//width=512 height=512

function color_calc(img_data,width,height) {
	for(y=0;y<height;y++) {
		for(x=0;x<width;x++) {
			img_data[y][x].r=img_data[y][x].g=img_data[y][x].b=255;
			dist=~~Math.sqrt((x-255)*(x-255)+(y-255)*(y-255));
			if(dist>255)dist=255;
			if(dist<0)dist=0;
			if(dist<128)dist=128;
			img_data[y][x].a=255-dist;
		}	
	}
}