Ellipse

Ellipse annotation

Since R2026a

Description

An Ellipse object labels an ellipse in a Viewer object. The properties of the object control the appearance and behavior of the ellipse annotation.

Viewer-based ROIs provide high-performance interactions with images and volumes you display in a Viewer object using imageshow and volshow, respectively. To draw an ellipse on an imshow image display, use drawellipse instead.

Creation

You can create an Ellipse object in these ways:

• Use the uidraw function to interactively draw the ROI in a specified viewer and create the ROI object.

• Use the images.ui.graphics.roi.Ellipse function described here. After you create the ROI, you can display it by adding the object to the Annotations property of a Viewer object.

Syntax

e = images.ui.graphics.roi.Ellipse

e = images.ui.graphics.roi.Ellipse(Name=Value)

Description

e = images.ui.graphics.roi.Ellipse creates an Ellipse object with default property values. Use e to query and modify properties of the Ellipse object after you create it.

e = images.ui.graphics.roi.Ellipse(Name=Value) sets one or more writable properties using name-value arguments.

Example: images.ui.graphics.roi.Ellipse(Position=[128 64 30 20)] creates an ellipse annotation centered at the xy-coordinate (128, 64). The ellipse has horizontal and vertical semiaxes of length 30 and 20, respectively.

Properties

Shape and Position

Position — Position of ROI

[0 0 0 0 0] (default) | 5-element numeric vector | 4-element numeric vector

Position of the ROI, specified as a 5-element numeric vector of the form [x y semix semiy theta] or a 4-element numeric vector of the form [x y semix semiy]. The center of the ellipse is at the xy-coordinate (x, y). The ellipse has horizontal and vertical semiaxes of lengths semix and semiy, respectively, before rotation. If you specify Position as a 5-element vector, the ellipse is rotated by theta degrees clockwise about its center.

The object always stores this property value as a 5-element row vector.

Center — Center coordinate of ROI

[0 0] (default) | 2-element numeric vector

Center coordinate of the ROI, specified as a 2-element numeric vector of the form [x y].

RotationAngle — Rotation angle

0 (default) | numeric scalar

Rotation angle around the center of the ellipse, specified as a numeric scalar. The angle is measured in degrees in a clockwise direction.

The value of RotationAngle does not impact the values in the SemiAxes property. The SemiAxes property represents the lengths of the semiaxes before rotation.

SemiAxes — Lengths of semiaxes

[0 0] (default) | 2-element numeric vector

Lengths of the semiaxes in the horizontal and vertical directions, specified as a 2-element numeric vector of the form [semix semiy].

Area — Area covered by ROI

Read-only: numeric scalar

This property is read-only.

Area covered by the ROI, represented as a numeric scalar.

Perimeter — Perimeter of ROI

Read-only: numeric scalar

This property is read-only.

Perimeter of the ROI, represented as a numeric scalar.

Color and Styling

Color — ROI color

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.

These are the RGB triplets for the default colors used in 2-D and 3-D Viewer scenes.

RGB Triplet	Appearance	Default Use
[0 0.5610 1]		This is the default color when creating an ROI in a 2-D Viewer object from the toolbar or by using the uidraw function.
[0.8660 0.3290 0]		This is the default color when creating an ROI in a 3-D Viewer object from the toolbar or by using uidraw. This color is also the default when you create an ROI in any Viewer object by using an object constructor function such as images.ui.graphics.roi.Circle.

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 (default) | scalar in range [0, 1]

Transparency of the ROI face, specified as a scalar 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.

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.

Label — ROI label

string scalar | character vector | ""

ROI label, specified as a string scalar or character vector. By default, the label shows the lengths of the horizontal and vertical semiaxes. If the parent Viewer has a nondefault value for the SpatialUnits property, such as "mm", then the label includes the specified units.

To display a custom label, specify the label to display as a string scalar or character vector. To display no label, specify the Label value as "".

Interactivity

Interactions — Interactivity of ROI

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

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 drag points on the boundary of the ROI, or translate (move) the entire ROI.
"none"	You cannot interact with the ROI or access the context menu. No drag points are visible.
"reshape"	You can reshape the ROI by changing the position of individual drag points on the boundary of the ROI.
"translate"	You can translate the entire ROI within the drawing area. No drag points are visible, and you cannot reshape the ROI.
"click"	You can click the ROI, but you cannot translate the ROI interactively. After you click the ROI, the parent Viewer broadcasts the AnnotationMoved event.

FacePickable — ROI face supports interactions

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

ROI face supports interactions, specified as "on" or "off", or as a numeric or logical 1 (true) or 0 (false). 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"	The ROI face supports interactions. If Interactions is "all" or "translate", then you can interact with the ROI by clicking the face and dragging the ROI.
"off"	You cannot interact with the face of the ROI.

Identifiers

Parent — ROI parent

Viewer object

ROI parent, specified as a Viewer object. You can create a Viewer object by using the viewer2d function.

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. This property has no effect on the display, and the viewer does not use this property. You can use this property to track annotations for app building.

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. This property has no effect on the display, and the viewer does not use this property. You can use this property to append metadata to the ROI for app building.

Object Functions

createMask

Create binary mask image from ROI

Examples

Create Ellipse Annotation Object

Display an image in a Viewer object. The viewer v is the parent of the image object created by imageshow.

img = imageshow("pears.png");
v = img.Parent;

Create an elliptical ROI on the image, using the Position property to specify the position of the ROI in the form [x y semix semiy theta].

position = [446 204 103 75 292];
e = images.ui.graphics.roi.Ellipse(Position=position);

Display the ROI by adding it to the Annotations property of the Viewer object.

v.Annotations = [v.Annotations; e];

Tips

• The uidraw and images.ui.graphics.roi.Ellipse functions create ROIs to display in a Viewer object. Viewer-based ROIs provide high-performance interactions with images and volumes you display in a Viewer object using imageshow and volshow, respectively. To draw an ellipse on an imshow image display, use drawellipse instead.

• The ROI supports this interactivity, including keyboard shortcuts.

  Behavior	Interaction
  Add annotation	To enable interactive drawing of an ellipse, call the uidraw function and specify the second argument as "ellipse". To draw the ellipse, click to place a point on the perimeter of the ellipse, drag to the opposite point on the perimeter, and release. To adjust the center, hold Ctrl while you drag.
  Make semiaxes directions snap at 15 degree angles	Hold Shift while drawing.
  Resize (reshape) annotation	Pause on the edge of the ROI until one of the four vertices defining the semiaxes appears. Drag the vertex to reposition it. Moving the vertex closer to or further from the center changes the corresponding semiaxis value. Moving the point along the perimeter rotates the ellipse.
  Move annotation	Hold Ctrl and drag the ROI.
  Edit annotation label	Right-click the edge of the ROI. From the context menu, select Edit label. When the label text highlights, type new text for the label. To finish editing, click outside the label.
  Remove annotation	Right-click the edge of the ROI and choose Remove annotation from the context menu. Select Remove all annotations to remove all annotations in the viewer.

Version History

Introduced in R2026a