Android Draw Circle Pixel by Pixel

问题:

I am trying to draw a circle on my bitmap, pixel by pixel, using this code:

for(int j = ((int)x - 20); j < ((int)x + 20); j++){
    for(int k = ((int)y - 20); k < ((int)y + 20); k++){

        int colorX = bitmap.getPixel((int)j, (int)k);

        if(Color.red(colorX) == 0 && Color.green(colorX) == 0 && Color.blue(colorX) == 0){

            if(Math.sqrt(((j - (( int ) x )) ^ 2 ) + ((k - (( int ) y )) ^ 2) ) <= 20)
                bitmap.setPixel(j, k, Color.YELLOW);
        }
    }

}

But there is a problem, this is not drawing a circle.. it looks like a triangle..

Can someone help me?

Thanks alot in advance ;)

回答1:

your mistake is a wrong use of ^ , its not a power operator

regarding xfer modes i mentioned in the comment, see this custom view:

class V extends View {
    Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    Xfermode mode = new AvoidXfermode(Color.BLACK, 0, AvoidXfermode.Mode.TARGET);

    public V(Context context) {
        super(context);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        int w = getWidth();
        float h = getHeight() / 3f;
        paint.setXfermode(null);
        paint.setColor(Color.RED);
        canvas.drawRect(0, 0, w, h, paint);
        paint.setColor(Color.BLACK);
        canvas.drawRect(0, h, w, 2*h, paint);
        paint.setColor(Color.GREEN);
        canvas.drawRect(0, 2*h, w, 3*h, paint);

        // draw the circle: it draws only in the middle black strip
        paint.setColor(Color.WHITE);
        paint.setXfermode(mode);
        canvas.drawCircle(w/2, 1.5f * h, h, paint);
    }
}

回答2:

I think you need to use sin/cos formula to draw circle, Following code is written in C++ but It can easily convert to Java/Android.

#include <stdio.h>
#include <graphics.h>
#include <stdlib.h>
#include <conio.h>
#include <bios.h>
#include <math.h>

void DrawCircle(int x, int y, int r, int color)
{
      static const double PI = 3.1415926535;
      double i, angle, x1, y1;

      for(i = 0; i < 360; i += 0.1)
      {
            angle = i;
            x1 = r * cos(angle * PI / 180);
            y1 = r * sin(angle * PI / 180);
            putpixel(x + x1, y + y1, color);
      }
}