Using the Chroma Key in shot editor

In order to use Chroma Key features, you must purchase Ustream Producer Studio. You can purchase it here:  http://www.ustream.tv/producer

Producer enables you to perform real-time Chroma Key operations on your sources during a broadcast. Chroma keying enables you to select a color and replace all occurrences of that color in an image with some other image. Click the Chroma Key button to open the Chroma Key controls. Click the Use Chroma Key checkbox to enable the controls. Use the sliders to set the Key Threshold, Black Clip, and White Clip. Click the Defaults button to restore all controls to their default state.

The key color is the color of the background you want to drop out of the resulting media. The Chroma Key configuration panel is designed to work through the configuration as a process, from top to bottom, fine tuning your key as you go. It is important to not over-adjust any one control. A good key color is obtained by finding the right balance in the controls.

To get the best key possible, select the key color using the mouse. To do this check the Pick key color with mouse checkbox. Use the mouse to click a point on the image in the preview area. As you do this watch the preview change in level of transparency, displaying the gray/white checkerboard pattern on the affected regions. Usually, clicking near the corners of the media provides the best key, but sometimes clicking near the main subject in the image is best. In the example below, you can see that I've selected the blue background behind me to change the transparency. 

Once you have the best key by picking the color, you can adjust the key threshold to drop out the background as completely as possible. The Key Threshold parameter enables Producer to determine how close the color in the video is to the key color. Adjust the key threshold (0 to 100) by using the slider.

Probably the most important part of getting a good key is getting good source material. Poor source material generates poor keys; no algorithm
can make up for this. Assuming you have a reasonably good camera, the most important part of getting a good key is good lighting. Invest in good lights and learn how to set them up properly. Here are some basics:

  • The background screen (green or blue) needs to be independently lit from other items in the shot.
  • Light the background screen evenly, with no shadows. The more evenly lit, the better the keying.
  • Light people evenly with no shadows. A fill light is very important. An office white board may be helpful in accomplishing this.

You should get reasonable quality out of most cameras on the market today. Producer has been tested with a low-end DV cameras to verify the quality of its Chroma Keying system. However, the better the camera the better the keying. If you use a cheap USB Web-cam, it does not give you enough source quality to get a good key. But if you have excellent lighting, an iSight camera does work well. 3CCD is better than 1CCD. If you have the choice, a PAL camera is better than NTSC, due to how data is captured internally.

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