.	event.EventData	NotifyAccess: private ListenAccess: public
DrawingFinished	ROI has been interactively drawn.	event.EventData	NotifyAccess: private ListenAccess: public
MovingROI	ROI shape or location is being interactively changed.	images.roi.ROIMovingEventData	NotifyAccess: private ListenAccess: public
ROIMoved	ROI shape or location has been interactively changed.	images.roi.ROIMovingEventData	NotifyAccess: private ListenAccess: public
ROIClicked	ROI has been clicked.	images.roi.ROIClickedEventData	NotifyAccess: private ListenAccess: public
AddingWaypoint	A waypoint is about to be interactively added to the ROI.	event.EventData	NotifyAccess: private ListenAccess: public
WaypointAdded	A waypoint has been interactively added to the ROI.	event.EventData	NotifyAccess: private ListenAccess: public
RemovingWaypoint	A waypoint is about to be interactively removed from the ROI.	event.EventData	NotifyAccess: private ListenAccess: public
WaypointRemoved	A waypoint has been interactively removed from the ROI.	event.EventData	NotifyAccess: private ListenAccess: public

Tips

• To draw the ROI interactively using the draw or drawfreehand function, position the cursor on the image and click and drag to draw the ROI shape. To finish drawing the ROI, release the mouse button.

• The ROI supports the following interactivity, including keyboard shortcuts.

  Behavior	Keyboard shortcut
  Cancel drawing the ROI.	Press Esc. The function returns a valid ROI object with an empty Position property.
  Finish drawing (close) the ROI.	Double-click, which adds a point at the pointer position and draws a line connecting this point to the first point drawn, closing the ROI. Right-click, which draws a line connecting the last point to the first point drawn. Position the pointer over the first point and click. Press Enter, which draws a line connecting the last point to the first point drawn.
  Resize (reshape) the ROI.	Position pointer over a waypoint and then click and drag. No assistance (snapping to edges) is available in this mode.
  Add a waypoint.	Position the pointer on an edge of the ROI, right-click, and select Add Waypoint. You can also position the pointer on an edge of the ROI and double-click.
  Remove a waypoint.	Position the pointer on a waypoint, right-click, and select Remove Waypoint.
  Move the ROI.	Position the pointer over the ROI. The pointer changes to the fleur shape. Click and drag to move the ROI.
  Delete an ROI.	Position the pointer on the ROI (not on a vertex), right-click, and select Delete Freehand from the context menu. You can also delete the ROI programmatically using the delete function.

Version History

Introduced in R2018b

R2026a: Specify colors using hexadecimal color codes

You can now specify colors, such as the Color, LabelTextColor, SelectedColor, and StripeColor properties, using hexadecimal color codes.

R2020a: UIContextMenu property is not recommended

Starting in R2020a, using the UIContextMenu property to assign a context menu to an ROI object is not recommended. Use the ContextMenu property instead. The property values are the same.

There are no plans to remove support for the UIContextMenu property at this time.