preview live data from image acquisition device -凯发k8网页登录

introduction

after you connect matlab^® to the image acquisition device you can view the live video stream using the video preview window. previewing the video data can help you make sure that the image being captured is satisfactory.

for example, by looking at a preview, you can verify that the lighting and focus are correct. if you change characteristics of the image, by using video input object and video source object properties, the image displayed in the video preview window changes to reflect the new property settings.

the following sections provide more information about using the video preview window.

instead of using the toolbox's video preview window, you can display the live video preview stream in any handle graphics^® image object you specify. in this way, you can include video previewing in a gui of your own creation. the following sections describe this capability.

opening a video preview window

to open a video preview window, use the preview function. the video preview window displays the live video stream from the device. you can only open one preview window per device. if multiple devices are used, you can open multiple preview windows at the same time.

the following example creates a video input object and then opens a video preview window for the video input object.

vid = videoinput('winvideo');
preview(vid);

the following figure shows the video preview window created by this example. the video preview window displays the live video stream. the size of the preview image is determined by the value of the video input object's roiposition property. the video preview window displays the video data at 100% magnification.

in addition to the preview image, the video preview window includes information about the image, such as the timestamp of the video frame, the video resolution, the frame rate, and the current status of the video input object.

stopping the preview video stream

when you use the preview function to start previewing image data, the video preview window displays a view of the live video stream coming from the device. to stop the updating of the live video stream, call the stoppreview function.

this example creates a video input object and opens a video preview window. the example then calls the stoppreview function on this video input object. the video preview window stops updating the image displayed and stops updating the timestamp. the status displayed in the video preview window also changes to indicate that previewing has been stopped.

vid = videoinput('winvideo');
preview(vid)
stoppreview(vid)

to restart the video stream in the video preview window, call preview again on the same video input object.

preview(vid)

closing a video preview window

to close a particular video preview window, use the function, specifying the video input object as an argument. you do not need to stop the live video stream displayed in the video preview window before closing it.

closepreview(vid)

to close all currently open video preview windows, use the closepreview function without any arguments.

closepreview

previewing data in custom guis

instead of using the toolbox's video preview window, you can use the preview function to direct the live video stream to any handle graphics image object. in this way, you can incorporate the toolbox's previewing capability in a gui of your own creation. (you can also perform custom processing as the live video is displayed. for information, see performing custom processing of previewed data.)

to use this capability, create an image object and then call the preview function, specifying a handle to the image object as an argument. the preview function outputs the live video stream to the image object you specify.

the following example creates a figure window and then creates an image object in the figure, the same size as the video frames. the example then calls the preview function, specifying a handle to the image object.

% create a video input object.
vid = videoinput('winvideo');
% create a figure window. this example turns off the default
% toolbar, menubar, and figure numbering.
figure('toolbar','none',...
       'menubar', 'none',...
       'numbertitle','off',...
       'name','my preview window');
% create the image object in which you want to display 
% the video preview data. make the size of the image
% object match the dimensions of the video frames.
vidres = vid.videoresolution;
nbands = vid.numberofbands;
himage = image( zeros(vidres(2), vidres(1), nbands) );
% display the video data in your gui.
preview(vid, himage);

when you run this example, it creates the gui shown in the following figure.

performing custom processing of previewed data

when you specify an image object to the preview function (see previewing data in custom guis), you can optionally also specify a function that preview executes every time it receives an image frame.

to use this capability, follow these steps:

function mypreview_fcn(obj,event,himage)
% example update preview window function.
% get timestamp for frame.
tstampstr = event.timestamp;
% get handle to text label uicontrol.
ht = getappdata(himage,'handletotimestamplabel');
% set the value of the text label.
ht.string = tstampstr;
% display image data.
himage.cdata = event.data

% create a video input object.
vid = videoinput('winvideo');
% create a figure window. this example turns off the default
% toolbar and menubar in the figure.
hfig = figure('toolbar','none',...
       'menubar', 'none',...
       'numbertitle','off',...
       'name','my custom preview gui');
% set up the push buttons
uicontrol('string', 'start preview',...
    'callback', 'preview(vid)',...
    'units','normalized',...
    'position',[0 0 0.15 .07]);
uicontrol('string', 'stop preview',...
    'callback', 'stoppreview(vid)',...
    'units','normalized',...
    'position',[.17 0 .15 .07]);
uicontrol('string', 'close',...
    'callback', 'close(gcf)',...
    'units','normalized',...
    'position',[0.34 0 .15 .07]);
% create the text label for the timestamp
htextlabel = uicontrol('style','text','string','timestamp', ...
    'units','normalized',...
    'position',[0.85 -.04 .15 .08]);
% create the image object in which you want to
% display the video preview data.
vidres = vid.videoresolution;
imwidth = vidres(1);
imheight = vidres(2);
nbands = vid.numberofbands;
himage = image( zeros(imheight, imwidth, nbands) );
% specify the size of the axes that contains the image object
% so that it displays the image at the right resolution and
% centers it in the figure window.
figsize = get(hfig,'position');
figwidth = figsize(3);
figheight = figsize(4);
gca.unit = 'pixels';
gca.position = [ ((figwidth - imwidth)/2)...
               ((figheight - imheight)/2)...
               imwidth imheight ];
% set up the update preview window function.
setappdata(himage,'updatepreviewwindowfcn',@mypreview_fcn);
% make handle to text label available to update function.
setappdata(himage,'handletotimestamplabel',htextlabel);
preview(vid, himage);