Usage: contrast(amount)
This filter increases or decreases the image contrast.
amount- -100 to 100 - The amount (in %) to change the image contrast. Positive numbers increase contrast and negative numbers decrease contrast.
http://localhost:8888/unsafe/filters:contrast(40)/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fexample.jpg
http://localhost:8888/unsafe/filters:contrast(-40)/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fexample.jpg
