How to start recording by pressing a certain key, without involving figures?

I'm collecting frames from a camera, trying to start and stop the data recording by pressing certain keys (eg. once I press "b", the recording starts). However, the functions I found until now, such as WindowKeyPressFcn and waitforbuttonpress both involves a figure, which is not part of my recording. I also tried getkey, which didn't seem to work.

Does anyone have ideas about how I can get to my aim without creating an extra figure? Your help is very appreciated!

My previous attempt is to use userInput and control the recording by typing "start" and "stop", but it apperently also waits for user input to record any frames, hence 0 frame was recorded when I stop it.

fprintf('Recording %s. Type "start" to begin recording.\n', taskname);

while true
    userInput = input('', 's');
    if strcmpi(userInput, 'start')
        ...
        
        tic;

        fprintf('Recording in progress. Type "stop" to end recording.\n');
        while true
            userInput = input('', 's');
            if strcmpi(userInput, 'stop')
                break;
            end
            ...

        s = toc;
        fprintf('Recording stopped for %s.\n', taskname);
        fprintf('%d frames\n', frames_num);
        fprintf('%f seconds\n', s);
        fprintf('%f fps\n', frames_num / s);

       ...
        
        break;
    else
        fprintf('Invalid input. Type "start" to begin recording.\n');
    end
end

Answer 1

[Pratyush Swain]

Hi Julia,

I see that you are looking for a non-blocking or non-waiting approach to start and stop your data recording process and you do not want to involve any figures.

One workaround for this issue would to use a library for detecting keyboard inputs:

You can refer to Matlab Input library fro keyboard from HebiRobotics on File Exchange: https://in.mathworks.com/matlabcentral/fileexchange/61306-hebirobotics-matlabinput.

It can be used in the following manner :

% Initialize Object for HebiKeyboard %
kb = HebiKeyboard();
while true
    % Read keystroke %
    state = read(kb);
    fprintf('Waiting for Input. Hit SPACE to start recording . \n');
    % If pressed button is SPACE - start recording %
    if state.SPACE
        tic;
        fprintf('Recording in progress. Hit ESC to end recording.\n');
        % Once recording is in progess, exit only on ESC key %
        while true
          state = read(kb);
          % Exit Recording on ESC Key %
          if state.ESC
              disp("RECORDING STOPS");
              break;
          end
        end
        % Exit while loop once recording process is done %
        s = toc;
        break;
        
    end    
end

The whole process is non-blocking and doesn't require creation/focus on any particular figure window. In the above demonstration I have assumed "SPACE" and "ESC" buttons for starting and stopping the recording process, the state struct also has 12 options for checking different key strokes as follows:

You can also use other keyboard characters in the following manner:

% Checking for 'b' to start recording - as mentioned in your query %
if find(state.keys('b'))
    % Recording in now in progress, proceed to next steps.
    ...
end

The installation for the same is also fairly simple:

1- Download latest source file(zip/tar) from https://github.com/HebiRobotics/MatlabInput/releases

2- Extract the folder containing the files and do add to path (including folder & subfolders) on MATLAB Desktop.

3- Use HebiKeyboard() in your code file.

For more information on HebiKeyboard Library, please refer to https://github.com/HebiRobotics/MatlabInput.

For other alternatives on Non-blocking keyboard reads, you can refer to:

1- https://in.mathworks.com/matlabcentral/fileexchange/30622-non-wait-detection-of-keyboard-input-including-mex-implementation

2- https://in.mathworks.com/matlabcentral/answers/291975-any-function-like-kbhit

Hope this helps.