Save a captured image with ov7670 into raspberry pi pico internal memory

Question

I try to capture and save a small photo with ov7670 camera module and save it to raspberry pi PICO internal memory as a BMP file format using circuitpython libraries. Any solution or sample code?

First, I tried to view color bars test pattern in this way:

import time
import board
import busio
import digitalio
import storage
from adafruit_bitmapsaver import save_pixels
from ulab import numpy as np

from displayio import (
    Bitmap,
    Group,
    TileGrid,
    Palette,
    ColorConverter,
    Colorspace,
)

from adafruit_ov7670 import (
    OV7670,
    OV7670_SIZE_DIV8,
    OV7670_SIZE_DIV16,
    OV7670_COLOR_RGB,
    OV7670_COLOR_YUV,
    OV7670_TEST_PATTERN_COLOR_BAR
)
with digitalio.DigitalInOut(board.GP10) as reset:
    reset.switch_to_output(False)
    time.sleep(0.001)
    bus = busio.I2C(board.GP1, board.GP0)
cam = OV7670(
    bus,
    data_pins=[
        board.GP12,
        board.GP13,
        board.GP14,
        board.GP15,
        board.GP16,
        board.GP17,
        board.GP18,
        board.GP19,
    ],  
    clock=board.GP11,  
    vsync=board.GP7,  
    href=board.GP21,  
    mclk=board.GP20,  
    shutdown=None,
    reset=board.GP10,
)
cam.size = OV7670_SIZE_DIV8
cam.colorspace = OV7670_COLOR_RGB
cam.test_pattern = OV7670_TEST_PATTERN_COLOR_BAR
bitmap = Bitmap(cam.width,cam.height, 65535)   
arr = np.zeros((2*cam.width, 2*cam.height), dtype=np.uint16)
cam.capture(arr)
palette=Palette(cam.width*cam.height)
for y in range(cam.height):
    for x in range(cam.width):
        palette[y*cam.width+x]=arr[x,y]     
x=0
y=0
for y in range(cam.height):
    for x in range(cam.width):
         bitmap[x,y]=y*cam.width+x
save_pixels('/test.bmp', bitmap, palette)
print("done")

But the result was this:

What's the problem? How can I get a clear image?