BGRToLab

Convert from BGR to Lab

Supported Technologies

MT

Synopsis

FwStatus   fwiBGRToLab_8u_C3R ( const Fw8u * pSrc, int srcStep, Fw8u * pDst, int dstStep, FwiSize roiSize );
FwStatus   fwiBGRToLab_8u16u_C3R ( const Fw8u * pSrc, int srcStep, Fw16u * pDst, int dstStep, FwiSize roiSize );

Parameters

dstStep   Destination buffer step size (width of the buffer in bytes).
pDst   Pointer to a location in a destination buffer.
pSrc   Pointer to a location in a source buffer.
roiSize   Specifies the height and width of an ROI.
srcStep   Source buffer step size (width of the buffer in bytes).

Description

These functions step through an ROI in a source buffer, convert the source data from the BGR color model to the Lab color space, and write the converted data to a destination buffer.

A gamma-corrected RGB image (pSrc) declared in the ROI is converted to a CIE Lab image (pDst) in two steps. 

1. Normalize the data to [0 to 1] and convert RGB to XYZ using the following formulas.

 X = 0.412453*R + 0.35758 *G + 0.180423*B
 Y = 0.212671*R + 0.71516 *G + 0.072169*B
 Z = 0.019334*R + 0.119193*G + 0.950227*B

 The formulas assume that R, G, and B values are normalized to [0 to 1] for integer data types.
 For floating point data types, the data must already be in the range [0 to 1].
 For integer data types, the converted image data is saturated to [0 to 1] and scaled to the data type range.
 For floating point data type, data is saturated to [0 to 1].

2. Convert( ) XYZ To Lab as follows.

D65 white point:

 Xn = 0.950455
 Yn = 1.0
 Zn = 1.088753

 CIE chromaticity coordinates:

  xn = 0.312713
  yn = 0.329016

 if ((Y / Yn) > 0.008856)
 L = 116 * (Y/Yn)^(1/3) - 16 else L = 903.3 * (Y/Yn)^(1/3)
 a = 500 * [f(X/Xn) - f(Y/Yn)]
 b = 200 * [f(Y/Yn) - f(Z/Zn)]

  where if (t > 0.008856) f(t) = t^(1/3) - 16 else f(t) = 7.787*t + 16/116

 The computed L, a, and b are in the range [0 to 100], [-128 to 127], and [-128b to 127], respectively.

 The computed image data is quantized and scaled to fit the 8-bit or 16-bit range.

8U data type:

 L = L * 255/100
 a = a + 128
 b = b + 128

16u data type:

 L = L * 65535 / 100
 a = (a+128) * 255
 b = (b+128) * 255