I have noticed lately some of the older movies like Titanic being sold on Blu-rays in high definition (HD). As I understand, high definition implies high level of detail, yet I do not understand what is the true difference in terms of quality. If I remember correctly, 35mm film already has a higher resolution than HD so 35mm frame has at least 3 million pixels, depending on the lens, and shooting conditions, while an HD frame has 2 million pixels, measured using 1920 x 1080 scan lines. Which, kind of steers me towards believing that original film should have better visual quality.
So what is the real quality-wise difference between digital HD recording and a 35mm film recording?