drawcrosshair

Create customizable crosshair ROI

Syntax

roi = drawcrosshair

roi = drawcrosshair(ax,___)

roi = drawcrosshair(___,Name,Value)

Description

roi = drawcrosshair creates a Crosshair object and enables interactive drawing of the crosshair region-of-interest (ROI) on the current axes. The crosshair ROI is made up of two perpendicular lines that are the full width and height of the axes.

To draw the ROI, move the cursor over the axes and click. For more information about using the ROI, including keyboard shortcuts and context menu options, see Tips.

Note

drawcrosshair is a convenience function for creating a crosshair ROI object. If you create the ROI object directly, by calling Crosshair, you must call the draw object function to enable interactive drawing of the ROI. In contrast, the drawcrosshair function creates the ROI object and calls the draw object function for you. After creating the ROI using either option, to customize its appearance and behavior, you must use object properties, object functions, and event notifications. For more information about using these capabilities, see Tips.

roi = drawcrosshair(ax,___) begins interactive placement of an ROI in the axes specified by ax.

roi = drawcrosshair(___,Name,Value) customizes the appearance and behavior of the ROI using one or more name-value pairs. Unspecified name-value pairs are set to the default value.

Examples

Create Crosshair ROI and Modify ROI Properties

This example shows how to create a crosshair ROI both interactively and programmatically.

Create Crosshair ROI

Read an image into the workspace and display it.

figure;
imshow('pears.png')

Create a crosshair ROI on the image using the drawcrosshair function interactively. Move the cursor over the image anywhere and click to draw the ROI. Click on the ROI Center (the point where the horizontal line crosses the vertical line) to move the ROI on the image.

h = drawcrosshair();

Create Crosshair ROI Programmatically

Read an image into the workspace and display it.

figure;
imshow('pears.png')

Create a crosshair ROI on the image using the drawcrosshair function. Use Name/Value pair arguments to specify the initial position of the ROI.

h = drawcrosshair('Position',[100 100]);

Modify Crosshair ROI Appearance Using Properties

Read an image into the workspace and display it.

figure;
imshow('pears.png')

Create a crosshair ROI on the image using the drawcrosshair function. Use Name/Value pair arguments to specify the initial position of the ROI.

h = drawcrosshair('Position',[100 100]);

Use properties of the Crosshair object to change the lines in the ROI to be striped. The drawcrosshair function returns a Crosshair object that supports many properties. Use the StripeColor property to specify the stripe color.

h.StripeColor = 'green';

Display Value of Pixel Specified By Crosshair ROI

Read an image into the workspace and display it.

img = imread('coins.png');
hAx = gca;
imObj = imshow(img,'Parent',hAx);
imObj.Parent.Visible = 'on';

Create a crosshair ROI.

h = drawcrosshair('Parent',hAx,'Position',[50 50],'LineWidth',1,'Color','y');

Use the addlistener object function to receive notification when the ROI moves. Specify the callback function to execute when the event occurs. When you move the crosshair ROI, the code displays the position of the crosshair in the title and displays the pixel value at the location in the ROI label.

addlistener(h,'MovingROI',@(src,data)displayInfo(src,data,hAx,img));

This is the callback function that displays the value of the pixel specified by the crosshair ROI.

function displayInfo(src,data,hAx,img)
pos = ceil(data.CurrentPosition);
pixval = img(pos(2),pos(1));
src.Label = mat2str(pixval);
title(mat2str(pos));
end

Input Arguments

ax — Parent axes of ROI

gca (default) | Axes object | UIAxes object

Parent axes of the ROI, specified as an Axes object or a UIAxes object. For information about using an ROI in a UIAxes, including important limitations, see Using ROIs in Apps Created with App Designer.

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: roi = drawcrosshair(gca,'Position',[1000 1000]);

'Color' — ROI color

blue (default) | ColorSpec value

ROI color, specified as the comma-separated pair consisting of 'Color' and a MATLAB^® ColorSpec (Color Specification) value.

Example: 'green'

Example: 'g'

Example: [0 1 0]

'ContextMenu' — Context menu

ContextMenu object

Context menu, specified as the comma-separated pair consisting of 'ContextMenu' and a ContextMenu object. Specify this name-value pair to display a custom context menu when you right-click the ROI. To create a custom context menu, create a ContextMenu object, by using the uicontextmenu function, and then configure context menu properties.

'Deletable' — ROI can be interactively deleted via context menu

true (default) | false

ROI can be interactively deleted via a context menu, specified as the comma-separated pair consisting of 'Deletable' and a logical value of true or false. When the value is true (default), you can delete the ROI via the context menu. To disable this context menu item, set 'Deletable' to false. When the value is set to false, you can still delete the ROI by calling the delete function, specifying the handle to the ROI as the input.

'DrawingArea' — Area of axes in which you can draw ROI

'auto' (default) | 'unlimited'

Area of the axes in which you can draw an ROI, specified as the comma-separated pair consisting of 'DrawingArea' and 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 area. x and y specify the upper left corner. w and h specify the width and height of the rectangle.

'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 the comma-separated pair consisting of 'HandleVisibility' and 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.

'InteractionsAllowed' — Interactivity of ROI

'all' (default) | 'none'

Interactivity of the ROI, specified as the comma-separated pair consisting of 'InteractionsAllowed' and one of the values in this table.

Value	Description
'all'	The ROI is fully interactable (default).
'none'	The ROI is not interactable, and no drag points are visible.

'Label' — ROI label

'' (default) | character vector | string

ROI label, specified as the comma-separated pair consisting of 'Label' and a character vector or string. By default, this property is set to an empty array ('') and the ROI has no label.

'LabelVisible' — Visibility of ROI label

'on' (default) | 'hover' | 'off'

Visibility of the ROI label, specified as the comma-separated pair consisting of 'LabelVisible' and one of these values.

Value	Description
'on'	Label is visible when the ROI is visible, and the Label property is nonempty (default).
'hover'	Label is visible only when the mouse hovers over the ROI.
'off'	Label is not visible.

'LineWidth' — Width of ROI edge

three times the number of points per screen pixel (default) | positive numeric scalar

Width of the ROI edge, specified as the comma-separated pair consisting of 'LineWidth' and a positive numeric scalar in points. The default value is three times the number of points per screen pixel.

'Parent' — ROI parent

Axes object | UIAxes object

ROI parent, specified as the comma-separated pair consisting of 'Parent' and an Axes or UIAxes object. For information about using an ROI in a UIAxes, including important limitations, see Using ROIs in Apps Created with App Designer.

'Position' — Position of ROI

1-by-2 numeric array

Position of the ROI, specified as the comma-separated pair consisting of 'Position' and a 1-by-2 numeric array of the form [x y]. The values x and y specify the x- and y-coordinates of the location where the horizontal line crosses the vertical line in the crosshair ROI. This value automatically changes when you draw or move the ROI.

'Selected' — Selection state of ROI

false (default) | true

Selection state of the ROI, specified as the comma-separated pair consisting of 'Selected' and true or false. You can also set this value interactively. For example, clicking the ROI selects it and sets this value to true. Similarly, if you Ctrl-click the ROI, deselects it and sets this value to false.

'SelectedColor' — Color of ROI when selected

'none' (default) | colorspec

Color of the ROI when selected, specified as the comma-separated pair consisting of 'SelectedColor' and a MATLAB ColorSpec (Color Specification). If 'SelectedColor' is set to 'none', then the value of Color defines the color of the ROI for all states, selected or not.

Example: 'green'

Example: 'g'

Example: [0 1 0]

'StripeColor' — Color of ROI stripe

'none' (default) | ColorSpec value

Color of the ROI stripe, specified as the comma-separated pair consisting of 'StripeColor' and a MATLAB ColorSpec (Color Specification) value. If you specify 'StripeColor', the ROI edge is striped. The striping consists of a combination of the value specified by 'Color' or 'SelectedColor' and this value.

Example: 'green'

Example: 'g'

Example: [0 1 0]

'Tag' — Tag to associate with ROI

character vector | string scalar

Tag to associate with the ROI, specified as the comma-separated pair consisting of 'Tag' and a character vector or string scalar. Use the tag value to find the ROI object in a hierarchy of objects by using the findobj function.

'UserData' — Data to associate with ROI

any MATLAB data

Data to associate with the ROI, specified as the comma-separated pair consisting of 'UserData' and any MATLAB data. For example, you can specify a scalar, vector, matrix, cell array, string, character array, table, or structure. MATLAB does not use this data.

'Visible' — ROI visibility

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

ROI visibility, specified as the comma-separated pair consisting of 'Visible' and 'on' or 'off', or as numeric or logical 1 (true) or 0 (false). A value of 'on' is equivalent to true, and 'off' is equivalent to false. Thus, you can use the value of this property as a logical value. The value is stored as an on/off logical value of type matlab.lang.OnOffSwitchState.

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

Output Arguments

roi — Crosshair ROI

Crosshair object

Crosshair ROI, returned as a Crosshair object.

Tips

• This table describes how to perform common tasks with a crosshair ROI.

  Task	Description
  Cancel the drawing operation.	Start drawing the ROI and press Esc before releasing the mouse. The function returns a valid ROI object with an empty Position property.
  Move the ROI.	Position the pointer over the ROI. The pointer changes to a fleur shape. Click and drag to move the ROI.
  Delete the ROI.	Position the pointer over the ROI and right-click to view its context menu. Select Delete Crosshair from the menu. You can also delete the ROI using the delete object function.

• The drawcrosshair function returns a Crosshair ROI object. After creation, use these capabilities of the object to customize the appearance and behavior of the ROI.

  • Object properties — ROI objects support properties that modify their appearance or behavior. For example, to change the color of the roi to yellow, set its Color property, roi.Color = 'yellow'.

  • Object functions — ROI objects support a set of object functions that operate on the ROIs. For example, if you want to pause the MATLAB command line after creating an ROI, call the wait object function.

  • Event notifications — ROI objects can notify your code when certain events, such as being moved or clicked, happen to the ROI. To receive these notifications, use the addlistener object function to set up listeners for specific events. In the listener, specify an event, such as 'ROIMoved', and a callback function that executes when the event occurs. When the ROI notifies your application through the listener, it returns data specific to the event. For example, with the 'ROIMoved' event, the ROI object returns its previous position and its current position.

  For an example of using event listeners with the Crosshair object, see Display Value of Pixel Specified By Crosshair ROI. For more details about these capabilities, see the Crosshair object documentation.

Compatibility Considerations

UIContextMenu name/value pair argument is not recommended

Not recommended starting in R2020a

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

There are no plans to remove support for the UIContextMenu name-value pair argument at this time.