如何在暗室中制作高ISO效果？

You basically need to increase "grain" size, and maybe flat out your noise spots.

It will be hard to obtain a natural-looking result, as those grain spots in a video are obtained from various types of interpolations over values obtained from the camera sensor (with specific noise that a sensor produces).
Take for example "analog" film cameras (the pictures taken with the film have that grainy natural look from the effective grains sizes/shapes of minerals that are used in the film itself). If it were so easy to produce a natural-looking film alike filter over digital images the film industry would not see the comeback that happens now.

With that said there are three things that come to my mind that could work:

Median Blur over (image + some noise):
https://docs.opencv.org/master/d4/d13/tutorial_py_filtering.html

使用 Perlin 噪声生成的图像，根据您的特定帧大小进行缩放 + 添加图像的颜色 * 某些因素：
https : //github.com/ruslangrimov/perlin-noise-python-numpy
* 因素 + 您的图像= 结果