用Python监控NASA TV直播画面的实现步骤
作者：Python中文社区
本文分享一个名为"Spacestills"的开源程序，它可以用于查看 NASA TV 的直播画面（静止帧）
演示地址：
https://replit.com/@PaoloAmoroso/spacestills
这是一个具有GUI的简单系统，它访问feed流并从Web下载数据。该程序仅需350行代码，并依赖于一些开源的Python库。
关于程序
Spacestills会定期从feed流中下载NASA TV静止帧并将其显示在GUI中。
该程序可以校正帧的纵横比，并将其保存为PNG格式。它会自动下载最新的帧，并提供手动重新加载，禁用自动重新加载或更改下载频率的选项。
Spacestillsis是一个比较初级的版本，但是它可以做一些有用的事情：捕获并保存NASA TV直播的太空事件图像。太空爱好者经常在社交网络或论坛共享他们从NASA TV手动获取的屏幕截图。Spacestills节省了使用屏幕捕获工具的时间，并保存了可供共享的图像文件。您可以在Replit上在线运行Spacestills。
开发环境
笔者用Replit开发了Spacestills。Replit是云上的开发，部署和协作环境，它支持包括Python在内的数十种编程语言和框架。作为Chrome操作系统和云计算爱好者，笔者非常喜欢Replit，因为它可以在浏览器中完全正常运行，无需下载或安装任何内容。
资源和依赖包
Spacestills依赖于一些外部资源和Python库。
NASA TV feed 流
肯尼迪航天中心的网站上有一个页面，其中包含精选的NASA视频流，包括NASA电视公共频道。feed流显示最新的静止帧并自动更新。
每个feed都带有三种尺寸的帧，Spacestills依赖于具有704x408像素帧的最大NASA TV feed流。最大更新频率为每45秒一次。因此，检索最新的静止帧就像从feed流的URL下载JPEG图像一样简单。
原始图像被垂直拉伸，看起来很奇怪。因此，该程序可以通过压缩图像并生成未失真的16：9版本来校正纵横比。
Python
因PySimpleGUI的原因需要安装 Python 3.6 版本。
第三方库
- Pillow：图像处理
- PySimpleGUI：GUI框架（Spacestills使用Tkinter后端）
- Request：HTTP请求
完整代码
from io import BytesIO
from datetime import datetime, timedelta
from pathlib import Path
import requests
from requests.exceptions import Timeout
from PIL import Image
import PySimpleGUI as sg
FEED_URL = 'https://science.ksc.nasa.gov/shuttle/countdown/video/chan2large.jpg'
# Frame size without and with 16:9 aspect ratio correction
WIDTH = 704
HEIGHT = 480
HEIGHT_16_9 = 396
# Minimum, default, and maximum autoreload interval in seconds
MIN_DELTA = 45
DELTA = MIN_DELTA
MAX_DELTA = 300
class StillFrame():
"""Holds a still frame.
The image is stored as a PNG PIL.Image and kept in PNG format.
Attributes
----------
image : PIL.Image
A still frame
original : PIL.Image
Original frame with wchich the instance is initialized, cached in case of
resizing to the original size
Methods
-------
bytes : Return the raw bytes
resize : Resize the screenshot
new_size : Calculate new aspect ratio
"""
def __init__(self, image):
"""Convert the image to PNG and cache the converted original.
Parameters
----------
image : PIL.Image
Image to store
"""
self.image = image
self._topng()
self.original = self.image
def _topng(self):
"""Convert image format of frame to PNG.
Returns
-------
StillFrame
Frame with image in PNG format
"""
if not self.image.format == 'PNG':
png_file = BytesIO()
self.image.save(png_file, 'png')
png_file.seek(0)
png_image = Image.open(png_file)
self.image = png_image
return self
def bytes(self):
"""Return raw bytes of a frame image.
Returns
-------
bytes
Byte stream of the frame image
"""
file = BytesIO()
self.image.save(file, 'png')
file.seek(0)
return file.read()
def new_size(self):
"""Return image size toggled between original and 16:9.
Returns
-------
2-tuple
New size
"""
size = self.image.size
original_size = self.original.size
new_size = (WIDTH, HEIGHT_16_9) if size == original_size else (WIDTH, HEIGHT)
return new_size
def resize(self, new_size):
"""Resize frame image.
Parameters
----------
new_size : 2-tuple
New size
Returns
-------
StillFrame
Frame with image resized
"""
if not(self.image.size == new_size):
self.image = self.image.resize(new_size)
return self
def make_blank_image(size=(WIDTH, HEIGHT)):
"""Create a blank image with a blue background.
Parameters
----------
size : 2-tuple
Image size
Returns
-------
PIL.Image
Blank image
"""
image = Image.new('RGB', size=size, color='blue')
return image
def download_image(url):
"""Download current NASA TV image.
Parameters
----------
url : str
URL to download the image from
Returns
-------
PIL.Image
Downloaded image if no errors, otherwise blank image
"""
try:
response = requests.get(url, timeout=(0.5, 0.5))
if response.status_code == 200:
image = Image.open(BytesIO(response.content))
else:
image = make_blank_image()
except Timeout:
image = make_blank_image()
return image
def refresh(window, resize=False, feed=FEED_URL):
"""Display the latest still frame in window.
Parameters
----------
window : sg.Window
Window to display the still to
feed : string
Feed URL
Returns
-------
StillFrame
Refreshed screenshot
"""
still = StillFrame(download_image(feed))
if resize:
still = change_aspect_ratio(window, still, new_size=(WIDTH, HEIGHT_16_9))
else:
window['-IMAGE-'].update(data=still.bytes())
return still
def change_aspect_ratio(window, still, new_size=(WIDTH, HEIGHT_16_9)):
"""Change the aspect ratio of the still displayed in window.
Parameters
----------
window : sg.Window
Window containing the still
new_size : 2-tuple
New size of the still
Returns
-------
StillFrame
Frame containing the resized image
"""
resized_still = still.resize(new_size)
window['-IMAGE-'].update(data=resized_still.bytes())
return resized_still
def save(still, path):
"""Save still to a file.
Parameters
----------
still : StillFrame
Still to save
path : string
File name
Returns
-------
Boolean
True if file saved with no errors
"""
filename = Path(path)
try:
with open(filename, 'wb') as file:
file.write(still.bytes())
saved = True
except OSError:
saved = False
return saved
def next_timeout(delta):
"""Return the moment in time right now + delta seconds from now.
Parameters
----------
delta : int
Time in seconds until the next timeout
Returns
-------
datetime.datetime
Moment in time of the next timeout
"""
rightnow = datetime.now()
return rightnow + timedelta(seconds=delta)
def timeout_due(next_timeout):
"""Return True if the next timeout is due.
Parameters
----------
next_timeout : datetime.datetime
Returns
-------
bool
True if the next timeout is due
"""
rightnow = datetime.now()
return rightnow >= next_timeout
def validate_delta(value):
"""Check if value is an int within the proper range for a time delta.
Parameters
----------
value : int
Time in seconds until the next timeout
Returns
-------
int
Time in seconds until the next timeout
bool
True if the argument is a valid time delta
"""
isinteger = False
try:
isinteger = type(int(value)) is int
except Exception:
delta = DELTA
delta = int(value) if isinteger else delta
isvalid = MIN_DELTA <= delta <= MAX_DELTA
delta = delta if isvalid else DELTA
return delta, isinteger and isvalid
LAYOUT = [[sg.Image(key='-IMAGE-')],
[sg.Checkbox('Correct aspect ratio', key='-RESIZE-', enable_events=True),
sg.Button('Reload', key='-RELOAD-'),
sg.Button('Save', key='-SAVE-'),
sg.Exit()],
[sg.Checkbox('Auto-reload every (seconds):', key='-AUTORELOAD-',
default=True),
sg.Input(DELTA, key='-DELTA-', size=(3, 1), justification='right'),
sg.Button('Set', key='-UPDATE_DELTA-')]]
def main(layout):
"""Run event loop."""
window = sg.Window('Spacestills', layout, finalize=True)
current_still = refresh(window)
delta = DELTA
next_reload_time = datetime.now() + timedelta(seconds=delta)
while True:
event, values = window.read(timeout=100)
if event in (sg.WIN_CLOSED, 'Exit'):
break
elif ((event == '-RELOAD-') or
(values['-AUTORELOAD-'] and timeout_due(next_reload_time))):
current_still = refresh(window, values['-RESIZE-'])
if values['-AUTORELOAD-']:
next_reload_time = next_timeout(delta)
elif event == '-RESIZE-':
current_still = change_aspect_ratio(
window, current_still, current_still.new_size())
elif event == '-SAVE-':
filename = sg.popup_get_file(
'File name', file_types=[('PNG', '*.png')], save_as=True,
title='Save image', default_extension='.png')
if filename:
saved = save(current_still, filename)
if not saved:
sg.popup_ok('Error while saving file:', filename, title='Error')
elif event == '-UPDATE_DELTA-':
# The current cycle should complete at the already scheduled time. So
# don't update next_reload_time yet because it'll be taken care of at the
# next -AUTORELOAD- or -RELOAD- event.
delta, valid = validate_delta(values['-DELTA-'])
if not valid:
window['-DELTA-'].update(str(DELTA))
window.close()
del window
if __name__ == '__main__':
main(LAYOUT)
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