How can I adjust the contrast of a live video?

I am using the Image Acquisition Toolbox to create a simple interactive GUI that should allow the user to preview the image and dynamically adjust the contrast of the video feed. I can get it to work in the command line using the following code:

vid = videoinput('winvideo',1);
src = getselectedsource(vid);
preview(vid);
src.Contrast = 200;

This works an allows me to adjust the contrast. For some reason, when I implement this in my GUI, MATLAB does not allow me to set the video input. I am trying this inside the slider function call:

vid = videoinput('winvideo',1);
src = getselectedsource(vid);
con = get(handles.contrast, 'Value');%gets slider value
src.Contrast = con;

When I try this, MATLAB says that I cannot adjust the contrast when the video is running.

"Error using imaqchild/subsasgn (line 34) Contrast: Property can not be set while Running is set to 'on'."

Does anyone have any idea how to get around this issue? It seems like the value can be changed while it's running in the preview window.

Thanks for any help.

[Sanjay Yadav]

I'm using the same logic but its not working out for me. Can you share your code if you have it yet?

Answer 1

[David Tarkowski]

There isn't really a good way to get around this. What happens when you set the contrast while the preview is running is that the camera stream is automatically stopped, the contrast is adjusted, and then the stream is then automatically restarted. When the object is running, the toolbox will not automatically stop the stream.

If you want to let the user change the contrast while the acquisition is running, you'll have to manually call stop before setting the contrast, and the call start to restart the object. This will cause any acquired data which has not been returned with getdata to be discarded.

[Marc]

That's kind of what I figured was going on based on the output of imaqtool. Thanks.

Answer 2

[Image Analyst]

Unlike the old analog frame grabbers, with digital cameras you often can't adjust contrast in very flexible ways. For example you can't have an offset (min value) or max value or slope. About the only thing you can do, if the camera adapter allows it, is to set the gamma.