Submitted by James Cole
In this article I'll talk about how to replace the backgrounds in your shots. Some shots are really easy and require only a simple sky replacement against a hard edge such as a building. Others require a bit more work, like this shot:
I replaced the background with a destroyed city and mountains behind that and some cool looking clouds to set the mood. As you can see, I didn't have a nice straight edge to work with either. The city in particular had to go behind a treeline. And to top it off, the whole shot had slight camera movement.
I have divided this article into three parts as there are quite a few elements involved in piecing this shot together. Part one is Destroying a City Without Killing Anyone. Part two will be Preparing the Shot for Compositing, and Part three will be Putting the Puzzle Together.
Destroying a City Without Killing Anyone
The first part of the process was to make the city look as if it had been destroyed by a meteorite collision. First I needed a still photo of a city. Living in Melbourne I thought it would be easiest to use a photo of Melbourne. Digital stills photographer, Emma Jaques took the photo (left) for me to use. I began by importing the photo into Adobe Photoshop and removing the sky and cropping out anything I didn't need. Once I was left with just the buildings I wanted, I began cutting chunks out with the eraser tool leaving jagged edges.I painted on some broken windows and even cut a hole in one building. Wherever I had an extruding chunk of building, I underlined it to give the impression that you could see the ceiling of the floor above, to add depth.
I added depth and detail to the buildings by cloning an image of another building that was already destroyed to add different floors and all the junk inside. This was still far from being finished though. The buildings were too clean and there just wasn't enough of them.
One thing I decided early on in pre-production was that I didn't want Lost set at any recognizable place. It is essentially a fantasy world. This gave me the freedom to add buildings that weren't really there. I wanted this to look like it was once a busy skyline. I took buildings from other photos and gave them the same destructive treatment as I had with the original photo. Now it was looking better. Still too clean though.
To dirty up the buildings, I created 3 layers. I had to think about how old these buildings were and how long they had been destroyed. The movie is set 33 years after the meteorite hit so there would be some sort of plant growth covering the buildings as well as the general dirt. The first layer, I just painted on browns and oranges and some blues. The second layer I used as the vegetation layer. I cloned areas of grass and distant trees and water from landscape photos. The final layer was just painted with smeary blacks and greys to dull everything down. The picture (left) shows the three layers and the final mix.
Once these layers were in place, I played around with the opacity and different blending modes to get the right look. I left the finished picture a little more colorful than it should be so that I would have more freedom once it was imported into Commotion to composite into the shot. I eventually desaturated the picture quite a lot to make it blend in better with the foreground.
To insert the city, mountains, and clouds into this shot, I first had to get rid of the sky. When I first looked at this shot I though it would be a simple key using Primatte Keyer in Commotion due to the fact that the sky was a completely different color from everything else in the shot. This however was not the case. As the sky was so bright compared to the dark trees that made up the skyline, the light wrapped around the edges giving them a very blue tint. Some areas of the trees disappeared due to the very fine detail.
In order to fix this I took a single frame into Adobe Photoshop. The first step was to erase the sky. I did this by using the magic wand tool and a little fine tuning with the freehand selection tool and then deleting it. Using the clone tool I erased the two characters as they will need to be walking in the finished shot. Continuing with the clone tool I corrected any area in the trees that had the background light wrapped around it. This also included filling in any gaps left by the leafless branches. With the smallest brush, I traced over the entire edge of the tree line keeping up the jagged edge that the foliage would normally create.
Now that I had a solid tree line, I had to reinsert it into the shot and make it look natural. The shot originally had a large pan in it but during editing I cut most of it out. However there was still a slight camera movement. To place the trees back into the shot seamlessly I had to first match the movement of the shot. I used the corner of the door on the house in the background and also the tip of a power pole you can see midway between the character and the right edge of the frame. This gave me the movement of the camera both up and down and also the rotation.
To blend the still frame into the shot I found a natural line running along the base of the trees and over the house. I made a rotospline that would follow this edge but made sure that the characters heads did not cross it. To complete the task I feathered the edge of the rotosplines and placed the still frame behind the main shot. As the trees were motionless I added noise to bring them to life. With the movement of the camera matched perfectly the work is unnoticeable.
Now that I had all the main pieces ready for the shot, it was time to put them all together. I began by inserting the city behind the trees. Using a combination of tools in Commotion such as levels and composite color matcher, I toned down the brightness and the contrast of the buildings to make them appear in the distance. I also added various blurs including super compound blur to marry the city to the original shot seamlessly.
I selected a photo of a mountain range that I thought looked pretty cool and also one that was lined with trees at the base so that it would match the shot better. I placed this behind the city layer and once again played with the color and contrast to blend and give the illusion of distance. I feathered the top edge of the mountains quite a lot to give the feeling they were almost touching the clouds. Finally I placed the footage of the clouds behind everything.
I went over the image to give it some fine tuning, including feathering the edge of the trees so they would blend into the new background. The trees on the left required more feathering to blend into the background mountains than the trees in front of the buildings. To do this I copied the layer of trees and using the rotosplines I cut them in half. This way I could add different amounts of feathering, blurring, and blending to the different sides. For a final touch I added different noise levels to both the still images of the mountains and the city buildings. By adding noise it adds to the feeling of movement of the otherwise still image. Most people will never notice this, however if it wasn't there I'm sure people would feel that something was wrong with the shot without being able to put their thumb on it.