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

在Replit上运行的Spacestills主窗口

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 sgFEED_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 = 300class StillFrame():"""Holds a still frame.The image is stored as a PNG PIL.Image and kept in PNG format.Attributes----------image : PIL.ImageA still frameoriginal : PIL.ImageOriginal frame with wchich the instance is initialized, cached in case ofresizing to the original sizeMethods-------bytes : Return the raw bytesresize : Resize the screenshotnew_size : Calculate new aspect ratio"""def __init__(self, image):"""Convert the image to PNG and cache the converted original.Parameters----------image : PIL.ImageImage to store"""self.image = imageself._topng()self.original = self.imagedef _topng(self):"""Convert image format of frame to PNG.Returns-------StillFrameFrame 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_imagereturn selfdef bytes(self):"""Return raw bytes of a frame image.Returns-------bytesByte 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-tupleNew size"""size = self.image.sizeoriginal_size = self.original.sizenew_size = (WIDTH, HEIGHT_16_9) if size == original_size else (WIDTH, HEIGHT)return new_sizedef resize(self, new_size):"""Resize frame image.Parameters----------new_size : 2-tupleNew sizeReturns-------StillFrameFrame with image resized"""if not(self.image.size == new_size):self.image = self.image.resize(new_size)return selfdef make_blank_image(size=(WIDTH, HEIGHT)):"""Create a blank image with a blue background.Parameters----------size : 2-tupleImage sizeReturns-------PIL.ImageBlank image"""image = Image.new('RGB', size=size, color='blue')return imagedef download_image(url):"""Download current NASA TV image.Parameters----------url : strURL to download the image fromReturns-------PIL.ImageDownloaded 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 imagedef refresh(window, resize=False, feed=FEED_URL):"""Display the latest still frame in window.Parameters----------window : sg.WindowWindow to display the still tofeed : stringFeed URLReturns-------StillFrameRefreshed 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 stilldef change_aspect_ratio(window, still, new_size=(WIDTH, HEIGHT_16_9)):"""Change the aspect ratio of the still displayed in window.Parameters----------window : sg.WindowWindow containing the stillnew_size : 2-tupleNew size of the stillReturns-------StillFrameFrame containing the resized image"""resized_still = still.resize(new_size)window['-IMAGE-'].update(data=resized_still.bytes())return resized_stilldef save(still, path):"""Save still to a file.Parameters----------still : StillFrameStill to savepath : stringFile nameReturns-------BooleanTrue if file saved with no errors"""filename = Path(path)try:with open(filename, 'wb') as file:file.write(still.bytes())saved = Trueexcept OSError:saved = Falsereturn saveddef next_timeout(delta):"""Return the moment in time right now + delta seconds from now.Parameters----------delta : intTime in seconds until the next timeoutReturns-------datetime.datetimeMoment 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.datetimeReturns-------boolTrue if the next timeout is due"""rightnow = datetime.now()return rightnow >= next_timeoutdef validate_delta(value):"""Check if value is an int within the proper range for a time delta.Parameters----------value : intTime in seconds until the next timeoutReturns-------intTime in seconds until the next timeoutboolTrue if the argument is a valid time delta"""isinteger = Falsetry:isinteger = type(int(value)) is intexcept Exception:delta = DELTAdelta = int(value) if isinteger else deltaisvalid = MIN_DELTA <= delta <= MAX_DELTAdelta = delta if isvalid else DELTAreturn delta, isinteger and isvalidLAYOUT = [[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 = DELTAnext_reload_time = datetime.now() + timedelta(seconds=delta)while True:event, values = window.read(timeout=100)if event in (sg.WIN_CLOSED, 'Exit'):breakelif ((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 windowif __name__ == '__main__':main(LAYOUT)

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