How to unify pictures?

Question

PHP

Hello!
This has already been discussed many times, but I would like to hear the opinion of those who actually conducted the experiments.
Of the most effective:

• exif substitution
  
• turn 3-4 degrees
  
• mirroring
  
• 5-7% change in contrast
  

From the obvious - changing the name of the picture and its size, but this does not work by itself.
The problem here is that competitors do exactly the same algorithm and already in issuing blows from "unique photographs".

Answer 1

Based on experience image recognition
Rotation does not work (except perhaps rotation with non-linear distortion)
Mirroring (in a highly asymmetric picture) - completely
exif - generally needs to be removed before uploading to the site
Contrast - it is doubtful
Color often (!) Does not participate in recognition
In general, everything that does not really visually change the picture does not fit
In relation to the PS - you need to test

Answer 2

No way.
habrahabr.ru/company/yandex/blog/158067
And here is a small experiment
www.maultalk.com/ipb.html?s=&showtopic=176632&view...
PS pays more attention to image quality.

Answer 3

Converter won't work?

Answer 4

Reduce/increase color?

Answer 5

Add a watermark