Python Tkinter mandelbrot设置比c更准确，没有明确的解释

下面的代码使用Tkinter的照片图像格式绘制Mandelbrot集，使用256次迭代和256色自定义调色板.

如果运行它，它会显示一个美丽的图像，其中有长长的烟雾状细丝，迭代界限之间没有同心边界.

如果我try 以相同的极限和精度重现c中的代码，我会得到迭代转义值之间的短丝和同心边界.

问题是，我错过了什么？为了获得更高的精确度，在我背后做了什么？

左边的附图是Python的

    # by Antoni Gual Via 4/2015

from tkinter import Tk, Canvas, PhotoImage,NW,mainloop 
from time import time

def mandel_pixel(c):   #c is a complex
  """ calculates the color index of the mandelbrot plane point passed in the arguments """
  maxIt = 256
  z =  c   
  for i in range(maxIt):
      a = z * z
      z=a + c  
      if a.real  > 4.:
         return i
      
  return maxIt

def mandelbrot(xa,xb,ya,yb,x,y):
    """ returns a mandelbrot in a string for Tk PhotoImage"""
    #color string table in Photoimage format #RRGGBB 
    clr=[ ' #%02x%02x%02x' % (int(255*((i/255)**.25)),0,0) for i in range(256)]
    clr.append(' #000000')  #append the color of the centre as index 256
    #calculate mandelbrot x,y coordinates for each screen pixel
    xm=list([xa + (xb - xa) * kx /x  for kx in range(x)])
    ym=list([ya + (yb - ya) * ky /y  for ky in range(y)])
    #build the Photoimage string by calling mandel_pixel to index in the color table
    return " ".join((("{"+"".join(clr[mandel_pixel(complex(i,j))] for i in xm))+"}" for j in ym))



#window size
x=640
y=480
#corners of  the complex plane to display  
xa = -2.0; xb = 1.0
ya = -1.27; yb = 1.27

#Tkinter window
window = Tk()
canvas = Canvas(window, width = x, height = y, bg = "#000000");canvas.pack()
img = PhotoImage(width = x, height = y)
canvas.create_image((0, 0), image = img, state = "normal", anchor = NW)

#do the mandelbrot
print('Starting Mandelbrot')
t1=time()
img.put(mandelbrot(xa,xb,ya,yb,x,y))
print(time()-t1, ' seconds')

mainloop()

还有C代码的相关部分

int mandel(double x0,double y0, int maxit){
  double x=x0,  xt=0.;
  double y=y0; 
  int i;
  for (i=1;((x*x+y*y)<4.) && (i<=maxit);i++){
   xt=x*x-y*y+x0;
   y=2.*x*y+y0;
   x=xt ;   
  } 
  return(i<maxit ? rgb(255.*(pow(((double)i/maxit),.25)),0,0):0);
}