Unable to use structName.?ClassName for VideoWriter class

I am finding the structName.?ClassName method of arguments validation does not work when trying to validate name-value arguments for the VideoWriter class.

I have a short function:

function myDemo(videoData,fileName,vwOpts)

arguments

videoData

fileName

vwOpts.?VideoWriter

end

disp(vwOpts);

videoObj = VideoWriter(fileName);

% Open video, write, close, etc.

end % MakeVideo

I would expect that calling this function with VideoWriter properties such as FrameRate would result in vwOpts being a struct that contains values passed to the function. However, I am getting the error:

>> myDemo(0:10,"myVideo.avi","FrameRate",20)

Error using myDemo

Too many input arguments.

I validated my approach by attempting to filter for arguments to the Bar() function by mimicking the matlab.graphics.chart.primitive.Bar example shown in the documentation and was successful.

I noticed I get this same error when failing to specify the full name of the Bar class, so looked for other methods of specifying the VideoWriter class, using matlab.audiovideo.VideoWriter as the full class name, with no success.

What am I missing about this approach? Is there a way of locating the full name of the VideoWriter class, or is there some reason I should not expect this example to work?

Thanks!

Answer 1

[Matt J]

The .?Classname syntax only works on the class properties with public SetAccess. Unfortunately, the VideoWriter class does not have any such properties. FrameRate is also not the name of a property at all:

mc=?VideoWriter; 
pl=mc.PropertyList;

table(string({pl.Name}'), string({pl.SetAccess}'), ...
            'Var', {'Property Name','SetAccess'})

ans = 12×2 table
           Property Name           SetAccess
    ___________________________    _________

    "Duration"                     "private"
    "Filename"                     "private"
    "Path"                         "private"
    "FileFormat"                   "private"
    "IsOpen"                       "private"
    "IsFilenameValidated"          "private"
    "IsWriteVideoCalled"           "private"
    "InternalFramesWritten"        "private"
    "NumFramesWrittenToDisk"       "private"
    "Profile"                      "private"
    "AllowedDataTypes"             "private"
    "FrameIntervalToCheckError"    "none"   

[Elizabeth]

Thanks for the insight. I had seen this before but was confused regarding how the properties listed via the method you show are radically different than the properties shown in a created object:

>> videoObj = VideoWriter("myFileName.avi")

videoObj =

VideoWriter

General Properties:

Filename: 'myFileName.avi'

Path: 'C:\Users\userName'

FileFormat: 'avi'

Duration: 0

Video Properties:

ColorChannels: 3

Height: []

Width: []

FrameCount: 0

FrameRate: 30

VideoBitsPerPixel: 24

VideoFormat: 'RGB24'

VideoCompressionMethod: 'Motion JPEG'

Quality: 75

Methods

After some digging in the VideoWriter class, it appears the properties shown in an actual video object are dynamic properties set from the assigned Profile. Which explains the issue I'm having, but is not exactly helpful for resolving it.

Thanks for your help.

[Matt J]

I think the best you can do is to let VideoWriter()'s argument validation do the work. You can use a try...catch if for some reason you want myDemo() to customize the error messaging,

function myDemo(videoData,fileName,vwNames,vwVals)
    arguments
        videoData
        fileName
    end
    arguments (Repeating)
        vwNames char
        vwVals
    end
    
    nv=[vwNames;vwVals];
    
    disp(struct(nv{:}));
    
    try
    videoObj = VideoWriter(fileName,nv{:});
    catch ME
    end
        
end % MakeVideo

[Elizabeth]

I'm mixing the option inputs to myDemo() with other option sets, which is why I was hoping to use the opts.?VideoWriter syntax. However, I think with your approach I can make a separate copy of the options cell array, remove options that I know aren't applicable, and let VideoWriter do the remaining work. Thanks for the suggestion.

Answer 2

[Chuguang Pan]

I think the options.?ClassName syntax is not supported within arguments block. You should specify each name-value pair individually.

function myDemo(videoData,fileName,vwOpts)
    arguments
        videoData
        fileName
        vwOpts.FrameRate
        % vwOpts.optionname //list distinct VideoWriter option name
        % ...
    end
    disp(vwOpts);
    videoObj = VideoWriter(fileName);
    % Open video, write, close, etc.
end % MakeVideo

myDemo(0:10,"myVideo.avi",FrameRate=20)

    FrameRate: 20

[Elizabeth]

Unfortunately, this use case is exactly the purpose of the options.?ClassName syntax--to automate the handling of properties so changes to ClassName can be seamlessly handled in other classes that use them. As discussed, I successfully used this syntax with the matlab.graphics.chart.primitive.Bar class, so problem is more specific to the class. I suspect it's in how the class is named in the myDemo() function, but I'm not sure because I haven't found the answer yet.