Before the advent of computers, blue/green screen effects were achieved through a process known as chroma key compositing. This technique involved filming an actor or object against a solid-colored background and then replacing that background with a different image or video.
In the case of blue and green screens, these colors were chosen due to their lack of prevalence in skin tones and clothing, making it easier to key out and replace the background. The process involved filming the subject against the blue or green screen and then using a film printer to superimpose the image onto another piece of film stock, which contained the desired background image.
This process was labor-intensive and required a high level of expertise and precision. The lighting of the subject and the screen had to be carefully controlled to ensure that the edges of the subject were clearly defined and that the color of the screen was even and consistent. If the lighting was not properly set up, the edges of the subject would appear fuzzy and the color spill from the screen would be visible, ruining the effect.
Once the footage was captured and printed, it was then edited and spliced together with the other elements of the shot. This was done using a film editing table, where the various pieces of film were physically cut and spliced together. The final result was then printed onto a new piece of film stock and could be shown in theaters or on television.
It's important to note that before the widespread use of chroma key compositing, special effects were accomplished through a variety of other techniques, such as rear projection, matte painting, and miniature models. Chroma key compositing was a significant advancement in special effects technology and paved the way for the development of more sophisticated techniques, such as computer-generated imagery (CGI).
In conclusion, blue/green screen effects were achieved through chroma key compositing before the advent of computers. This technique involved filming the subject against a solid-colored background and then using a film printer to superimpose the image onto another piece of film stock, which contained the desired background image. The process was labor-intensive and required a high level of expertise and precision, but it was a significant advancement in special effects technology and paved the way for the development of more sophisticated techniques.