Chroma Key

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Intro

  • A basic chroma keyer can make hard noisy edges, advanced versions can create more detailed keys. Other types keyers a used to create soft edges with a range of grey values of fine translucent detail.
  • This refers to a family of keyers that do not use the RGB colour-space but others ones like UVW or HLS. These colour-spaces separate the chromainance from the luminance of the image.
  • The term Chroma Keyer can refer to all types of keyers, rather than just the one specified above
  • it is good at selecting specific hues, often several of them are combined together to patch together different selections together to create an ok key very quickly as they are they are good at selecting ranges of colour to get a rough solid key.
  • As this type of keyer does not usually
  • Often used as a 'core key' (also know as gash/inner/holdout) and other types of soft keyers are combined with it for soft edge detail.

Types

Simple chroma keyer.

http://sourceforge.net/projects/simplechromakey/(external link) (cache)
float ComputeOpacity(float hue, float stauration, float lightness) {
if (saturation < min_saturation) return 1.0f;
if (lightness < min_lightness) return 1.0f;
if (lightness > max_lightness) return 1.0f;
float delta_hue = fabsf(hue - target_hue);
float opacity = 1.0f - (delta_hue / tolerance_hue);
if (opacity < 0.0f) opacity = 0.0f;
return opacity;
}

Sophisticated chroma keyer

Video Chroma Keyer Video Demystified By Keith Jack(external link) (cache) Uses YCbCr colour space result = (fg_pixel_value - blue_backing_contribution) + Kbg*bg_pixel_value
Kbg is generated via a 2d coordinate system. Hue Difference keyer Alternatives to the RGB Difference key is the Hue Difference key used in after effects. see http://www.bildfehler.de/fx/cdk.html(external link)(external link) (cache)


Links

External Links


Created by ome. Last Modification: Monday 12 of January, 2009 06:44:17 GMT by ome.