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Computer Editing: Video Filter Effects

Computer Editing: Video Filter Effects

Applying electronic filters in post-production can cure many of your footage's ailments.

When new technology replaces old, it is interesting how it's mirrored in the terminology we use. Very few people dial a phone or play records anymore, but the terms persist. Video case in point: the term "filter."

In traditional still and motion-picture photography, a filter was a specialized piece of glass affixed to the lens to alter (and hopefully improve) the image. Today, the term "filter" more often refers to a small program or subroutine running inside an editing application. But, just like optical filters, electronic filters help you manage the look of your footage.

Since covering all the aspects and uses of a single filter could easily require an entire article, we'll cut the subject into manageable chunks and look at a couple of commonly used corrective filters in this article and examine special effects filters next month.

Correction Filters
A corrective filter takes an aspect of a scene that has gone wrong and makes it right. At the top of the corrective filters list are two groups that are commonly known as image control filters and color correction filters.

As a videographer, your goal is to try to control the quality and amount of that light that falls on your scenes. But sometimes, forces conspire against you and you have to deal with footage that's badly over- or under-lit, or that combines light sources that are wildly unmatched in color temperature. When that happens, a corrective video filter is your last defense against having to live with otherwise unacceptable footage.

Let's break down two common shooting situations and see how applying a couple of video filters can help you turn poor footage into something suitable.

Filtering Backlit Scenes
Figure 1A illustrates the common circumstance of a backlit scene. The best solution in this situation is to use your iris control (or the backlight compensation control if your camcorder has one) to force the lens open so that the faces of your subjects are properly exposed during the original shoot.

Faces are important because, in any picture, you're programmed to look at the people first. But if the damage is already done and you're facing a session in the edit bay with backlit scenes, image control filters in your editor could still save the day.

Filtering for Brightness and Contrast
Almost every type of editing software has basic brightness and contrast filters. The brightness control affects the overall apparent luminance of the scene. In other words, as you bring up the general brightness values, everything appears lighter.

The problem with this approach is that since the background in a backlit scene is already too bright, simply raising the overall brightness level will usually wash out the scene. So you also need to tweak the image with the contrast control in concert with the brightness. The contrast control determines the difference between the darkest and lightest parts of a scene. In a backlit situation like ours, lowering the contrast can have a beneficial effect on the footage.

In essence, what you're telling the software to do is to decrease the difference between the over-lit background and the under-lit foreground subjects. Figure 1B shows the same scene after processing. By decreasing the contrast, the bright background has becomes less bright and the dark foreground faces have become less dark. Combined with a slight increase of the brightness control, this is often enough to make marginal footage more acceptable.

Color Correction Filters
The next pair of figures illustrates another common problem that your built-in filters can help fix. While recording baseball game footage, the shooter started out with an indoor white balance setting, but forgot to change it when he went to the game. Many public parks have old-style field lights that put out light that looks fine to the naked eye, but anything but fine to a camera.

Post-production revealed that the footage suffered from a horrible blue glow. Obviously, this scene had much too much blue information and not enough red and yellow. So opening up the image control filters again, there was a choice of how to put the footage back to a better-balanced state. Figure 2A shows such a scene with the adjoining color controls in their original settings.

In Figure 2B, the color correction has been applied by manipulating the individual color channel sliders to de-emphasize blue and increase the red content in the scene. The result is a much more realistic color balance. In many packages, you can also apply a phase re-adjustment, which is another way to alter the balance of the red, green and blue components of your scenes. These are typical examples of the power of well-crafted video filters.

A Filter for Every Job
Common corrective filters include those that affect the color balance, brightness and contrast, as well as others such as blur filters that can subtly lessen the appearance of wrinkles on older faces.

You'll also find some specialized video filters in this category including those that de-interlace video still captures and increase the apparent sharpness of the picture.

Each package has it's unique set of built-in filter capabilities, but many programs also allow you to purchase separate filter plug-ins to add more capabilities to the base set that comes with your editor. Check with the manufacturer for detailed information on compatibility and support.

As you explore the world of video filters, you'll quickly appreciate the power they give you over nearly every aspect of your footage's appearance. Given all of this image processing power, all that's left is to harness that power and use it wisely.

[Sidebar: Lens Filters]
As sophisticated as today's video cameras are, they still have limitations in the way they record scenes. But you can overcome these limitations with traditional filters.

Suppose you want to shoot a commercial in a natural setting. You travel up into the mountains and set the product on a tree stump in the forest. Your setting is lovely including beautiful puffy white clouds in a blue sky. The problem is, even using hard reflectors in order to bounce direct sunlight on the product, you can't get enough light to properly expose the sky in the background. The solution: take a glass polarizing filter and slide it halfway down into a matte box so that it darkens only the overhead portion of the shot.

In that situation, if you try to darken the sky with a filter in post, you would get poor results since the sky was completely blown out and lacked detail.

You could apply a luma key effect in post and replace the sky with stock sky footage, but that would be much more trouble than just using a filter to get the sky exposed properly in the first place.

Any time you can fix things as you shoot, so you don't have to apply filters in post, you're ahead of the game.

[Sidebar: Stacking Filters]
When you work with filter effects, remember that you aren't limited to using only one filter at a time. You can stack effects in order to either achieve more dramatic results, or better target the effect on a portion of your scene.

If you've placed one filter to color correct your scene, there's no reason you can't use a second filter to improve the brightness and contrast of the same scene. Keep in mind that the order of the filters can be critically important, so if one arrangement doesn't work, try reversing the filters.

[Sidebar: White Balance in Post]
In the past, if you had a scene that was shot with an improper white balance, you would need to open the color correction toolset and adjust the ratio of red, green and blue in your signal by eye (or by 'scope if you had one available.)

Today, you can click on some part of your scene known to be white and the software will not only shift the white pixels to proper white, it'll use the color offset information derived from the white pixels to calculate a new global white balance for the entire scene.

One click and it's almost as if you can go virtually back in time and white balance correctly in the field. Now that's digital progress.

Tags:  October 2002
Bill
Davis
Tue, 10/01/2002 - 12:00am