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Filed under: Cameras •Hardware •Production •

Greenscreen Primer Part 2

In the first installment, we looked at the basic set up for shooting Greenscreen. Most of what I covered is exactly what we see people skip everyday. What I’m going to talk about next is what everyone else ignores at their peril. We’re talking about capture systems and codecs.

As with color correction, capturing greenscreen plates is more than just resolution. Color depth and, more importantly, color scaling make a big difference. How you capture your precious cargo can make the difference between getting by and getting ahead.

Most people capture to their camera’s internal storage - Generally - P2, SxS, HDV Tape, or SD tape. Getting video onto these tiny cards (or tapes0 requires some compromise. Here’s why…

When a camera’s HD chip captures the world in front of it, the data pipe can be roughly 180 megabytes to over 400 megabytes per second. At this rate, most memory cards, if they could write that fast, would fill up in less than a minute. To get more data onto the card, and to allow it to keep up at all, color scaling and compression is applied. For many video projects, these manipulations are not apparent. Most of what we focus on in an image is the luma detail (black and white information) and the primary subject of the video. Much of this is left intact. BUT as compositors, we need more. When a camera throws away color data, it is throwing away exactly what we need to do our work.

Here’s the filesize differences between popular capture formats. While size isn’t everything… it’s something to consider.

Color Scaling

We’ve all heard the terms… “4:1:1”, “4:2:2”, “4:4:4”, “4:2:0”. For many, this is some course measure of quality. For greenscreen compositors, this is the terrain: From the smooth slopes of 4:4:4 to the rocky coasts of 4:1:1. Color Scaling will set the tone of your composite and ultimately define how far you will go, and is the first “handle” that you will have to get ahold of when determining the quality and file size of what you capture.

In a 4:1:1 frame, you have luma information for every pixel across the image, but color information for only one quarter of the pixels (above). This may work okay for normal footage when it is not as obvious (our eyes are more sensitve to luma than to chroma), but produces a terrible color key (below).

Color Scaling is simple enough. We start with YUV, Y is the Luma, U and V represent the color information. The magic numbers represent the ratio of YUV. So, if every 4 pixels of luma are matched with 2 in the UV… we have 4:2:2.

We generally keep all of the luma information (because it will look soft if we don’t); we then throw away color information. If you’re lucky, the process only throws away half the information but it’s usually more. HDV is 4:2:0, as is XDCAM. This is an overview. Adam Wilt has covered this in great detail in the past in his various camera reviews; you should read his articles on the subject if you want to dig deeper.

Half the color information in 4:2:2 means you are keying an image half the size of the full frame and then mapping that back to your selection. When you see the stair steps down the side of your next key, this is why.

There are 4:4:4 options. Sony F-950s, F-23s, and F-35s along with a few others all shoot 4:4:4. But you pay a dear price this color scale. Most of these camera bodies start at $100k or more. The Red is one exception but it reaches 4:4:4 at the cost of definition… we’ll talk about that later.

Color Depth

But it’s worse than all of this… 4:2:2 is the least of your problems. In fact, a large amount of our keying is done on 4:2:2 footage. The difference in our case is that the footage is uncompressed. Uncompressed?? You thought 4:2:2 was compression… no, that’s just scaling the color channels. You actually have four handles that control the quality and file size. These handles are Color Scaling, Color Depth, Temporal Compression, and Spatial Compression. We just covered Color Scaling, where the camera or system actually scales the color information (throwing away up to 75% of it) to save space. This is generally not something you can choose to do after you choose what type of camera or capture system that you are using.

Color Depth is the number of bits used to describe each pixel. Typically this is 8 bit or 10 bit in capture formats. 8 bits provide 256 levels in each channel. 10 bits provides 1024 levels in each channel. Why does this matter? If the footage is well shot (see Part 1 of this series), it doesn’t really matter. BUT, if the footage has a large variance of lighting across the screen, more bit depth gives you more data to work with.

Compression

The last two handles are temporal and spatial compression. Temporal compression looks at successive frames and tries to intelligently throws away data used in all the frames. This can create relatively small file sizes but it also can create files that are difficult to edit because of the lack of keyframes in important areas. Spatial compression looks at the colors within a single frame and tries to clump similar colors together as discretely as possibly. Unfortunately, this can often leads to square edges on subjects as the macro blocks (the clumped areas) are designed not to noticeable in uncorrected images. When we change the color levels, these perceptual cheats show up. We change the levels a lot in keying.

Whew… why do we need to know all of this? Because it factors into all of our shooting calculations. As we all know… life, and production, is imperfect. We really get to operate in a pristine environment. We need to make compromises. Understanding all of the factors allows us to compromise effectively. For instance…

next page: our setups

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Awesome. I listened to TWIM before Daisy and had a tough time figuring out what was being said. Thanks Alex!

Posted by node1.me  on  09/12  at  07:35 PM

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Hey Alex. Great info as usual.
Just wondering how apple’s new codec
ProRes 4444 fairs in all of this?
Thanks.

Posted by .(JavaScript must be enabled to view this email address)  on  09/13  at  03:39 PM

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Um… where’d part 1 go? The page is empty: http://provideocoalition.com/index.php/pvcpostpipeline/story/greenscreen_primer_part_1/

Posted by node1.me  on  09/16  at  03:31 PM

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Oh. I found it. Here: http://provideocoalition.com/index.php/alindsay/story/greenscreen_primer_part_1/

Posted by node1.me  on  09/16  at  03:33 PM

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ProRes 4444 is working very well (we’re using it currently for a film project).

a

Posted by Alex Lindsay  on  10/05  at  06:08 PM

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Great article!
With ProRes 4444 does capturing uncompressed still makes sense? I heard ProRes 4444 is basically indistinguishable from uncompressed.
And do you think its worth it to bother capturing Cineform on a Mac?

When can we expect part 3?

Posted by .(JavaScript must be enabled to view this email address)  on  02/22  at  10:28 AM

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We’re starting to capture with APR 4x4. I don’t have a definitive decision yet.

Posted by Alex Lindsay  on  02/22  at  11:48 AM

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