question about pause function,and real time drawing

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alex am 27 Okt. 2014

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Kommentiert: Geoff Hayes am 30 Okt. 2014

Akzeptierte Antwort: Geoff Hayes

Hello!

I use a drawing tablet and i manage to get and save x,y coordinates.

if i want to see what i draw, i use the command

line(x, y, 'Color','k','LineWidth', 1);

If i want to see in real time what i am drawing i use

line(x, y, 'Color','k','LineWidth', 1);

pause(0.001);hold on

The problem is when i use pause, i miss some packets from the tablet,so the drawing is kind of squared and it is different from the non real time drawing.

Any idea what to do at the real time drawing?

Thank you very much!

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Answer 1

Geoff Hayes am 28 Okt. 2014

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Alex - what you are working on sounds cool.

When you call

line(x, y, 'Color','k','LineWidth', 1);

for the real time drawing, do x and y contain all coordinates or only the most recent ones that were sent from the tablet? If the former, then you could try updating the line object instead of creating a new one with each call to line. Presumably you have created a figure (or are drawing on an axes within a GUI), so you could initialize your line as

hLine = line(NaN,NaN, 'Color','k','LineWidth', 1);

Now, when you receive new coordinates, you could just update the current line as

 x = [x ; newX];
 y = [y ; newY];
 set(hLine,'XData',x,'YData',y);

Then, remove the pause and hold statements and replace both with just the drawnow as

 x = [x ; newX];
 y = [y ; newY];
 set(hLine,'XData',x,'YData',y);
 drawnow;

The above might speed things up a bit so that you are less likely to miss packets. Try it and see!

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Geoff Hayes am 29 Okt. 2014

The line of code

set(hLine,'XData',x,'YData',y);

replaces the x and y data for the line graphics object (referenced by hLine) with the new set of x and y data, and you call this line whenever the pentouschestablet is true. When it is false (or rather when pennottouschestablet is true), the code resets x and y to the empty matrices, and the next time the pen touches the tablet, we call

set(hLine,'XData',x,'YData',y);

where x and y now only have the first position of where the pen touches the tablet. All information that we had before (for the previous letter) is replaced with the new point (and future points) for the new letter.

What you will need to do, is to create another graphics object for the next letter as

 if pennottouschestablet
     x=[];
     y=[];
     hLine = line(NaN,NaN, 'Color','k','LineWidth', 1);
 end

With the above, every time the pen leaves the tablet and then returns to it, we will use a new (line) graphics object to draw a shape, letter, etc..

(With the old code, it kept creating new line graphics objects so you never lost any data...you just ended up with perhaps more graphics objects than necessary.)

So try adding the third line to your second if body, and see what happens. I don't think that you will need to do a hold on for your axes (a quick test showed that I didn't for line objects) so it should work.

It may be worthwhile creating an array of these line handles, just so you have the ability to reference them in the future (say if you want to undo the last character/letter drawn, then just delete the handle for it and it will disappear from your axes). You could probably do something like

 hLines = line(NaN,NaN, 'Color','k','LineWidth', 1);
 while 1
     if pentouschestablet
        x = [x ; newX];
        y = [y ; newY];
        set(hLines(end),'XData',x,'YData',y);
        drawnow;
     end
     if pennottouschestablet
         x=[];
         y=[];
         hLines = [hLines line(NaN,NaN, 'Color','k','LineWidth', 1)];
     end
 end

Note how we just use hLines(end) to get the last handle in our array, and we just concatenate hLines with the new handles whenever the pen leaves the tablet.

Out of curiosity, what happens if there is a five second delay between when the pen leaves the tables and when it touches it again? Does the while loop continue to run? I only ask, because it it does, then we will be creating new handles when we don't need to. You may want to guard against this by assuming that if x and y are empty, then there is no need to create a new handle

      if pennottouschestablet
         if ~isempty(x)
             x=[];
             y=[];
             hLines = [hLines line(NaN,NaN, 'Color','k','LineWidth', 1)];
         end
     end