Fingerprint collection fell into three categories: visible prints, molded prints, and latent prints. The first two could be seen with the naked eye, while the latter were invisible and needed to undergo certain treatment before their original form could be revealed and extracted. At crime scenes, latent fingerprints were the most common.
There were many methods for collecting such prints, such as the powder method and the magnetic powder method. The former used contrasting colored powder to adhere to the print, while the latter used iron powder, then a magnetized brush to reveal the fingerprint. The powder method was the most widely used. In addition, if fingerprints were left on absorbent surfaces such as paper, cards, leather, or wood, they had to undergo chemical treatment before they could be revealed and extracted.
There were also quite a few chemical methods: iodine fuming, ninhydrin, the silver nitrate method, fluorescent reagent methods, and so on.
All of these required materials, as well as equipment to clearly scan the revealed fingerprints into a computer. Put the two together, and that was fingerprint extraction. The subsequent comparison involved marking the fingerprint's characteristic points in the police system's fingerprint database, using software to assist with filtering, then patiently examining them one by one.
Under current circumstances, very few people's fingerprints were not in the database. You needed fingerprints to get an ID card, to clock in at work, and even internet cafes could use fingerprints in place of ID cards. Over the past two years, facial recognition had begun to be used as well, directly connected to the police system.
Just as Wang Shouyi had said, the things needed to extract fingerprints were not expensive; they were just hard to gather all at once. Besides, even if they did not cost much, if they could