Pretty As A Pixel

If you have a piece of video content, whether you have created it or purchased it, and you load it into a media server to use it in a show, and you think the digital lighting programmer’s work is finished, think again.

Video content needs to be assessed for its performance and maximized for its playback quality on the video output device on which it is to be used. Since no two devices are the same, how do you best go about doing that? There is no real shortcut. The best way is to load the content into the media server, display it on the output device, and observe it. Since you usually can’t do that ahead of load-in, you may find that you need to make last minute adjustments to the content in order for it meet the client’s expectations.

Some of the adjustments that can be made to content include gamma, brightness or black levels, and contrast. These adjustments can be made on the fly from a console in order to increase the quality of the image once the content has been loaded on the server. Each of these adjustments will increase or decrease the luminance of an image. But what is luminance?

Luminance

Luminance can simply be described as the amount of light being emitted by a display at a given angle. It’s basically a measure of how bright a display will appear from a particular viewing angle. Video display manufacturers of LED screens, plasma displays, LCD screens and CRTs use a measure of luminance called a “nit” to describe the brightness of their products. A nit is one candela per square meter, and a typical computer display emits from around a hundred to a few hundred nits. An LED display typically emits several thousand nits, much to the chagrin of the lighting director.

In addition to luminance, there are a variety of factors that contribute to the appearance of an image as it is viewed on a display device. In this month’s column, we will examine gamma, brightness and contrast.

Gamma

Gamma is a color management tool that is used to correct any non-linearities in display devices. Non-linearities may be introduced for various reasons, but the bottom line is that a non-linearity changes the relationship between the luminance value of a pixel as it is input to the display device and the luminance value as it is actually displayed. For example, if the luminance for a certain pixel is supposed to be 50% and there is no gamma correction, then the nonlinearity in a display might make the displayed value change to something other than 50%. Any distortions caused by this non-linearity in the display device can be corrected by applying a gamma correction so that your eye perceives the correct brightness as it was intended across the range of luminosity. In some media servers, the gamma correction applies only to the combined red, green and blue (RGB) content, while in more sophisticated media servers gamma correction can be applied individually to each of the three color signals allowing more precise corrections. The main point to remember is that when you use gamma to adjust the brightness of an image, the way the colors are displayed in the image can look different than the way they were intended to look in the original content.

Brightness (Black Levels)

Brightness refers to the visual perception of luminance in an image. It differs from luminance in that it is a non-qualitative reference to the physiological perception of light. In other words, it’s how bright an image seems to be, not necessarily how bright it actually is. To illustrate, look at the graphic of White’s illusion on the left (Figure1). Which of the two columns of grey bars is brighter? The truth is they are exactly the same; they only look different in the context of the colors that border them. Don’t believe me? Check out the graphic at the end of this article (Figure 3).

Video display devices depend on this illusion for contrast. When a television screen, for example, is off, the screen appears dark grey. But when it’s on and a pixel is blacked out, it appears to be black, not grey.

When the brightness level on a display is adjusted, an offset is factored into the red, green and blue video components so that the black levels of the image are changed. What then would be the ideal setting for brightness? A good rule of thumb is to adjust the black levels so that black picture content displays as true black on your display device.

Incorrect adjustment of the brightness in an image is a very common problem and it can result in poor image quality when the image is displayed. Take care to make sure this adjustment is correct. It is also important to pay close attention to the proper adjustment of the black levels on the display device so that the brightness of the image will not have to be distorted beyond reasonable values. If the brightness in an image is set too low, a large range of input signals will be “crushed” or compressed beyond usability. If the brightness is set too high then no input signal can achieve true black, which will cause the image to be based on values of gray. The overall contrast ratio will be lost, and the image will appear washed-out and dull.

Contrast

The contrast ratio of an image is the ratio of light to dark in an image. It is a major determining factor in how the quality of an image is perceived. If an image has a high contrast ratio, it will appear to be sharper than a picture with a lower contrast ratio, even if the lower contrast picture has substantially more measurable resolution. Lower contrast ratios can appear gray or hazy whereas higher contrast ratios can cause the white areas of an image to become washed out. Because every image can have varying amounts of light and dark, it will be necessary to make adjustments to the contrast of each image to achieve the optimum display settings (Figure 2 – at the top of the web page).

Making adjustments to contrast, gamma and brightness in real time is a truly unique function of a media server because it allows the digital lighting programmer to make adjustments to each piece of content on the fly and those values can be recorded into a cue and played back from a lighting console. And going a step further, should the display device have to be swapped for a different device in a different venue, the content itself does not have to be re-rendered. Basic attribute palettes that have been stored for contrast, brightness and gamma can be updated and all cues in the show referencing those palettes will automatically update.

Figure 3: