While calling the custom preview function, use the prefix 'app.'. Also, while defining the 'vid' variable, define it as a property. This ensures that it can be accessed anywhere.
app.vid = videoinput('winvideo',1);
% Create blank image
hImage = image(app.UIAxes,zeros(720,1280,3)); %If image resolution is 1280x720
pause(2)
% These lines set proper aspect ratio
app.UIAxes.XLim = [0,1280];
app.UIAxes.YLim = [0,720];
app.UIAxes.XTick = [];
app.UIAxes.YTick = [];
pbaspect(app.UIAxes,[1280,720,1])
preview(app.vid,hImage)
% Set up the update preview window function.
setappdata(hImage, 'UpdatePreviewWindowFcn', @app.preview_fcn);
% Make handle to text label available to update function.
setappdata(hImage,'HandleToTimestampLabel',app.TimeStamp);
Refer to the original answer for further reference:
Details about the custom function “preview_fcn” can be found out from the link below:
