Tristan Thompson was all smiles before the shooting at the White House Correspondents Dinner … TMZ obtained video of him on his way in, and he was stopping to take selfies with fans.

Check out this video from Saturday’s event in Washington D.C. … Tristan is on his way to take an escalator up to the dinner when he stops to snap photos with some folks.

Tristan is wearing a bedazzled and sparkling jacket as he pauses for some pics … and he even has to retake a selfie with a woman who whipped out her phone.

This all went down about 30 minutes before shots rang out at the hotel and sent guests scrambling for cover.