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December 2004 HelpLine

Here are a few tips to help you achieve a successful chroma-key:
• First, light the background evenly, top to bottom and left to right.
• Second, keep your foreground object as far away from the background as possible to minimize color spill.
• Third, light your foreground with the new background in mind. In other words, if you're trying to key someone into a new background that has a window in the corner with the sun coming through it, it's important that your foreground object be lit so it looks like the sun is shining in the same direction.
• Finally, when shooting, don't use any diffusion on the lens (fog, mist, etc.). These filters will just mix the light and cause a bad key.

Cropping Problems?

Q)  I use an image-editing program to crop and make minor adjustments on some of my files. Does cropping just discard the extra pixels or does it actually recopy the file? In other words, does cropping create the possibility of artifacts degrading the image? I like to crop one side at a time, but I wouldn't want to do it that way if cropping four times would increase the possibility of adding artifacts, rather than doing it in one pass.

Jim Grabill
Via e-mail


A)  As long as you stay in the "native" file format of your image-editing program or you use a TIFF file, you shouldn't have problems with cropping one side at a time. A problem might occur if you open a JPEG file, crop one side, close the image as a JPEG, then reopen the file at a later time and crop another side and resave as a JPEG, and so forth. JPEG throws out data to compress files, then rebuilds that data later when the file is opened. Low levels of compression used only once have little effect on an image. However, multiple compression cycles (which happens every time a file is opened, then resaved) can degrade the image. In short, when working with compressed files, don't keep resaving them in a compressed file format. All image-processing programs work the same way concerning this issue.

 

 

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