Changing stroke color dynamically

Question

I am using stylus input to draw lines in a canvas. I want to change the color of the stroke with the pen pressure. so I used:

DrawingAttributes dattribute = new DrawingAttributes();
inkcan.EditingMode = InkCanvasEditingMode.Ink;

if (stylusInput.pressureFactor < 0.5)
dattribute.Color = Colors.Red;
else
dattribute.Color = Colors.Blue;

inkcan.DefaultDrawingAttributes = dattribute;

but I have found that the color changes only when I lift off and retouch the pen to tablet surface. I am not sure how to fix that problem.

Any help would be greatly appreciated.

I just tried with key down, but is has the same problem. the color only changes if I lift off the pen and reintroduce to the tablet. it doesn't change dynamically with the pressure. — MKS, Aug 03 '11 at 15:50

Emond · Accepted Answer · 2020-02-27T10:34:54.113

Look at this question: InkCanvas Eraser

In the MSDN it states:

If you change the EraserShape, the cursor rendered on the InkCanvas is not updated until the next EditingMode change.

The effect you are experiencing might be caused by the EditingMode being changed when you pull the pen off the canvas and put it back down.

If so, you could toggle the EditingMode property as I suggested in the linked answer.

EDIT

Have a look at this a 3rd down it says:

Off course, life is never as simple as that, so there is one more little issue to handle. Apparently, the InkCanvas uses different renderers for the final result compared to while the strokes are being drawn. To show a transparency based on the pressure while the drawing action is still in progress, we need to use the protected property called DyamicRenderer which gets/sets the object used to render the strokes on a drawing context while the stroke is being drawn. This rendering object must be a descendent of DynamicRenderer. All you need to do here is override the OnDraw method and change the brush that is used. When you assign a new value to this property, the InkCanvas actually changes an internal 'PlugIn list' which is called whenever data is entered using the stylus.

This might be it.

I think your answer is the correct one for my problem. I tried with your code, but it doesn't work properly. When I reset the EditingMode, it doesn't show any ink (pen / color / drawing) in the canvas. I need to lift off my pen, and then it display the ink again. I am using the following code: var editMode = inkcan.EditingMode; inkcan.EditingMode = InkCanvasEditingMode.None; inkcan.EditingMode = editMode; — MKS, Aug 03 '11 at 17:31
@MKS, are you changing the color inbetween these lines like this: var editMode = inkcan.EditingMode; inkcan.EditingMode = InkCanvasEditingMode.None; dattribute.Color = Colors.Blue; inkcan.EditingMode = editMode; — Emond, Aug 03 '11 at 17:37
yes... those are working properly (pressure value), but only the problem is color doesn't change with the pressure value. If I lift off the Pen and reintroduce to tablet surface, then it only works. — MKS, Aug 03 '11 at 17:42

score 1 · Answer 2 · answered Aug 03 '11 at 15:53

The if condition is evaluated only once, so there is no reason for the colour to change while you are drawing. Unfortunately, there seems to be no "onpressurechanged" event, so you would probably have to set up a loop that checks the pressure every x milliseconds and adjusts the colour accordingly. Since you don't want to freeze the UI, you would either need to run it in another thread and delegate the colour change back to the UI thread, or you can queue the pressure checks onto the window dispatcher with "applicationIdle" priority. This would look something like:

void checkPressure(InkCanvas inkcan)
{
//return if touch is lifted code here

DrawingAttributes dattribute = new DrawingAttributes();
if (stylusInput.pressureFactor < 0.5)
dattribute.Color = Colors.Red;
else
dattribute.Color = Colors.Blue;
inkcan.DefaultDrawingAttributes = dattribute;
this.Dispatcher.BeginInvoke(new MyPressureDelegate(checkPressure), DispatcherPriority.ApplicationIdle, inkcan);
}

assuming your stylusInput shares scope with the function, of course. Otherwise you'd need to pass it in along with the canvas in an object array.

I already have a method called updateState() and I called that method when any kind of stylus movement (StylusMove,StylusDown, StylusUp). I place my code under that method and check whether it is getting correct pressure value (bellow or above 0.5). I have found that it display the pressure value correctly, but it doesn't change the ink color. — MKS, Aug 03 '11 at 17:07

Changing stroke color dynamically

2 Answers2