How does converting a video to 4K actually work?

Converting a video to 4K expands each frame from 1,920 × 1,080 pixels to 3,840 × 2,160 (or 4,096 × 2,160 for DCI 4K), requiring interpolation that guesses new pixel values because original detail cannot be recovered. Professional upscales rely on the highest-quality source material, as demonstrated by the Star Trek: The Next Generation restoration, which used original 35 mm negatives equivalent to 20 megapixels and cost over $12 million.