Ellipse

R2026b

Elliptical region of interest

Description

An Ellipse object specifies the shape and position of an axes-based elliptical 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 this ROI, including keyboard shortcuts, see Tips.

Creation

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

• Use the drawellipse 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, or programmatically by using name-value arguments.

• Use the images.roi.Ellipse 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 or programmatically by modifying properties of the object.

Syntax

roi = images.roi.Ellipse

roi = images.roi.Ellipse(ax)

roi = images.roi.Ellipse(___,Name=Value)

Description

roi = images.roi.Ellipse creates an Ellipse object with default property values.

roi = images.roi.Ellipse(ax) creates the ROI on the axes specified by ax.

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

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

AspectRatio — Aspect ratio of ellipse

positive number

Aspect ratio of the ellipse, specified as a positive number. The value of this property changes automatically when you draw or resize the ellipse, or by setting the SemiAxes property. The Ellipse object calculates this value as height/width. The default value is (1+sqrt(5))/2.

Center — Center of ROI

[] (default) | 1-by-2 numeric vector

Center of the ROI, specified as a 1-by-2 numeric vector of the form [x y]. The values x and y are the coordinates of the center point of the ROI. The value of this property changes automatically when you draw or move the ROI.

RotationAngle — Angle around center of ROI

0 (default) | nonnegative numeric scalar

Angle around the center of the ROI, specified as a nonnegative numeric scalar. The angle is measured in degrees in a clockwise direction. The value of this property changes automatically when you draw or move the ROI.

The value of RotationAngle does not impact the value of Position. The Position property represents the initial position of the ROI, before rotation. To determine the location of a rotated ROI, use the Vertices property.

SemiAxes — Lengths of semiaxes of ellipse

1-by-2 numeric vector

Lengths of the semiaxis of the ellipse, specified as a 1-by-2 numeric vector of the form [semiaxis1 semiaxis2]. The Ellipse object assigns the length of the semiaxis that is closest to the x direction to semiaxis1. Note however that the shape and orientation of the ellipse can change through interaction. The value of this property changes automatically when you draw or reshape the ROI.

Vertices — Locations of points on perimeter

Read-only: n-by-2 numeric matrix

This property is read-only.

Locations of points on the perimeter of the ellipse, returned as an n-by-2 numeric matrix.

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.

FixedAspectRatio — Aspect ratio remains constant

false or 0 (default) | true or 1

Aspect ratio remains constant during interaction, specified as a numeric or logical 0 (false) or 1 (true). When the value is true, the aspect ratio remains constant when you draw or resize the ROI. When the value is false, you can change the aspect ratio when drawing or resizing the ROI. You can change the state of this property using the default context menu.

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".

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 Ellipse 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
wait	Block MATLAB command line until ROI operation is finished

Examples

Create Elliptical ROI Non-interactively

Read an image into the workspace and display it.

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

Create an elliptical ROI on the image, using the Center property to specify the location and the SemiAxes property to specify its shape. 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.Ellipse 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.Ellipse(gca,Center=[1000 1000],Semiaxes=[350 150]);

Set Up Listeners for Ellipse ROI Events

Read an image into the workspace.

I = imread('cameraman.tif');

Display the image.

imshow(I);

Draw an elliptical 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.Ellipse(gca,'Center',[100 100],'Semiaxes',[50 80]);

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 current center and the current semiaxes of the Ellipse ROI.

function allevents(src,evt)
evname = evt.EventName;
    switch(evname)
        case{'MovingROI'}
            disp(['ROI moving Current Center: ' mat2str(evt.CurrentCenter)]);
            disp(['ROI moving Current SemiAxes: ' mat2str(evt.CurrentSemiAxes)]);
        case{'ROIMoved'}
            disp(['ROI moved Current Center: ' mat2str(evt.CurrentCenter)]);
            disp(['ROI moved Current SemiAxes: ' mat2str(evt.CurrentSemiAxes)]);
    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 Ellipse 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.EllipseMovingEventData	NotifyAccess: private ListenAccess: public
ROIMoved	ROI shape or location has been interactively changed.	images.roi.EllipseMovingEventData	NotifyAccess: private ListenAccess: public
ROIClicked	ROI has been clicked.	images.roi.ROIClickedEventData	NotifyAccess: private ListenAccess: public

Tips

• To draw the ROI interactively using the draw or drawellipse function, position the cursor on the axes and click and drag to create the shape. To finish 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.
  Fine-tune width of ellipse as you are drawing.	As you draw the ellipse, use the scroll wheel to make small changes to the width of the ellipse.
  Rotate the ROI.	Position the pointer near a vertex. The pointer changes to the rotate pointer. Click and rotate the ROI on its center. To make the rotation snap at 15 degree angles, press Shift as you rotate.
  Maintain aspect ratio while drawing.	Hold the Shift key as you draw. Creates a circular ROI. To lock the aspect ratio, position the pointer on the ROI, right-click, and select Fix Aspect Ratio from the context menu
  Resize (reshape) the ROI.	Position pointer over a vertex and then click and drag. To main the aspect ratio as you resize, hold the Shift key.
  Move the ROI.	Position the pointer over the ROI. The pointer changes to the fleur shape. Click and drag to move the ROI.
  Delete the ROI.	Position the pointer anywhere in the ROI and right-click. Select Delete Ellipse from the context menu. You can also delete the ROI programmatically using the delete function.

• To draw a circular ROI, use the Circle object.

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.