How to draw with an "inverted" paint in Android Canvas?

Question

I draw some stuff on a canvas, over I want to draw a circle in inverted color :

canvas.drawCircle(zx, zy, 8f, myPaint);

How to configure myPaint for circle pixel to be in the inverted color of the underlying pixels ?

Thanks

Trapes

Answer 1

try this

float mx [] = {
             -1.0f,  0.0f,  0.0f,  1.0f,  0.0f,
             0.0f,  -1.0f,  0.0f,  1.0f,  0.0f,
             0.0f,  0.0f,  -1.0f,  1.0f,  0.0f,
             1.0f,  1.0f,  1.0f,  1.0f,  0.0f 
    };
ColorMatrix cm = new ColorMatrix(mx);

p.setColorFilter(new ColorMatrixColorFilter(cm));

canvas.drawCircle(zx, zy, 8f, p);

Cornstarch

I tried with no success :( I get a black circle, whatever the under picture is. – Trapes

Sorry, I will try to find other solution. – Cornstarch

No problem. It seems like the background color is not taken into account, even when using things like setColorFilter(new PorterDuffColorFilter(Color.BLACK, PorterDuff.Mode.XOR)) or setXfermode(new PorterDuffXfermode(PorterDuff.Mode.XOR)) – Trapes

Now I get a static white circle. Do I need to set another âraùeter? Thanks – Trapes

Answer 2

I'd say a color matrix for inverting should look like this:

float mx [] = {
         -1.0f,  0.0f,  0.0f,  0.0f,  255.0f,
         0.0f,  -1.0f,  0.0f,  0.0f,  255.0f,
         0.0f,  0.0f,  -1.0f,  0.0f,  255.0f,
         0.0f,  0.0f,  0.0f,  1.0f,  0.0f 
};

Here is more information for the matrix:

Mosasaur