Make Film Work

Critter Watching

Posted by Jason on August 7th, 2010

Our friends, Justin & Elise, asked us to watch their friends while they went camping for a few days. Everything here was shot on iPhone 4 and finished in Final Cut Studio.

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iPhone 4 & Beverly

Posted by Jason on July 5th, 2010

A quick video shot and edited on iPhone 4 in Beverly, MA. If you have trouble with the video above, watch it on Vimeo.

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Our eyes don’t see all of the subtle variations between every color. We do, however, see the difference in shades of a color. In video, this is called luminance. The color is referred to as chroma, or chrominance. Since the dawn of video, cameras have been using a technique to reduce the amount of color information captured to reduce the amount of bandwidth needed to deliver it. Since our eyes won’t see these colors anyway, we’re not missing much.

How is it done?

The most common model of color subsampling uses an area that’s 4 by 1. Four pixels wide and one pixel high. For each of those pixels measurements are taken, and values assigned. The first is for how luminous each is, measuring once for each pixel. When we’re done we’ll have four unique measurements for each of our four pixels.

Next, color information is measured. We already know that our eyes aren’t going to see all of the color that we’ trying to record, so instead of getting a color value for each pixel we’ll be averaging the colors together. This dramatically reduces the amount of data needed to transport the image. Colors accuracy is also significantly reduced. When we’re done we’ll have one measurement that represents four pixels.

This example assumes we’re using 4:1:1 sampling. It’s commonly found in consumer DV cameras and for most applications it’s just fine. The real problem starts when you get into color correction and — even more so — when working with green screen. It’s then that the more color information you have to work with the better off you’re going to be.

Below are brief descriptions for some of the most popular color sampling methods.

4:1:1

DV video, represents a highly subsampled image. Sample area is four pixels wide and one pixel high. Found in cameras like the DVX-100.

4:2:0

Some HD cameras use this including the Sony EX-1 and EX-3. Technically, its the same as 4:1:1. The camera is still averaging the color of four pixels down to one. The difference is that the sample size is two pixels high by two pixels wide. Commonly used in HDV cameras.

4:2:2

Vastly superior to 4:2:0 and 4:1:1. Each color sample size is further broken down from four pixels to two, yielding much higher color accuracy. Used by DVCPRO HD and Digital Betacam.

4:4:4

Every color is represented as accurately as possible. No subsampling. Used by RED, Viper and Genesis camera systems.

4:4:4:4

Same as 4:4:4, but includes an alpha channel, typically used for chroma keying.

Why care?

Maybe you’ve noticed artifacts in your image; blockiness where colors bump up against each other. This is because the same algorithms are trying to average the colors the best it can. Sometimes, especially in these transitional places, it may guess wrong.

This is important to understand when doing color correction, and absolutely essential when doing any kind of chroma keying. If you’re working with a camera that has low color sampling resolution you’ll need to know what kind of limitations you’re up against. You may not actually be able to get the seamless VFX shot you originally had planned. But that’s filmmaking, right?

Now what?

I encourage you to figure out what color sampling method your camera uses. The more you know about your camera, the more you’re going to be able to get out of it. You’re also going to make things easier on yourself once you get to editing.

Additional resources

Color Subsampling on DVXuser

Subsampling notation PDF

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Halloween

Posted by Jason on December 27th, 2009

Halloween in Salem, MA, 2009. Occurring on a Saturday, it was said to have been the largest event in town for nearly a decade. An estimated 100,000 people showed up to celebrate.

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