Why is it that Line objects can be instances of two different classes? How can I use findobj to detect only chart primitives?

Using findobj, I have ended up with two arrays of Line graphics handles,

load handleArrays

hL

hL = 
  5x1 Line array:

  Line
  Line
  Line
  Line
  Line

hD

hD = 
  5x1 graphics array:

  Line
  Line
  Line
  Line
  Line

However, the arrays do not appear to belong to a common class,

whos hL hD

  Name      Size            Bytes  Class                                   Attributes

  hD        5x1                40  matlab.graphics.primitive.Data                    
  hL        5x1                40  matlab.graphics.chart.primitive.Line              

Moreover, even though hD clearly has all the same properties as hL, it cannot be accessed with standard indexing, e.g.,

isprop(hD(1),'Color')

ans = logical
   1

hD.Color

Unrecognized method, property, or field 'Color' for class 'matlab.graphics.primitive.Data'.

What is the function of these two different handle graphics types, and how do I tell findobj to retrieve only the hL kind and not the hD kind?

[Malay Agarwal]

Hi Matt,

Could you provide more information on how you are using "findobj" to get the two arrays?

[Matt J]

Hi @Malay Agarwal,

I am using,

h=findobj(ax,'Type','line','-or','Type','functionline');

The current axes was a member of a tiledlayout, but I'm not sure that matters.

[Fangjun Jiang]

Can you clarify how hD and hL are obtained, and what are you expecting? matlab.graphics.chart.primitive and matlab.graphics.primitive are packages, not class.

Answer 1

[Steven Lord]

Handle Graphics objects support the creation of heterogeneous arrays. So for example you can store a figure handle and an axes handle in the same array. [This is more often used to preallocate an array of Handle Graphics objects using gobjects and storing various types of object handles inside.]

f = figure;
ax = axes;
h = [f, ax]

h = 
  1x2 graphics array:

    Figure    Axes  

If you look at the class of the h it is neither Figure nor Axes; it is a common (undocumented) base class from which both Figure and Axes inherit, as per this line from the documentation for heterogeneous arrays I linked to above: "The class of a heterogeneous array is that of the most specific superclass shared by the objects of the array."

class(h)

ans = 'matlab.graphics.Graphics'

What your code shows me is that hD is such a heterogeneous array, something along the lines of h2 in the example below:

L1 = plot(1:10);
hold on
L2 = fplot(@sin);

h2 = [L1; L2]

h2 = 
  2x1 graphics array:

  Line
  FunctionLine

class(h2)

ans = 'matlab.graphics.primitive.Data'

[Matt J]

Thanks, Steve. However, it is a bit odd to me that a heterogeneous array cannot generate comma-separated lists, and throw a very misleading error message that the property does not exist.

f = figure;
ax = axes;

h = [f, ax]

h = 
  1x2 graphics array:

    Figure    Axes  

h(1).Color

ans = 1x3

1 1 1

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

h(2).Color

ans = 1x3

1 1 1

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

h.Color

Unrecognized method, property, or field 'Color' for class 'matlab.graphics.Graphics'.

[Steven Lord]

It's true that the Figure and Axes classes each define a property named Color. But does their common base class define a Color property?

f = figure;
ax = axes;

h = [f, ax];
properties(h)

No properties for class matlab.graphics.Graphics.

No, their common base class (matlab.graphics.Graphics) has no public properties. So the error message was accurate.

Now if we took two objects that inherit from a common base class that does have properties, like two different types of axes:

f2 = figure;
ax2 = polaraxes;

h2 = [ax, ax2]

h2 = 
  1x2 graphics array:

    Axes         PolarAxes

class(h2)

ans = 'matlab.graphics.axis.AbstractAxes'

Does this common base class define any public properties?

properties(h2)

Properties for class matlab.graphics.axis.AbstractAxes:

    Units
    Position
    InnerPosition
    OuterPosition
    PositionConstraint
    TightInset
    Colormap
    Alphamap
    ColorOrder
    ColorOrderIndex
    LineStyleOrder
    LineStyleOrderIndex
    LineStyleCyclingMethod
    NextSeriesIndex
    FontUnits
    FontSize
    FontSizeMode
    TitleFontWeight
    SubtitleFontWeight
    TitleFontSizeMultiplier
    SortMethod
    Clipping
    NextPlot
    Toolbar
    Interactions
    InteractionOptions
    Children
    Parent
    Visible
    HandleVisibility
    Layout

Their common base class has several properties, so we can use one of those to generate a comma-separated list.

h2.Units

ans = 'normalized'

ans = 'normalized'

By default, most of their properties will happen to be the same. One that isn't in this case is Parent.

h2.Parent

ans = 
  Figure (1) with properties:

      Number: 1
        Name: ''
       Color: [1 1 1]
    Position: [671 558 577 433]
       Units: 'pixels'

  Use GET to show all properties

ans = 
  Figure (2) with properties:

      Number: 2
        Name: ''
       Color: [1 1 1]
    Position: [671 558 577 433]
       Units: 'pixels'

  Use GET to show all properties

Answer 2

[Fangjun Jiang]

load handleArrays.mat
get(hD,'Color')

ans = 5x1 cell array
    {[               0 0 0]}
    {[     0 0.4470 0.7410]}
    {[0.8500 0.3250 0.0980]}
    {[0.9290 0.6940 0.1250]}
    {[0.4940 0.1840 0.5560]}

get(hD,'Type')

ans = 5x1 cell array
    {'line'}
    {'line'}
    {'line'}
    {'line'}
    {'line'}

get(hL,'Type')

ans = 5x1 cell array
    {'line'}
    {'line'}
    {'line'}
    {'line'}
    {'line'}

class(hD)

ans = 'matlab.graphics.primitive.Data'

class(hD(1))

ans = 'matlab.graphics.chart.primitive.Line'

class(hL)

ans = 'matlab.graphics.chart.primitive.Line'

class(hL(1))

ans = 'matlab.graphics.chart.primitive.Line'

class([hD,hL])

ans = 'matlab.graphics.primitive.Data'

doc matlab.graphics.primitive.Data

Error using matlab.internal.capability.Capability.require (line 94)
This functionality is not available on remote platforms.

Error in matlab.internal.webwindow (line 161)
Capability.require(Capability.WebWindow);

Error in matlab.internal.doc.ui.CshDocPageHandler/openCshWebWindow (line 87)
cefWindow = matlab.internal.webwindow(url, matlab.internal.getDebugPort());

Error in matlab.internal.doc.ui.CshDocPageHandler/openBrowserForDocPage (line 39)
win = openCshWebWindow(obj, url.string);

Error in matlab.internal.doc.ui.DocPageHandler/openBrowser (line 4)
success = openBrowserForDocPage(obj, url);

Error in matlab.internal.doc.ui.DocPageLauncher/openDocPage (line 30)
success = openBrowser(obj.Handler, getNavigationUrl(activePage));

Error in doc (line 79)
launcher.openDocPage;

In R2023b, it says matlab.graphics.primitive.Data is an undocumented builtin constructor.

[Matt J]

Thanks, it's good to see how the property data can be retrieved, but my question was more about how these primitive.Data objects arise, and how to avoid them.

[Fangjun Jiang]

In R2023b, it says matlab.graphics.primitive.Data is an undocumented builtin constructor.

doc matlab.graphics.primitive.Data

[Fangjun Jiang]

For single handle, you can use .Property, like hD(1).Color. But for array of handles, I always use get(). I never thought hD.Color should work.