Inserting explosions into your movies is the most common special effect among filmmakers. But to make a credible explosion it is necessary to take into account all the details: in this article we see a procedure to give the scene maximum realism.
Once the explosions of the films were true: a building was built, it was filled with explosives, and it was blown up in front of the cameras. We often made models, to take fewer risks and have less costs, but sometimes we worked at full size. Today, real explosions are still used, but only in small and harmless cases. For the rest, the explosions are made in computer graphics. In high-budget films, the scene is reconstructed using camera tracking with a program such as Blender (or other commercials) and an explosion is simulated in a big way, with smoke and flame simulators. Instead, when the budget is lower, ready-made films are used, which simply have to be superimposed: this is the case that we will see in this article. These films are usually real explosions filmed on a uniform background: with the chroma key technique the background is removed digitally. In general, the explosions are all the same: basically it is a bubble of gas and smoke, it does not have a precise orientation. This means that if the situation is not too complex, the technique of superimposing the video of an explosion can give excellent results. There is a problem, however: simply superimposing the explosion on our movie does not provide a good result: firstly because being positioned above the original video it will also cover objects that should instead be in the foreground. For example: usually an actor is blown up in front of the explosion, to make it more likely: by superimposing the explosion on the original movie, the actor will be covered by the blaze. This problem can be solved with rotoscoping: it is sufficient to superimpose the explosion on the original film, and in turn the original film on the explosion. The last level, however, must be masked, so that you only see the objects that must be in the foreground, in front of the explosion. There is, however, one further question to be taken into account: the crater. If an explosion occurs on the ground, a hole will form. It is therefore evident that a hole must also appear in our video. The simplest method is to shoot the original movie with the crater, and then digitally make a version of the same movie with the terrain properly reconstructed. In the end, with a simple transition between the two versions of the movie, the crater will appear at the right time. Reconstructing the ground is simpler than you might ask: what we will do is to superimpose the original movie itself: in the superimposed layer a mask must be applied to make visible only a part of the video, the one we intend to use to reconstruct the ground. In our example, we have made a portion of the grass visible. Using the composite transition, it is possible to move the superimposed video track to cover the crater: of course, the operation must be repeated until the entire crater is completely covered by grass. Also in this case it will be necessary to apply an additional layer on top of all the others, to make sure that the actor is not in turn hidden by the grass that we have copied and pasted. You can find the example video at the following address: http://www.youtube.com/watch?v=uLKYmRmpsF8
Where to find the effects
In our examples we used two ready-made effects found on the site http://detfilmshd.com/ . This site provides dozens of movies of various special effects, of decidedly high quality. The videos are available in two versions: the original one, with a blue or green background, and the one with the chroma key already applied (therefore with a transparent background). We, in our example, have decided to use the PK versions, that is PreKeyed, without background: this in fact allows us to skip a rather annoying passage. The effects of detfilmshd are not free, but being able to buy each effect individually it will not be necessary to spend large amounts: with $ 28 you can have the effect of the main explosion ( https://www.detfilmshd.com/G_Collections_PK_Explosions_01.html ) and for the same amount the cloud of smoke ( https://detfilmshd.com/G_Collections_PK_LandMines01.html ).
All the clips we need
We prepare the necessary to realize the effects
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Let’s put on a patch
Let’s rebuild the ground where the crater is present in the movie
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In the foreground
We bring the actor back to the fore
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End with a bang
The last operation consists in superimposing the explosion on the film
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