Why do movie stunts need to be performed on a blue background to be modified?

First, the application principle of blue/green screen

The purpose of blue/screen shooting is to clearly distinguish and separate from the foreground and performance objects in post-production. The better the pre-shooting of blue/green screen is, the more assured the quality of the post-keying will be. In order to achieve this goal, it is important to understand the principle.

the important principle of blue/green screen shooting is color exclusivity. The basic structure of film is composed of three layers: red, green and blue. From the root, red, green and blue can be used as background colors. In order to separate the background from the foreground, color exclusivity is very important. At the same time, the exposure sensitivity of red, green and blue colors to the film, color overflow, environmental color similarity and so on are all considerations in choosing the background color before shooting. The basic structure of film is shown in Figure 1. Modern film consists of nine layers, with a total thickness of .65 inches, that is, .165 mm thick. The order of arrangement is blue, green, red and base layer, with blue at the top. The coating between the three colors is to protect their respective colors from being affected.

The basic principle of post-keying is to find out the difference between the color and lightness of the scene before and after the picture. For example, use chroma key to select blue or green samples on the background, and then select all the colors in the picture that are the same or similar to the samples for separation. There are many methods of keying, such as luma key, color key, difference matte, spill suppressor, linear color key and so on. These techniques are used separately or comprehensively according to the different shooting materials in the early stage. If you don't grasp the pre-shooting, it will be difficult to make it wonderful in the later stage.

Give two examples. For example, it is necessary to shoot military-related scenes. Soldiers wear green uniforms and are painted with green camouflage patterns. Under this condition, you need to choose a blue background screen; For another example, in the future, shots with sci-fi themes will use a lot of blue tones, so it is necessary to consider using green as the background. Only by careful consideration can the foreground object elements be better separated from the background.

Second, why can blue/green screens exist at the same time? What is the difference between them?

Whether to use blue or green has been debated at home and abroad. There are many sayings: foreigners have blue eyes, so they need to use a green background; The color of the sky is blue, and it is necessary to use blue for location shooting. There are many different opinions, of course, there are some reasons.

here, the characteristics of blue/green color are objectively explained from the aspects of film, film camera and video.

For film, the grain structure of the green layer is the finest. In the video NTSC format, the green channel has the best sampling rate. In film, the sharpness of blue layer is the best, and the grain structure is also the finest (the same as that of green layer). In the video format, the blue channel is the noisiest. If you shoot in DV video (4:1:1) format, the best choice is green background.

Red, green and blue can be used as the background, and blue is frequently selected for the following reasons: blue is the complementary color of our skin color, and in most scenes, the tone is similar to the skin color; The logical choice is to use relative colors and ignore the use of close colors. From a historical point of view, movie cameras and films are very sensitive to blue light, but from the perspective of today's development, these views are not very accurate. Green has its own advantages. When there are blue objects in the foreground, it has obvious adaptability when painting the mask part. In the process of drawing mask, green has obvious reflection ratio, which is easier than blue. At the same time, video cameras are usually more sensitive to the green channel, and the best resolution and picture details are generally in the green channel. However, the disadvantage of green is color overflow, and even in a small area, the undesirable green influence can be found. For blue, sometimes the overflow blue is really not obvious.

sometimes (in fact, it is normal, but sometimes it is not obvious) the background color will be reflected to the foreground object. When shooting with a blue background, there is a slight blue infection at the edge of the foreground object. This is considered as a blue overflow. From the point of overflow, blue is slightly better than green. The use of red as a background is very rare, which has a lot to do with its high color overflow.