folds

After the autumn rain, the air was filled with the damp fragrance of osmanthus blossoms. As Luo Yan stuffed her laptop into her backpack, the lab clock struck five in the afternoon. The screen still displayed the test interface for the "Tennis Motion Recognition System V2.0," the green recognition box bouncing nimbly in the retired player's motion video, its accuracy consistently at 92%—the result of three sleepless nights she and Qi Sheng had worked on.

"Really not coming together?" Zhang Qi stood at the door with a folder in her arms, glancing towards the direction of the sports college next door. "I just saw Senior Qi coming this way."

Luo Yan paused in her hand as she zipped up her backpack: "No, he said he wanted to watch Lele's training. I can go back by myself."

Zhang Qi smiled but didn't expose him: "Then I'll be going now. Don't be late for next week's exhibition match. I heard they've invited retired athletes from the provincial team."

The exhibition match is an important testing component of the project and a showcase of the collaborative efforts between the School of Physical Education and the Department of Computer Science. Luo Yan wasn't entirely confident—the system has a high accuracy rate in recognizing beginner movements, but it hasn't yet been tested in real-world scenarios against the "non-standard yet effective" personalized movements of retired athletes.

Her shoulders ached from the tight backpack straps, and as she walked out of the computer building, she indeed saw Qi Sheng at the intersection. He was carrying a black sports bag and looking down at his phone, his fingers rapidly swiping across the screen, as if replying to a message.

“I’ve fine-tuned the system’s ‘dynamic fault tolerance’ parameters.” Luo Yan stepped forward, opened her phone, and showed a screenshot of the parameters. “During the test just now, there was still a slight delay in recognizing left-handed players.”

Qi Sheng looked up, then took her phone, zooming in on the screenshot with his fingertip. He inadvertently touched the back of her hand, like feeling a cool drizzle. "Let me see," he said, his voice lower than usual, his finger tapping the corner of the screen. "The threshold here can be relaxed by 2%. Their wrist rotation range is generally too large."

"You've tried it?" Luo Yan asked, somewhat surprised.

“I watched their match video last night.” He handed the phone back, the screen still showing the parameters. “Lele said that her forehand shots always go out of bounds, which might be due to the grip angle. Do you want to go and take a look? It’s a good opportunity to test the system’s real-time recognition of children’s movements.”

Luo Yan's heart skipped a beat. This was the first time he had used an "invitation" tone instead of a tone about project collaboration. She unzipped her backpack and took out the laptop she had just put inside: "I just happened to bring the equipment."

Outside the wire fence of the training field, Lele was practicing hitting the ball against the wall, her pink sportswear standing out against the dusty background. Her movements were more standardized than in last week's final, but her hitting point was always too early, causing the ball to hit the wall and bounce back out of bounds, making her little brows furrow.

"Your grip is too high," Qi Sheng shouted from outside the net, his voice carrying a muffled, filtered sound as it passed through the wire mesh. "Move it down one centimeter, try it."

Lele obediently adjusted her grip, and when she hit the ball again, it landed steadily in the red circle on the wall. She turned around excitedly, saw the computer in Luo Yan's hand, and her eyes lit up: "Sister, can your system recognize my movements just now?"

"Try it and you'll see." Luo Yan opened the software and clicked "Real-time Recognition." The green recognition box immediately framed Lele's figure, and as she swung her racket, a series of parameters popped up on the right side of the screen: "Knee bend angle 120 degrees (Excellent), body rotation 30 degrees (Good), hitting point height 1.2 meters (Excellent)."

"All good scores!" Lele leaned over to look at the screen, her ponytail brushing against Luo Yan's hand. "Teacher Qi said that if I can hit ten good shots in a row, she will teach me that 'lazy serve'."

Qi Sheng stood to the side, twirling the silver-gray racket in his hand. When he heard "lazy serve," the tips of his ears turned slightly red. Luo Yan's gaze fell on his fingers as he twirled the racket, and she suddenly remembered that when she was debugging the system last week, he said, "The essence of the slice serve is not in the power, but in the 'inch power' of the wrist." At that time, his fingertips were typing parameters on the keyboard, and his nails were neatly trimmed, with a healthy pink hue.

“Lele will also be watching the exhibition match next week,” Qi Sheng suddenly said, interrupting her reverie. “She said she wants to see how retired players play.”

Luo Yan nodded: "The system just needs to be tested for its ability to recognize high-level movements." She paused and pulled up a set of data, "But I'm a little worried that there are many 'non-standard' but effective details in the movements of retired athletes, such as unconventional wrist rotations, which the system might misjudge."

“I found some of their competition videos from back then.” Qi Sheng took out a USB drive from his sports bag. “It’s marked which ones are ‘personal signature moves’ and which ones are real mistakes. It might help you optimize your algorithm.”

The USB flash drive has a matte silver casing and a small tennis ball pendant.

"Thank you." When Luo Yan took the USB drive, her fingertips touched his hand again. This time, neither of them recoiled as if electrocuted as before. They just paused quietly for half a second, like two drops of water about to merge.

Lele called out "Teacher Qi" from the court, breaking the brief silence. Qi Sheng responded and went over to her, correcting her turning movements hand in hand. His sleeve slid down to his elbow, revealing a light brown mark on his forearm, a callus from years of holding a racket, its edges already fading white, like a faded memory.

For the next three days, the lab became Luo Yan's second dormitory. She dissected the competition videos Qi Sheng had given her frame by frame, added a "personalized action library" to the algorithm, and marked the retired players' "non-standard actions" as "acceptable deviations." She was always alone in the lab late at night, until the early hours of the third day when a message suddenly popped up on her screen from Qi Sheng: "Still busy?"

Luo Yan glanced at the time; it was already 2:30. She replied with an "Mm," along with a screenshot of the system's misjudgment—a retired player, left-handed, hit a shot with the effect of a forehand using a backhand grip, but the system stubbornly judged it as an "incorrect move."

Five minutes later, there was a knock on the door. Luo Yan opened the door and saw Qi Sheng standing in the corridor, his hair a little messy, carrying a thermos in his hand, as if he had just come from the training ground.

"Here you go." He handed over the thermos, which was still warm. "Zhang Qi said you haven't eaten properly for three days."

It was seafood congee, the rice cooked until soft and tender, the savory aroma of dried scallops mixed with the slight spiciness of ginger, dispelling the chill of the late night. Luo Yan scooped up a spoonful and suddenly remembered the sandwiches they shared in the lab last time, which also carried this kind of clumsy concern.

“That left-handed player,” Qi Sheng stood in front of the screen, pointing to the backhand grip, “has a shoulder injury and can’t do a standard rotation, so he compensates with his wrist.” He stretched out his left hand to simulate the movement, “You see, his wrist rotation angle is 20 degrees larger than the standard value, but with footwork adjustments, he can hit tricky shots.”

Luo Yan's spoon paused at her lips. The system's sensors only detected "excessive wrist rotation," but failed to recognize the underlying "compensatory mechanism"—the wrinkles beyond the data, hiding a story known only to those who truly understand football.

"Can the algorithm add a 'history of sports injury' tag?" she asked, her fingertips hovering over the keyboard.

“It’s difficult,” Qi Sheng shook his head, “but we can add a ‘dynamic error tolerance’ module to adjust the judgment criteria in reverse based on the effect of the shot.” ​​He leaned closer to the screen, his fingertips sliding across the parameter panel, “For example, as long as the ball lands in the valid area and the power and spin meet expectations, we can relax the requirements for the details of the action.”

This was exactly where she had been stuck for so long. Luo Yan watched his fingertips working and suddenly felt that Qi Sheng was like her "dynamic fault-tolerant module"—he could always point out the way out in the simplest way when she was trapped in the data maze, with an unquestionable certainty.

By the time they finished their porridge, the sky was already beginning to lighten. After helping her adjust the algorithm parameters, Qi Sheng got up to leave, his gaze falling on the dark circles under her eyes: "Don't go to the exhibition match today, go back and get some sleep."

“No,” Luo Yan shook her head, “We agreed to do a real-world test.”

He didn't try to persuade her any further, but instead took out an eye mask from his bag. It was dark blue and had a tennis racket pattern on it: "Use this during your lunch break, don't push yourself too hard."

The exhibition match venue was packed. Luo Yan sat at the technical table with her laptop, watching the recognition frame on the screen jump flexibly with the retired players' movements, with an accuracy rate consistently above 95%. The anxiety she had felt for three days finally subsided.

Qi Sheng sat in the first row of the coach's bench, wearing a white shirt and with his hair neatly combed. Whenever the system recognized a "personalized action" and marked it as "valid," he would glance towards the technical table, and when his gaze met Luo Yan's, he would nod slightly, as if exchanging a silent signal.

During halftime, Lele ran over with a bottle of mineral water in her arms, looked up at her, and asked, "Sister, can my movements be entered into your system?"

"Of course you can." Luo Yan pulled up Lele's training data. "Look, this is your forehand shot from last week, and this is today's. You've made great progress."

Lele's attention, however, was drawn to a small window in the corner of the screen—a loop playing videos of Qi Sheng's backhand shots, data collected last time. "Teacher Qi's movements are so beautiful," she whispered, "like he's dancing."

Luo Yan had never described tennis moves like this before, but she felt that this metaphor was particularly apt—Qi Sheng's backhand shot, with its wrist rotation, had a smooth rhythm, like a dancer gliding on ice, concealing both power and restraint.

The final match was between two retired players. The left-handed player indeed used that "non-standard" shot; the system's recognition box turned green, and a "Valid Personalized Move" label appeared next to it. As the audience gasped in amazement, Luo Yan saw Qi Sheng stand up and walk towards the technical table, half of a dark blue blindfold peeking out of his shirt pocket.

"The results are good." He stood by the technical table, his voice tinged with amusement. "It's more stable than I expected."

“Thanks to the video you provided.” Luo Yan turned the laptop towards him, the real-time data on the screen still scrolling. “Especially that ‘dynamic fault tolerance’ module, it played a big role.”

His gaze lingered on the screen for a moment, then he suddenly said, "Are you free tonight? Lele's coaching team is treating us to dinner, saying they want to thank you for your system that helped them with the motion analysis."

Luo Yan thought of the mountains of code piled up in the lab and was about to refuse when she saw the expectation in his eyes. "Okay," she heard herself say, her voice as soft as a sigh.

The dinner was held at a casual restaurant near the gymnasium, and the private room was noisy. Lele's coaches took turns toasting Luo Yan and Qi Sheng, saying things like "The system is so useful" and "We'll rely on it for all our training from now on." Luo Yan wasn't a big drinker and only took a small sip each time, but Qi Sheng drank several glasses for her, explaining that "she has to modify code tomorrow."

Luo Yan went to the restroom halfway through her journey and ran into Qi Sheng in the hallway when she returned. He was leaning against the wall making a phone call, his voice low, his profile appearing exceptionally gentle in the warm yellow light. "...Um, the system test was successful...No need, I can handle it...Okay, I'm hanging up."

Upon seeing Luo Yan, he hung up the phone, his finger swiping across the screen as if deleting something. "My mom," he explained, "asked when I'm coming home."

"Haven't been back in a long time?" Luo Yan leaned against the opposite wall. The wind in the corridor carried the fragrance of osmanthus, which tickled her cheeks.

“It’s been almost half a year.” He looked down at the ground. “The project has been busy, I haven’t been able to leave.” He paused, then suddenly looked up. “What about you? Are you going back to Lin’an for winter break?”

“We should go back.” Luo Yan nodded. “My mom said the wintersweet in the yard should be blooming soon.”

"The wintersweet in Lin'an is very famous." Qi Sheng's voice was very soft, as if he was talking to the air. "I passed by once, and the fragrance could fill an entire street."

Luo Yan's heart skipped a beat. She had never told him about the winter plum blossoms of Lin'an, yet he remembered them. Just as he remembered the wrist angle for a slice serve, the compensatory movements of left-handed players, and those subtle details hidden in the folds of data.

Back in the VIP box, Lele was twirling around with the trophy in her hand, her pink tracksuit skirt fluttering like a happy butterfly. The coaches clapped and laughed, and someone shouted, "Teacher Qi, give us one too!" Qi Sheng smiled and waved, but was pushed to the center of the field.

He didn't pick up a racket; he simply made a backhand stroke. The moment his wrist rotated, the lights in the booth cast shimmering spots of light on his face, like scattered stardust. Watching his movement, Luo Yan suddenly recalled what he'd said late at night in the lab: "The core of dynamic fault tolerance is acknowledging imperfection"—perhaps relationships between people are like that too; they don't need perfect parameters, just the right angle to fit together within each other's folds.

When leaving the restaurant, Qi Sheng escorted Luo Yan back to her dormitory. The two walked slowly along the ginkgo-lined path, their shadows stretched long by the streetlights, occasionally overlapping in the piles of fallen leaves.

"The project will conclude next week," Luo Yan said, kicking at a ginkgo leaf, her voice slightly unsteady, "and maybe... we won't have to keep running to the sports academy anymore."

Qi Sheng paused, said nothing, but took something out of his pocket and handed it to her—a silver bookmark with a simple tennis racket pattern engraved on it, the edges of which were polished very smoothly, as if someone had rubbed it repeatedly.

"This is for you." His voice was soft. "The project is finished, but the system still needs maintenance." He paused, then added, "If you have any problems, feel free to contact me."

Luo Yan took the bookmark; the coolness of the metal traveled through her fingertips, yet it felt as warm as a fire. She watched his retreating figure as he turned and left, his white shirt gleaming softly under the streetlights.

The distance that once stood between them, the past that had been sealed away by time, all quietly unfolded into new shapes in the patterns of this bookmark, in the code of the lab late at night, and in their unspoken understanding.