An original shot from The Arsenal of Freedom, upscaled to HD from the original 1988 videotape master. The planet is blurry because the original was a matte painting transferred into a low-res bitmap, whilst the Enterprise lacks detail due to coming off the SD video master tape. The jagged lines are down to less-effective mixing technology.
The same shot from the new Blu-Ray version. The original, low-res planet bitmap has been replaced by a 3D CG model, based on the original but far more detailed. The Enterprise is actually the exact same shot as in the original episode, with the extra detail emerging from the original film footage rather than any modern enhancements.
The original series of TNG was shot on 25,000 reels of 35mm film, the standard for television and film production. In retrospect, this was a thankful decision, since 35mm film can be blown up and projected onto massive cinema screens and therefore needs to contain vast amounts of information, far more than can be displayed on high definition TV screens. This means that any TV series or film shot on 35mm can be ultimately 'saved' and transferred to HD, with varying degrees of work done. Conversely, any show filmed directly on video (such as most of the original Doctor Who), cannot be transferred to HD (or rather it could, but would look awful).
However, whilst the show was shot on film, it was edited and mastered on video. The film reels (of both live-action footage with actors and elaborate model shots of the Enterprise and other spacecraft) were run through a video editing suite, with music, sound effects, credits and visual effects (such as phaser blasts and transporter effects) added. The resulting master tape contained the 'finished' episode...but only on video. These video masters are the source of the original television transmissions and the later VHS and DVD releases.
Whilst good for their day, the master tapes were not in high definition and were mastered for the then-standard square-shaped, 525-line NTSC American standard. This had two drawbacks. Firstly, it meant that the directors of TNG - and other US TV shows of the time - were frequently only concerned with the 'middle' of whatever image they were shooting, and left boom mikes and other equipment in the left and right edges of the shot, knowing they would not be seen. Whilst fine for that time, it does mean that most TV shows of that era cannot be displayed in the now-standard widescreen aspect ratios, as it would contain things we were never meant to see (such as light stands or actors waiting to enter the scene). Secondly, and more irritatingly, the image can appear to be soft, slightly unfocused and even blurred when viewed on other standards, such as the PAL system used in the UK and numerous other countries.
Wesley Crusher saves the day for the first of way, way too many times.
To overcome these issues, CBS initially decided to upscale the existing master tape images. A more thorough and complex version of the system used by Blu-Ray players to upscale standard-definition DVD images, this produced some good results but was inconsistent and distractingly variable in quality (the Blu-Ray documentary shows some comparisons of this process to show how it panned out). CBS ultimately decided this was unsatisfactory and they would have to start over by returning to the original film elements and re-editing the entire series from scratch.
This sounds simple but in truth is a daunting task. A typical 44-minute episode of Star Trek spent as long in editing and mastering (basically, the entire post-production process) as it did in shooting in the first place. A substantial portion of each episode's budget and time went on this, and CBS were now ordering it to be done again for every single episode of the series. This required a staff of dozens working as hard - and in some cases harder - as the original series editors to reassemble each episode. Fortunately, Star Trek: The Next Generation was filmed before the advent of widespread use of CGI, meaning that most of the space scenes featured miniatures filmed on 35mm which could be easily re-used. CGI in the 1980s and 1990s never passed through the filming process and was output directly onto video, meaning that in order to be upgraded to HD for Blu-Ray, CGI from that era needs to be completely junked and re-rendered from scratch at a higher resolution, which is astronomically expensive. This makes a Blu-Ray release of, say, Babylon 5 virtually impossible (although B5's situation is complicated by the loss of some of the original film stock) and even makes Deep Space Nine and Voyager dubious, unless TNG's re-release is hugely successful. Fortunately, TNG used pure-CGI scenes only a couple of times during its run (in the first season, most notably for the Crystalline Entity in Datalore) and these can easily be re-shot.
As the documentary on the Blu-Ray states, the re-mastering process is essentially straightforward in theory, since all of the artistic decisions (composition of shots, which takes to use etc) were made back in 1987-88 and thoroughly catalogued and noted. For the most part, the new editors simply had to rescan the film negatives for each take and re-compile them according to the notes (or simply comparing them to the existing DVD/VHS versions). However, they almost immediately ran into problems of missing effects footage. Because video images degrade each time they are copied, every time the original editors decided to re-use an existing flyby shot of the Enterprise, they returned to using the original film footage. This meant that, in storage, the film footage moved from episode to episode as each shot was re-used. In some cases, they found shots of the Enterprise belonging to the pilot episode, Encounter at Farpoint, in the same storage boxes as episodes from the third and fourth seasons. More inexplicably, entire scenes would sometimes be misplaced. A 12-second sequence from Sins of the Father in the third season wound up in a totally different episode box. In some cases the new editors spent weeks tracking down a single shot of just a few seconds to reunite it with its original episode. This hard work paid off: out of the 1,150 minutes of the first season, only 2 seconds of footage from We'll Always Have Paris could not be found, and the original SD video had to be used and upscaled. If you can spot it, I'll be impressed.
We come in peace, shoot to kill!
Another problem also reared its head immediately, although this was anticipated: as effects were added on video, the new editors had to redo every single beaming effect, every single phaser blast and every single visual effect of any kind, both in the live-action footage and in the space scenes. Every single scene involving bluescreens also had to be re-composited. What was not anticipated was that the a lot of the original matte paintings, particularly of the planets that the Enterprise orbits, had gone missing in the intervening time and the surviving low-res bitmaps were not of any use. So almost every single planet in the series (barring the one in 11001001, which is a high-res matte painting of Earth taken from The Search for Spock) has been replaced by a highly-detailed 3D model, based on the corresponding matte painting but with far more detail and, noticeably, animation (i.e. the planets spin as the Enterprise orbits them). This also allowed the effects team to fix a few instances of blatantly the same planet being re-used by altering terrain details and a few colours.
A happy by-product of this approach is the total elimination of matte lines. In the original video master, lines around and mild discolouration of the spaceship models can sometimes be seen, a result of the inferior editing technology of the time. By re-compositing scenes with state-of-the-art equipment, the effects sequences are now flawlessly displayed. This is most notable in the opening of the pilot, with a beauty shot that starts low and then moves up to the underside of the Enterprise, over the front of the ship and into the bridge. The original version, with the contrast turned up, displays clear matte lines around the Enterprise which are now completely absent. The HD presentation also radically improves the image quality of the models themselves, resulting overall in impressive effects sequences that would look excellent in a modern movie.
The same applies to effects sequences involving actors. Phaser blasts are more vivid, composited scenes dispense with the more obvious 'bluescreen-ness' elements (fuzzy lines around actors being vapourised or whatever) and people beaming in and out of scenes are no longer accompanied by subtle shifts in lighting or colour (or at least not as noticeably as before). One drawback of HD is that in a few cases the use of blatant stunt doubles is more noticeable than before.
Overall, the Next Generation remastering project - the largest remastering project of its kind ever attempted - is a stunning piece of work. It takes an old show from the 1980s and makes it look like it was filmed yesterday. If successful, this remastering technique could be used to 'save' many shows from the 1970s onwards that would otherwise be forever condemned to inferior SD upscaling. Certainly the new Blu-Rays need to be a success for this to happen and other shows to follow.
Season 1 of ST:TNG is out on Blu-Ray now. Season 2 will follow in a few months. CBS hope to release all seven seasons by 2015, with a possible update of Deep Space Nine to follow if the re-release of TNG is successful. My own review of TNG Season 1 will follow in a few weeks.
Fuller coverage of the Blu-Ray remastering, including interviews with the people working on it and numerous comparison screenshots, can be found on the TrekCore site.