Android Paint: how to get "airbrush" effect?

Question

I am following the "FingerPaint" demo in the API Demos.

I would need to get an "airbrush" effect, in the sense that when I draw over the same spot it gets darker and darker.

Please see the image:

as you can see the center is darker because I passed with the paint on the same spot more than one time.

Please how do I get the same effect, of getting darker a spot if drawn over more than one time?

EDIT EDIT EDIT

the suggested

mPaint.setAlpha(0x80) 

kind of work, but only if I release touch and then touch again, if I do not release and keep the finger on the screen the effect is not reached.

The point is that you do not reach the effect if you do not release your finger from the screen, if you keep on drawing without releasing the touch it doesn't get darker when paint over. If you release the touch and then draw again you get the effect

This is the result I get. And I do not want:

this would be the desired result:

---

this is is the code taken form the API Demos:

public class FingerPaint extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(new MyView(this));

    mPaint = new Paint();
    mPaint.setAntiAlias(true);
    mPaint.setDither(true);
    mPaint.setColor(0x44FF0000);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeJoin(Paint.Join.ROUND);
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeWidth(12);

}

private Paint mPaint;

public class MyView extends View {

    private static final float MINP = 0.25f;
    private static final float MAXP = 0.75f;

    private Bitmap mBitmap;
    private Canvas mCanvas;
    private Path mPath;
    private Paint mBitmapPaint;

    public MyView(Context c) {
        super(c);

        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(0xFFAAAAAA);

        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

        canvas.drawPath(mPath, mPaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            touch_start(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_MOVE:
            touch_move(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_UP:
            touch_up();
            invalidate();
            break;
        }
        return true;
    }
}

}

Answer 1

I made only few minor changes in your code.

 mPaint.setColor(Color.BLACK);// changed color to balck
 mPaint.setAlpha(0x80); // only change    

Activity class

public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(new MyView(this));

    mPaint = new Paint();
    mPaint.setAntiAlias(true);
    mPaint.setDither(true);
    mPaint.setColor(Color.BLACK);
    mPaint.setAlpha(0x80); // only change
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeJoin(Paint.Join.ROUND);
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeWidth(12);

}

private Paint mPaint;

public class MyView extends View {

    private static final float MINP = 0.25f;
    private static final float MAXP = 0.75f;

    private Bitmap mBitmap;
    private Canvas mCanvas;
    private Path mPath;
    private Paint mBitmapPaint;

    public MyView(Context c) {
        super(c);

        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(0xFFAAAAAA);

        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

        canvas.drawPath(mPath, mPaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            touch_start(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_MOVE:
            touch_move(x, y);
            invalidate();
            break;
        case MotionEvent.ACTION_UP:
            touch_up();
            invalidate();
            break;
        }
        return true;
    }
}

}

snap shot

Answer 2

This approach is more of a simulation the way something like Photoshop would do it: Integrate along the path and draw individual paint splashes with an adjustable spacing inbetween.

public class DrawView extends View {
public Paint mPaint;

private Bitmap mBitmap;
private Canvas mCanvas;

private int strokeRadius;
private ShapeDrawable mBrush;
private Paint mBitmapPaint;

private float mPreviousX, mPreviousY;

public DrawView(Context context, AttributeSet attrs) {
    super( context,  attrs);

    mBitmapPaint = new Paint(Paint.DITHER_FLAG);

    int strokeWidth = 20;

    strokeRadius = strokeWidth/2;

    Shape brushShape = new OvalShape();

    mBrush = new ShapeDrawable(brushShape);

    Paint paint = mBrush.getPaint();

    // radial gradient shader with a transparency falloff, if you don't want this,
    // just set a color on the paint and remove the setShader call
    Shader shader = new RadialGradient(strokeRadius, strokeRadius, strokeRadius,
            Color.argb(255, 0, 0, 0), Color.argb(0, 0, 0, 0), Shader.TileMode.CLAMP);

    paint.setShader(shader);
    paint.setAlpha(0x10);
}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);
    mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
    mCanvas = new Canvas(mBitmap);
}

@Override
protected void onDraw(Canvas canvas) {
    canvas.drawColor(0xFF00B8F5);

    canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
}

private void touch_start(float x, float y) {
    mPreviousX = x;
    mPreviousY = y;
}

private void touch_move(MotionEvent event)
{
    float x = event.getX();
    float y = event.getY();

    // get vector from previous to current position
    float xdist = x - mPreviousX;
    float ydist = y - mPreviousY;

    // get the length
    float segmentLength = (float) Math.sqrt(xdist * xdist + ydist * ydist);

    // derive a suitable step size from stroke width
    float stepSize = Math.max(strokeRadius / 10, 1f);

    // calculate the number of steps we need to take
    // NOTE: this draws a bunch of evenly spaced splashes from the start point
    // to JUST BEFORE the end point. The end point will be drawn by the start point of the
    // next stroke, or by the touch_up method. If we drew both the start and
    // end point there it would be doubled up
    int steps = Math.max(Math.round(segmentLength / stepSize), 2);

    for(int i = 0; i < steps; ++i)
    {
        int currentX = (int) (mPreviousX + xdist * i / steps);
        int currentY = (int) (mPreviousY + ydist * i / steps);

        drawSplash(currentX, currentY);
    }

    // update the previous position
    mPreviousX = x;
    mPreviousY = y;
}

private void touch_up(MotionEvent event) {
    drawSplash((int) event.getX(), (int)event.getY());
}

/**
 * draws the brush to the canvas, centered around x and y
 * @param x
 * @param y
 */
private void drawSplash(int x, int y)
{
    mBrush.setBounds(x - strokeRadius, y - strokeRadius, x + strokeRadius, y + strokeRadius);

    mBrush.draw(mCanvas);

}

@Override
public boolean onTouchEvent(MotionEvent event) {
    float x = event.getX();
    float y = event.getY();

    switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN:
        touch_start(x, y);
        invalidate();
        break;
    case MotionEvent.ACTION_MOVE:
        touch_move(event);
        invalidate();
        break;
    case MotionEvent.ACTION_UP:
        touch_up(event);
        invalidate();
        break;
    }
    return true;
}
}

Edit: snap shot (Raghunandan). Result Testing with White Background and Black Color paint.