This needs 4 taps total: first the 2 ends of a red reference card's long edge (standard business-card size, laid flat on the fish, same distance from the camera), then the fish's nose and tail tip. We'll try to auto-detect the card so you may only need the last 2 taps. Nothing shows until all 4 points are placed.

✓ Use

↩️ Undo Point 🔄 Reset