When doing this project, the focus was the greenscreen component. This was taking someone from an existing piece of footage and placing them into new scenery. The goal of this was learning tips on how to record green screen footage, and chroma keying, in adobe after-effects. We also did extra elements, like making the “main character” more believable within the new scenery. These include light wraps, and lens aberrations.
Chroma Keying, and the Alpha Channel
Every digital image is made up of 4 channels. These channels are Red, Blue, Green, and Alpha. The alpha channel communicates with the other three channels, and can switch off the other components (i.e. red, blue, and green). With an alpha channel, the white pixels are “activated”, and the black “deactivated”. This was told to us by referring to binary, with white being 1 and black being 0.
When chroma keying, one of these alpha channels is selected, and removed from the picture. This is why green and blue screens are used when filmmaking. When we chroma keyed our project, we cut out around the “main character” and the greenscreen using a rotobezier, and then selected the opposite of the cut footage. This was so that the components on the border of the image weren’t visible (i.e. the man holding the green screen). After this, the main character, greenscreen, and corner wall were visible. This created a mask that “deactivated” the unneeded components within the alpha channel.
Using a keylight keyer, an effect within our program, we now removed the greenscreen. This was done by using an eye dropper to remove the green component. This deactivated the pixels in the alpha channel. Viewing the alpha channel, we increased the “clip the black” value within the effect which removed the grey elements of the alpha channel. This is where the green pixels weren’t deactivated. Then, we inserted our new image behind the footage. This was the factory background. The new background image was modified o better fit the scene (rotated, mirrored, changed in scale).
Colour corrections, and Background Matching
To match the new background, we used the RGB curves effect. We matched the neutral colours using certain indicators between the separate footage. For example, the black coat represents how the black colour in the footage should look. This was then compared to blacks within the background image and amended accordingly.
Lightwraps, and Making a Convincing Image
We added a camera lens blur effect to the background and modified the blur radius. This made the background look “out of focus”, so the background and foreground didn’t look bizarre and out of place.
When an object is exposed to a certain degree of light, it bends around its edges. This can be replicated with the “lightwrap”.By exaggerating the screen softness around the footage, it blended with the background. Then we copied the composition footage and gave it the silhouette alpha effect. This uses the alpha channel to punch a hole in the background. Then, an adjustment layer was added to act as the “light wrap”.
Additionally, we added a film grain effect, which is available in after-effects, to emulate the film grain of camera footage.
Final Footage
The Future
When starting this specific project, I had done chroma keying and such for personal and university projects. Whenever I did it, the final effect always looked “off”. Now having undertook this module, I can recognise that I had basically finished at the first step. Now I know how to create a more convincing final greenscreen image. With the amount of movies nowadays that use green screen in filming, this’ll significantly help my professional career.