It is explained in an article, Colorization Using Optimization giving several examples.
A tool on GitHub allows also to color a sketch, with another image as color sample: Style2Paint .
Wisely by removing the "noise", ie the image defects such as dots and stripes, and it needs to know to interpret that, the algorithm gives life to a cliche damaged or washed out.
It is implemented in a plugin for Gimp, G'Mic. It can also automatically make HDR, ie increase the brightness of light areas and darken the dark parts.
Intelligent algorithm to resize images maintaining the appearance of objects within it. It is actually implemented in the resizing functions of the main graphical utilities like Gimp with the Liquid Rescale plugin, ImageMagick with the -liquid-rescale option.
This algorithm is available both as a Gimp plugin to install or in Photoshop.
This algorithm is implemented in the software Sketch To Photo . From the article, you can download the command line program.
More information about this esthetic trick algorithm .
深度学习 PixelNN: Example-based Image Synthesis
PixedNN. Appearing more recently in 2017, turns a sketch into a photo!
pix2pix 来自卡内基梅隆大学的 Aayush Bansal 、Yaser Sheikh、Deva Ramanan，联合研究了一种基于示例图像合成清晰照片的技术 — PixelNN，把模糊的低分辨率或者是有马赛克的图片，使用表面法线贴图和草图等，合成出高分辨率的逼真图像。
这玩意可以提高驾驶技术：用 GAN 去除（爱情）动作片中的马赛克和衣服。
Animating a photo, a sketch. This algo of artificial intelligence makes the subject walk on a picture! Not available in a toolbox for now.
Image improvements. Demos of various algos for processing pictures to improve or restore them (the sources files are not linked).