Guide to Great Web Video: Preprocessing Video
Color Correction and Level Adjustments
Color correction and level adjustments are often subjective settings and highly dependent on the source and the way it is captured. They are also important considerations for preprocessing your Web video because of the inherently different characteristics of desktop monitors versus television sets. Here’s rundown of the parameters you need to consider and how they can improve the overall quality of your Web video.
Gamma Correction – This is the first adjustment you will want to experiment with, especially if you have video destined for viewing on both PCs and Macintosh computers. Different viewing devices have different gammas, a non-linear function that generally affects the very dark and very light areas of an image while maintaining levels throughout the mid-section. An example of a gamma curve is shown in 10. PC monitors tend to be darker, and so you may want to raise the gamma of your video, giving it a brighter appearance without making the whites bloom or clip.Brightness and Contrast – These are generally sliders that allow a linear adjustment across the entire range of luminance values in the video. They can be used to make modifications to the lighting of a scene. Our eyes tend to see more details in darker areas of an image than in lighter areas, so raising the brightness can mask noise. Keep in mind, however, that a better approach is a good noise reduction filter, since the codec will see the noise details even when we can’t.Hue and Saturation – These are also sliders that affect the chrominance component of your video, across its entire range. A slight color shift can be compensated for using the hue adjustments. Richer colors that may not be "legal" NTSC levels can be achieved by increasing saturation.Black and White Restore – Sometimes you just need to make black be black or white be white, depending on the camera and lighting settings used, or if you’ve made some adjustments to the overall brightness of your video. For web video, flat color areas are easier to code because there are no small variations for the codec to spend bits on. A black restore function will take all values that are near black to true black, and similarly for the white restore function.Brightness, Contrast, Hue, and Saturation are the basic characteristics that almost all video applications, and most video capture devices, will allow you to adjust. A traditional TBC will also accomplish some of these tasks, but more flexible tools are available from today’s software applications. VirtualDub only has built in filters for brightness and contrast, so if you’re capture device doesn’t give you the full flexibility you need, you might consider a higher end product like Adobe Premiere or Cleaner 5. Our tutorial on Media Cleaner Pro 4 goes into more detail on how to apply these functions to your video using that application.
It’s a wrap
Putting video on a web page is easy, but making it look good is still a challenge given the limited bandwidth connections of the end user. To make the most of any encoder, you need to feed it the highest quality video and take advantage of all the tools and techniques available for preprocessing that we’ve discussed here. VirtualDub addresses many of these features, but falls short in the areas of inverse telecine and color correction. For experimentation, as a learning tool to see how and why these processing function work, it is a good application to have in your video processing arsenal. Sometimes, you just need to de-interlace or resize the video for web delivery, but other times you’ll need to tap into all the features of a good encoding application to achieve the highest quality. The key is to remove unwanted details, so the codec can spend its precious bits on the information that our eyes will see, and that is vital to the message of the content.