bent backward
The word straight does have an antonym, which is crooked. So perhaps a crooked line is the opposite of a straight line.
The antonym for crooked is straight.
An antonym for creased is straight or smooth
the opposite of straight is crooked or wavy.
straight
A synonym for straight is unbent.
tunnel? Or straight path.
oval, or straight
Straight line
Antonym means the opposite. So the opposite of angle is a straight line.
bent backward
curved, twisted