效果图：
15689482562051.png
主要通过修改图片的alpha通道值实现，离图片中心点越近alpha值越大，最大255，距离中心点越远alpha值越小最小0
public Bitmap changeAlpha(Bitmap bitmap){
int w = bitmap.getWidth();
int h = bitmap .getHeight();
Bitmap result = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_4444);
int ratio = w > h ? h * 32768 / w : w * 32768 / h;
int r,g,b,a,color;
int[] oldPx = new int[w * h];
int[] newPx = new int[w * h];
float Size =0.5f;
int cx = w >> 1;
int cy = h >> 1;
int max = cx * cx + cy * cy;
int min = (int)(max * (2 - Size));
int diff = max - min;
bitmap.getPixels(oldPx, 0, w, 0, 0, w, h);
for (int x = 0; x < w; x++) {
for (int y = 0; y < h; y++) {
color = oldPx[x * h + y];
r = Color.red(color);
g = Color.green(color);
b = Color.blue(color);
a = Color.alpha(color);
int dx = cx - x;
int dy = cy - y;
if (w > h){
dx = (dx * ratio) >> 15;
}
else{
dy = (dy * ratio) >> 15;
}
int distSq = dx * dx + dy * dy;
float v = ((float)distSq / diff) * 255;
a = (int)(a+(v));
a = (a > 255 ? 255 : (a < 0 ? 0 : a));
newPx[x * h + y] = Color.argb(a,r, g, b);
}
}
result.setPixels(newPx, 0, w, 0, 0, w, h);
return result;
}
