Long-time fans of the Mario Kart series might remember coins. Collecting them increased a kart's top speed, but they've only appeared as a gameplay element in two of the six Mario Kart titles. Now Mario Kart 7 is bringing them back, and Nintendo has changed its tune on how important they are to the legacy of the series.

"Looking back now, I think [coins are] an essential element," program director Yusuke Shiraiwa said in the latest Iwata Asks. "There aren't many obstacles in the air or underwater, so I was worried that players might find themselves with their hands free. The coins are floating, so it's fun to get them."

He says that in past games, the team would try to add coins near the end of development when it no longer fit. This time, Shiraiwa told them to include it in the prototypes from the very beginning.

His biggest challenge, though, came from the demands of making three different images with a high frame rate. Asked what is biggest programming challenge was, Shiraiwa said it was easily achieving 60 frames per second. "And the Nintendo 3DS system requires two images, one for the right eye and one for the left," he said. "In addition to that, there's the bottom screen, so we have to do three.

"That alone made the burden bigger than ever before, but the hardware was new, and the planners were saying, 'We want to put in more characters,' and the designers were saying, 'We want the visuals to look better.' It was our first time to handle the hardware, so achieving that balance was awfully hard. And it was the first time to connect eight Nintendo 3DS system, so we had to try a number of things with regard to how much processing capacity to use."

Mario Kart 7 is due this Sunday, December 4, for the Nintendo 3DS.