AE Keying Technology in Film and Television Post-production

AE Keying Technology in Film and Television Post-production

AE keying technology has been widely used in many fields related to film and television processing, and has become an indispensable link in later film and television processing. This paper introduces AE keying technology in film and television post-production in detail from the definition, material preparation and basic technology of post-keying, and finally further analyzes the application of AE keying technology through practical cases.

1, key definition

Keying refers to extracting a specified image from a picture or video picture in post-processing and synthesizing the extracted image into a new scene, thus increasing the vividness of the picture, which is called keying in technical terms. Generally speaking, keying refers to taking a certain color value or brightness value in an image to define a transparent area. All pixels with similar colors or brightness values in the picture become transparent, so as to extract the subject. Nowadays, key imaging technology has become an indispensable link in most post-film and television processing.

2. Material preparation

For the video material to be keyed, the early shooting and preparation work is very important, which almost determines the final effect of keying. According to the principle of keying technology: a certain color in the video picture is converted into transparency and the Alpha channel is extracted, so keying technology usually requires the background color of the lens picture to be as single as possible. Therefore, in the actual shooting process, the subject is usually required to complete the action performance of the film in front of a relatively single and high-purity color background. Theoretically, the background color can be any color that does not exist in the foreground color. We usually use blue or green background in real shots, because people's bodies do not contain these two colors. The skin color of China people is mostly yellow, and the complementary color of yellow and blue background is usually used. After the sample is sent, the skin color will be whiter, which can achieve better results, while the color effect of the yellow skin before the green background is slightly worse than that of the blue background. In western countries, because many Europeans and Americans have eyes close to blue, they often use green backgrounds when shooting people. Usually, the shots to be keyed will be shot in a professional solid color studio. At present, the most widely used is the "blue box" shooting. In this way, the blue background can be removed in post-production and then the foreground can be synthesized with other materials. Blue box technology is widely used at present, such as the movie Avatar. The shot material is collected in the computer and processed by post-production software.

3. The basic technology of late keying.

In the post-production of keying, the collected materials are mainly keyed by film and television post-production software, keying special effects or plug-ins, so as to preserve the foreground and remove unnecessary background. Through key treatment, its new background material is synthetic. In the later period of keying, the commonly used keying techniques are: chroma key (blue screen or green screen keying), brightness key, difference key and so on.

3. 1 chroma key

Chromaticity key, also known as color key, is a technology that software automatically removes a color and its similar colors from an image by specifying a color. Blue screen keying and green screen keying are commonly used, but in principle, the background is not necessarily blue or green, and other solid colors can be selected. Generally speaking, the greater the contrast between the color to be keyed and the color to be retained, the better the color keying. In After Effects, the adjustment of special effects is a very delicate process. A good keying effect can not only keep the foreground pixels completely, but also remove the background pixels cleanly. At the same time, if the foreground contains semi-transparent pixels and soft areas, this information should also be kept intact in order to prepare for the subsequent post-synthesis. The method and main parameters are adjusted as follows:

(1) Select a picture or video to adjust and use the command effects/keying/color. Use the eyedropper tool to absorb the color that needs to be deducted (that is, the color that needs to become transparent).

(2) Adjust the value of color tolerance, that is, control the color tolerance. The higher the value, the more transparent the specified color is.

(3) Adjust the edge feathering value. Edge feathering is the feathering effect of the control area, which can be adjusted slightly, but not too big.

3.2 brightness key

The technology of keying by brightness difference is called brightness keying. That is, select parts with similar brightness to make them transparent. When the subject is very bright, put it in front of a dark background to shoot, and the light will only illuminate the subject, thus shooting a picture with a completely black background and a bright foreground. Then the channel is extracted according to the brightness difference by brightness keying technology. This matting method is usually used to shoot scenes with large contrast between foreground and background light, such as cutting black characters from white paper. The method and main parameters are adjusted as follows:

(1) Select the image or video to adjust. By using the "Effect/Keying/Brightness" key command, first select the keying type. By specifying a value for the key type, for example, selecting the key highlight view mode, pixels with brightness values higher than the specified value will become transparent.

(2) Adjust the tolerance value, which is used to set the range of the specified brightness value. The lower the value, the closer the extracted brightness value is to the specified brightness value.

(3) Adjusting the edge feathering value to set the extracted edge feathering degree. The larger the value, the softer the edges. However, if the value is large, it will also lead to a long rendering time.

(4) Adjust the value of "Edge Thin" and set the thickness of the edge of the extraction area through positive and negative values. A positive value will increase the transparent area, while a negative value will decrease the transparent area.

3.3 Difference key

The transparent area defined based on the basic background image is called the difference key. Through the differential key, not only the solid background can be dug out, but also any background can be dug out. It uses two different colors to key the image. In actual shooting, when there is no condition for blue screen or green screen shooting, or the shooting action can not achieve the effect of solid color keying, you can use the difference key to key. Differential keys also have some disadvantages. Due to the subtle changes of shooting light, video noise, film particles and other possible factors, the background before and after shooting is not the same, so it is difficult to extract pure channels. In practical application, differential key is also a very practical key technology. The method and main parameters are adjusted as follows:

(1) Select the picture or video to be adjusted and use the command effect/key/color difference key. In fact, when we use this command, we can see from the picture that the background is basically transparent, but the local effect may not be very good and needs further adjustment.

(2) In the view mode, select the mask mode and view/mask correction. Among the three eyedropper tools, the second eyedropper is used to specify the transparent area and the third eyedropper is used to specify the opaque area. Then we can get better results by fine-tuning the parameters related to partial A in **/partial A out**.

3.4 Key Light Keying

Key light plug-in is a very effective, convenient and powerful keying tool, which has been acquired by Adobe and turned into an internal plug-in. Plug-ins can identify pictures by selecting key colors and remove the selected colors. By adjusting the three colors of black, white and gray in the screen mask mode. Black means completely transparent, white means completely opaque, and gray means translucent. By adjusting the alpha channel, we can select the satisfactory effect. The method and main parameters are adjusted as follows:

(1) Select the picture or video to be adjusted, and use command effect/keying/key light to add key light effect. In the "Screen Color" option, use the eyedropper tool to absorb the color that needs to be deducted (that is, the color that needs to become transparent).

(2) Adjust the value of the screen pre-blur parameter, and don't adjust it too much. If it is too large, the details of the image edge will be lost. The numerical value should be adjusted to the best effect according to the actual situation, that is, the edge of the image is softer.

(3) Switch to the screen mask option to further adjust the tuning range. The white area represents the reserved part, and the black area represents the cut part. By adjusting the values of the two parameters of clip black and clip white, the gray part in the material can be turned into black or white. If there is less gray, you can directly adjust the screen gain to adjust the color.

(4) Adjust the values of Alpha bias and desppillbias parameters, and adjust the image edge for anti-overflow.

4. Take the basic key technology color key as an example.

The following uses color key keying technology to complete a "background replacement of cartoon pictures".

1) Start AE software, import two materials as shown in the figure below, and create a new composition in the project window, named "cartoon picture background replacement". Select two material pictures in the project window, add the pictures to the newly created synthetic "cartoon picture background replacement", and rename the two layers as "cartoon picture" and "background" respectively.

2) Select "cartoon picture", select the command effects/keying/color key, select the eyedropper tool in the key color column, and click in the blue part of the composition window to absorb the color removed by the key, as shown in figure 1.

5. Concluding remarks

In After Effects, there are many kinds of keying techniques, and each keying method should have its own characteristics and advantages. In practical application, the most suitable keying method should be selected according to the actual situation of materials. Sometimes, for complex keying processing, it may be necessary to combine two or more methods to obtain satisfactory results. Finally, it is necessary to adjust the color saturation of the keyed picture and the background scene to ensure that the final fused picture can have similar color environment. Using AE keying technology can finally provide us with clearer, more vivid and realistic visual effects.

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