The Assemble Lab component assembles an image from the supplied L, a and b channels according to the CIE 1976 L*a*b color model. L stands for lightness, and a and b are chromaticity coordinates. For more information on the CIELab color model, see this Wikipedia article on the L*a*b color model. Assemble Lab is a map component, it can be located in the Channels category on the Components Bar.
For RGB-to-Lab conversion, the component first converts the input RGB values (assumed to be defined in the sRGB space) to the CIE XYZ color space using the illuminant D65, then the resulting CIE XYZ color is converted to the CIE Lab color space using the illuminant D50. For Lab-to-RGB conversion, the component converts the input CIE Lab values to the CIE XYZ color space using the illuminant D50, then the resulting CIE XYZ color is converted to the sRGB color space using the illuminant D65.
Since all the channels are supplied by map inputs, their values can be controlled separately for different image areas by connecting a map component to the corresponding input. In this case the output brightness of the map component defines the channel level: black areas correspond to the minimum channel level, white areas to the maximum level, and channel levels in-between are represented by intermediate brightness levels.
The alpha channel of the image produced by this component is always set to 100%; to set a different alpha channel, use the Set Alpha component.
L: Map Input
Defines the value of the Lightness channel in the range between 0 and 100. Value of 0 yields black, value of 50 produces pure, saturated color, and value of 100 yields white. Since L is a map input, its value can be controlled separately for different image areas by connecting a map component to this input.
a: Map Input
Defines the color position on the chromaticity axis a in the range between -128 and 127. Since a is a map input, its value can be controlled separately for different image areas by connecting a map component to this input.
b: Map Input
Defines the color position on the chromaticity axis b in the range between -128 and 127. Since b is a map input, its value can be controlled separately for different image areas by connecting a map component to this input.
