Las Vegas is the town that tips built, so it's fitting that Stephen Campbell and Tim Baldwin chose the gambling mecca's backbone, the Strip, to launch a new mobile payments app called Tip.ly. Vegas is a city driven by cash--you can't tip out your valet, bellhop, or housekeeping service with a credit card. But Campbell and Baldwin are betting they can bring tipping into the 21st century with an app that combines Venmo-like peer-to-peer money transfers with a dash of facial recognition.

Say you want to tip your valet after picking up your car but forgot to bring money. Tip.ly offers a strange workaround: Take a photo of your valet and pick a dollar amount to send. The tip recipient has to sign up for a Tip.ly account to collect, but it doesn't matter if the sign-up happens before or after you send the cash. To create an account and access the funds, your valet has to upload a profile shot to the app. Tip.ly then compares the photo you snapped and verifies that the same person is collecting the tip.

It sounds a little confusing at first, and your valet might be a little creeped out when you try to take a photo, but it's all in the name of showing appreciation. The facial recognition feature is only necessary if the valet doesn't already have a Tip.ly account. You can search for Tip.ly users at businesses nearby to see if your tip recipient is already on the app. And if the money isn't collected within 14 days, it stays in your account.

Cofounder Campbell told Macworld that the app, which has been in development for more than a year, has been in testing for months and that service industry workers seem to be embracing it.

"We've been going up and down in the Strip in some major hotels and have had successfully completed tips to doormen, bellmen, valets," Campbell said. "We've heard from the valets in particular--they're looking at it like this is a way to make more money more often."

Not another Venmo clone

Tip.ly isn't designed for sending money to friends, like Venmo and Square Cash. The facial recognition feature exists so the recipient doesn't have to hand over personal information like a username, e-mail address, or phone number. And unlike other mobile payment apps, Tip.ly lets you give a star rating and review to the person you're sending cash to, which Campbell considers one of the app's most important features.

"Service professionals are working their tails off to provide good service and we wanted to provide a way, whether they were tipped or not, to show appreciation, much like you see on Uber," Campbell said.

"We're not out to rate people publicly," Baldwin added. "If it's less than a 3-star rating, it's automatically a private comment. If it's a 4 or 5-star, it can be public or private. We do allow people to search for a business and see all the service professionals associated with it."

The app wouldn't work in a restaurant environment or any other situation where a tipping method already exists. This is for hair stylists, massage therapists, valets, babysitters--people who deserve tips but don't process credit card payments. You pay a convenience fee for your forgetfulness: 5 percent of the tip plus 50 cents, to cover the transaction fee. Tip recipients don't have to pay anything to collect the tip--which is as it should be. Whether the app gains traction or not depends on the workers like the Vegas valets who rely on tips to make a living.