Archive for December, 2008

Had a comment about this on my previous post so thought I’d put abit of info in on green screen. Everything herein is pretty much self taught as I’ve not really been able to find much info on chroma-key (as it’s also known) so I’m sure there’s plenty of omissions, but here goes…..

So first off you need a green screen. My first attempt involved painting some boards green with the brightest green paint that my nearest DIY store could mix. Despite being a cheapo option this actually worked!….well, kinda. Here’s a still from my first stop motion animation ‘Real 10‘.

OK, so doesn’t look too bad?  Well actually this will be (and was) a pain to get to work.  Obviously there’s a large section to the right which the green screen doesn’t cover.  This isn’t actually as much of a problem – I routinely place masks over chroma key shots (i.e. placing a green section of image over the area to cover the section).  These are usually .pngs as the images allow you to have transparent areas within the image, allowing the section you want to keep to show through.

The problem with the rest of the image is the shadows and uneven shades of green across the image.  The bottom is much darker than the top.  I ended up having to repair the greenscreen frame by frame in photoshop for this shot, which isn’t ideal – it’s rather time consuming.

The key to getting shots ot work is to make sure you have an even colour across the background so that the computer can easily eliminate it and replace it with the background shots you want.

A background with a flat background colour will make it easier for the computer to isolate the green screen sections.

The main way to achieve this is decent lighting. I used halogen security lights on my first animation.  These are good because they’re cheap, however they also do not produce even levels of light – as the element heats up the colour they emit changes and the levels fluctuate.  Because of this one frame of your animation will be brighter or a slightly different colour than the others.  You end up with a flickering effect when the frames are played back.

I’m now using photography lights which seem much better, though they are a little more expensive.  I’ve also got a fabric greenscreen and stand which can be had from ebay for about £100.

The last thing you need is some software.  I’m currently using Pinnacle Studio which is abit of a cheapo program, but it does combine the chroma key tool with an editing interface.  Adobe After Effects will also do the job.  I’m sure there’re others.

Here’s a short video showing some of the green screen elements for my animation Real 10:

So been a while since I posted anything, not because I haven’t been doing anything (well it’s partially that I’ve been lazy!) but also because the shots I’m currently working on are some of the more complicated involving meshing stop motion animation shots of the robot with live action.

As it turned out the shots I filmed without the green screen were much better than those with.  I think this was mostly down to the lighting levels which were altered when the green screen was put up and blocked out alot of the natural light which was coming in through the windows.

Unfortunately I didn’t have enough lights to match the levels I’d need to replicate the correct levels, which just goes to show the budget I’m working on (I think I spent about £250 on lights, not exactly a massive amount!).

Because of this it’s been more difficult to integrate the live action footage with the animation shots, in most cases requiring me to remove the windows in the shots where the robot can be seen outside frame by frame and then inserting footage of the robot behind where the windows were.

The shot where Tom and Pete see the robot and panic is 64 frames long, and each frame was done by hand one after the other in Photoshop.  The shot where the robot punches through the wall was about 30 frames, but was slightly more complicated as I had to animate the wall breaking and then composite falling debris over the shot as well.