我正在使用pycairo，下面是完整的代码段：
from math import pi
from cairo import PDFSurface, Context, Matrix
WIDTH = 6 * 72
HEIGHT = 4 * 72
s = PDFSurface('example1.pdf', WIDTH, HEIGHT)
c = Context(s)
# Draw a circle
c.save()
c.set_line_width(0)
for i in range(1000):
c.arc(1 * 72, 3 * 72, 0.5 * 72, 0, 2 * pi)
c.set_source_rgb(1.0, 0, 0)
c.fill()
c.arc(1 * 72, 3 * 72, 0.5 * 72, 0, 2 * pi)
c.set_source_rgb(1.0, 1.0, 1.0)
c.fill()
s.finish()
我希望生成的pdf为空白。但是，您会看到一个红色圆圈。我知道这是由抗锯齿引起的，很久以前在这里也问过同样的问题： Two exactly same SVG paths not drawn exactly the same way 但是答案并没有解释抗锯齿是如何引起这种效果的。
如果有人可以告诉我它是怎么发生的，将不胜感激。
谢谢！
答案 0 :(得分：0)
想象一下，您想用红色绘制半个像素：执行cairo_rectangle(cr, 42, 42, 0.5, 1);得到一个半个像素宽的矩形。然后设置红色的源颜色并将其绘制（在白色背景上）。
预期结果是什么？
好吧，一半像素是白色（(1, 1, 1)），一半像素是红色（(1, 0, 0)）。因此，逻辑上的做法是将红色和白色混合在一起，得到(1, 0.5, 0.5)之类的颜色。
接下来，您要用白色绘制相同的“半像素”。像素的“左半部分”为红色而“右半部分”为白色的信息未保存在任何地方。因此，像素只有这种“红白色混合”。因此，再次需要一些混合：(1, 0.5, 0.5)的一半(1, 1, 1)和(1, 0.75, 0.75)的一半[W 23:00:48.600 LabApp] Error loading server extension ipyparallel.nbextension
Traceback (most recent call last):
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/notebook/notebookapp.py", line 1546, in init_server_extensions
mod = importlib.import_module(modulename)
File "/Users/MyName/anaconda3/lib/python3.7/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "", line 1006, in _gcd_import
File "", line 983, in _find_and_load
File "", line 953, in _find_and_load_unlocked
File "", line 219, in _call_with_frames_removed
File "", line 1006, in _gcd_import
File "", line 983, in _find_and_load
File "", line 965, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'ipyparallel'
[I 23:00:48.605 LabApp] JupyterLab extension loaded from /Users/MyName/anaconda3/lib/python3.7/site-packages/jupyterlab
[I 23:00:48.605 LabApp] JupyterLab application directory is /Users/MyName/anaconda3/share/jupyter/lab
[W 23:00:48.608 LabApp] JupyterLab server extension not enabled, manually loading...
[I 23:00:48.609 LabApp] JupyterLab extension loaded from /Users/MyName/anaconda3/lib/python3.7/site-packages/jupyterlab
[I 23:00:48.609 LabApp] JupyterLab application directory is /Users/MyName/anaconda3/share/jupyter/lab
[I 23:00:48.611 LabApp] Serving notebooks from local directory: /Users/MyName
[I 23:00:48.611 LabApp] The Jupyter Notebook is running at:
[I 23:00:48.612 LabApp] http://localhost:8888/?token=8b2eb70aca4d7230313e1ec5178745ea84875981d0482c16
[I 23:00:48.612 LabApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
[C 23:00:48.616 LabApp]
Copy/paste this URL into your browser when you connect for the first time,
to login with a token:
http://localhost:8888/?token=8b2eb70aca4d7230313e1ec5178745ea84875981d0482c16
[I 23:00:49.252 LabApp] Accepting one-time-token-authenticated connection from ::1
[W 23:00:49.856 LabApp] Could not determine jupyterlab build status without nodejs
[E 23:00:49.862 LabApp] Uncaught exception GET /api/contents/?content=1&1543561249855 (::1)
HTTPServerRequest(protocol='http', host='localhost:8888', method='GET', uri='/api/contents/?content=1&1543561249855', version='HTTP/1.1', remote_ip='::1')
Traceback (most recent call last):
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/tornado/web.py", line 1592, in _execute
result = yield result
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/tornado/gen.py", line 1133, in run
value = future.result()
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/tornado/gen.py", line 326, in wrapper
yielded = next(result)
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/notebook/services/contents/handlers.py", line 112, in get
path=path, type=type, format=format, content=content,
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/notebook/services/contents/filemanager.py", line 431, in get
model = self._dir_model(path, content=content)
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/notebook/services/contents/filemanager.py", line 337, in _dir_model
if self.should_list(name) and not is_file_hidden(os_path, stat_res=st):
File "/Users/MyName/anaconda3/lib/python3.7/site-packages/notebook/utils.py", line 145, in is_file_hidden_posix
stat_res = os.stat(abs_path)
OSError: [Errno 62] Too many levels of symbolic links: '/Users/MyName/config'
[W 23:00:49.870 LabApp] Unhandled error
[E 23:00:49.871 LabApp] {
"Host": "localhost:8888",
"Pragma": "no-cache",
"Accept": "*/*",
"Authorization": "token 8b2eb70aca4d7230313e1ec5178745ea84875981d0482c16",
"Accept-Language": "en-us",
"Accept-Encoding": "gzip, deflate",
"Cache-Control": "no-cache",
"Content-Type": "application/json",
"User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_1) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0.1 Safari/605.1.15",
"Connection": "keep-alive",
"Referer": "http://localhost:8888/lab",
"Cookie": "username-localhost-8888=\"2|1:0|10:1543561249|23:username-localhost-8888|44:YmRiOThiODMxODA3NDM1NDhmMmNlYjZiMDM2YmFhZWI=|4cec00ae573ce2c242270e1ad51ea5b8bf633143c50175883402000209508f9e\"; username-localhost-8889=\"2|1:0|10:1543560086|23:username-localhost-8889|44:ZmQyNTdmMjU3OTQ4NDg4MjllNWE5MThiMDY4YmMxOTk=|0516e335b3a1b0666b7b03fe9e19820005734c51f9cff9aaa4d19bbe5ac3b72c\"; username-localhost-8890=\"2|1:0|10:1543352001|23:username-localhost-8890|44:MTlkMTFlNGYyNWQ5NDE3NDg0NDk4MTMyMmExMDRhMzY=|1f0e8c67a9bfa9bfe8367f0bc3fef515c76590f21c0f0f26576e466bfa7bd8b6\"; _xsrf=2|d4da0c6c|43dea648c15b852bca19be92af6821cf|1543351994"
}
[E 23:00:49.871 LabApp] 500 GET /api/contents/?content=1&1543561249855 (::1) 13.31ms referer=http://localhost:8888/lab
，或者1/4的红色和3/4的白色的混合。
因此，即使我们先用红色然后用白色填充了相同的几何图形，红色仍然部分可见。
这与抗锯齿有什么关系？圆并不完全适合圆网格。因此，在边缘附近，您会得到一些仅部分位于圆内的像素。抗锯齿意味着完成了与我上面解释的类似的操作：背景色和圆的填充色混合在一起。因此，这种混合意味着在再次“擦除”圆之后，某些第一色仍保持可见。