a sample example of Shader. according to http://www.curious-creature.org/2012/12/11/android-recipe-1-image-with-rounded-corners/

RoundedDrawable.java

public class RoundedDrawable extends Drawable {
    private static final boolean USE_VIGNETTE = true;
    private RectF mRect = new RectF();
    private final Paint paint;
    private final int mRadius;
    private final BitmapShader bitmapShader;

    public RoundedDrawable(Bitmap bitmap, int radius) {
        bitmapShader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        paint = new Paint();
        paint.setAntiAlias(true);
        paint.setShader(bitmapShader);
        mRadius = radius;
    }

    @Override
    protected void onBoundsChange(Rect bounds) {
        super.onBoundsChange(bounds);
        mRect.set(0, 0, bounds.width(), bounds.height());

        if (USE_VIGNETTE) {
            RadialGradient vignette = new RadialGradient(
                    mRect.centerX(), mRect.centerY() * 1.0f / 0.7f, mRect.centerX() * 1.3f,
                    new int[]{0, 0, 0x7f000000}, new float[]{0.0f, 0.7f, 1.0f},
                    Shader.TileMode.CLAMP);

            Matrix oval = new Matrix();
            oval.setScale(1.0f, 0.7f);
            vignette.setLocalMatrix(oval);

            paint.setShader(
                    new ComposeShader(bitmapShader, vignette, PorterDuff.Mode.SRC_OVER));
        }
    }

    @Override
    public void draw(Canvas canvas) {
        canvas.drawRoundRect(mRect, mRadius, mRadius, paint);
//        canvas.drawCircle(mRect.right / 2, mRect.bottom / 2, mRadius, paint);
    }

    @Override
    public void setAlpha(int alpha) {
        paint.setAlpha(alpha);
    }

    @Override
    public void setColorFilter(ColorFilter cf) {
        paint.setColorFilter(cf);
    }

    @Override
    public int getOpacity() {
        return PixelFormat.TRANSLUCENT;
    }
}