Fluff from the naval of my mind

There are few things in life important enough for me to venture out onto the British roads during a Bank Holiday weekend.

The sense of horror that washes over me when I merely contemplate the plunge into the tarmac terror that is Bank Holiday traffic is enough to persuade me that, short of life threatening circumstances, the project can be delayed.

Why do I raise this issue now? Well, first, and most obviously, it is a Bank Holiday weekend. Secondly, while playing with chroma-key video editing (you know, the green/blue screen technique for separating and transplanting one image into another) I discovered that, while I could achieve reasonable results with a very rough set-up, what I really need is better lighting.

This all stems from my need, not for perfection, but to get the best result I can from my system. The masks I was getting, even with the limited system I was using, were not too bad. The tricky parts of masks tend to be fine detail like hair. One ends up with either flat hard edged hair that looks like someone has cut you out of a magazine, or you get a distracting flickering halo at the edge of the hair as the algorithm struggles to show the wispy hair as semi-transparent. Neither result met my criteria for ‘best result possible’.

The difference between generating a reasonable mask and a good mask using consumer video cameras are two-fold:

1. Good software
2. Good lighting

The good software comes, for me, in the form of Apple’s Final Cut Pro and a superb plug-in dvMatte Pro (more on these in a moment).

The good lighting proved to be a problem. You see, I’m not a big fan of lights. I have exactly three lamps in my house; one beside the bed, one I use as a video light in the study and a ‘daylight’ lamp I have on the desk. All other lighting in the house is provided by 60W ceiling bulbs.

The upshot of the paucity of flexible lighting (or even bright lighting) makes it tricky to brightly and evenly light the green backdrop and makes correctly lighting the foreground (me) almost impossible. The different types of light make colour balance tricky and the inability to evenly light the backdrop made it even more difficult to produce a clean mask.

The standard matte tools in Final Cut Pro are designed for use with high quality video. Consumer cameras tend to produce low quality video. Low quality in the sense that it is compressed and this compression causes artefacts in the image imperceptible to the human eye but all to evident to software algorithms. The practical upshot of this being that it is nigh on impossible to produce a sharp mask with the built in matte tools. dvMatte Pro to the rescue!

dvMatte Pro is nothing short of astounding. Designed specifically for creating mattes from DV and HDV footage this plug-in worked wonders with even the roughly lit footage I produced while experimenting.

The conclusion that the problem was lighting was arrived at over several experiments. The best mask I could achieve used three lights; the overhead 60W incandescent bulb, a halogen lamp lighting the foreground subject and the daylight lamp (which has a greeenish tint anyway) lighting the backdrop from below. Using three different types of light creates ‘interesting’ challenges for colour balance too, but that’s another story.

Oddly, the best results also used the plain magnolia wall of my study and not the green screen. I suspect this is because the wall shows less variation across its surface when lit with just one lamp. Also, the daylight lamp produces a green tint on the wall (this tint is only apparent on video, it looks white to the naked eye).

The problem areas in the resulting mask were as follows.

1. Hair was being picked out much better than previous attempts, but still remained ill-defined and resulted in an odd shimmering halo when pushed too far.
2. Tops of shoulders had some odd highlight artefacts and even more peculiar a shadowy halo on the raw footage. I suspect this was partially because the screen was lighter around this area and the lack of a back light and weak foreground lighting conspired to produced the odd shadows in the video.
3. Ears also had problems for some reason. Getting ears clearly defined meant pushing the hair too far into the flickering halo effect.

All in all the results were comparable with green screening circa 1980. The foreground image showed haloing and did not integrate well with the scene into which it was dropped. If I hardened all the edges by cutting my hair or wearing a hat I have no doubt I could get a good result (this seems to be a common tactic among YouTubers), but that would be defeatist talk!

To get reasonable results I think I need better lighting to produce better defined raw footage and help the matte algorithm. Specifically I need to address the following.

1. Bright even light on the green screen.
2. Brightly lit subject.
3. Back light on subject

Bright even lighting of the green screen should make it much easier for the algorithm to identify. This much is obvious.

Brightly lighting the subject will help to sharpen the image and reduce DV artefacts due to poor light. The more light the camera has to work with the better and brightly lighting the background while ignoring the foreground will produce a problem with back lighting (a bit like filming someone against a window the image blows out unless compensated for in camera, but this in turn causes the backlit screen to overexpose which causes a whole other set of problems).

Back lighting the subject is, for this project, icing on the cake. It should help to eliminate spill from the green screen, help to eliminate boundary shadows on the subject and help punch the subject out of the background.

Having looked around at specalist video lights I decided they were too expensive for what I need. Frankly, I’m not looking at producing broadcast TV here. I just want to produce reasonable quality video. The solution seems to be 500W halogen work lights. They’re cheap, bright and flexible. I need two to light the screen, one for a back light and one for the foreground light. Four lamps, total cost around £80 (less than the cost of a single video light and stand).

Sorted then? Not quite. As I said, it’s a Bank Holiday weekend. The closest store I can find on-line that I know has these lamps in stock is a large DIY depot half an hour away and this project can wait.

I am not going to a DIY store, of all places, on a Bank Holiday weekend!