applycform results interpretation?

Before asking the question, allow me to reiterate my gratitude for any of the help received in adance :-)

I'm doing something in apperance very mundane :

1) Read RGB image using imread()

2) Convert RGB to Lab using cform

Yet, the results are not what I expect... Here's my code :

m = imread('3x4_RGB.tif')

cform = makecform('srgb2lab');

Lab = applycform(m,cform);

Here's my m ( 4×3×3 uint8) array :

m(:,:,1) =

128 255 255

0 0 234

245 255 0

230 0 32

m(:,:,2) =

128 0 255

255 255 181

245 0 0

229 192 0

m(:,:,3) =

128 0 0

0 255 163

245 255 255

139 64 128

And here's the Lab (4×3×3 uint8) array :

Lab(:,:,1) =

137 138 249

224 231 200

246 153 75

228 174 34

Lab(:,:,2) =

128 209 112

49 77 146

128 222 196

119 67 170

Lab(:,:,3) =

128 198 221

209 113 145

128 67 16

172 178 64

I suspect I have some massaging of numbers to do at this point?

The first RGB element reads 128,128, 128.

It's converted CIE Lab value would 137, 128, 128.

Well, I expected Lab = 54, 0, 0.

I'm missing something here.... I suspect the output values are still 'RGB-based'? For use back in some imshow() or other image display functions?

[Roger Breton]

This post on StackOverflow helped : Why I got wrong outputs for converting RGB to l*a*b* color values using MATLAB? - Stack Overflow

Basically (I'll have to research this further) but, using rgb2lab gives the expected numeric results :

Lab(:,:,1) =

53.5850 53.2408 97.1393

87.7347 91.1132 77.9855

96.5375 60.3242 32.2970

89.2650 68.0698 14.7631

Lab(:,:,2) =

0 80.0925 -21.5537

-86.1827 -48.0875 16.7037

0 98.2343 79.1875

-12.5051 -66.1018 48.1573

Lab(:,:,3) =

0 67.2032 94.4780

83.1793 -14.1312 16.5673

0 -60.8249 -107.8602

43.9621 51.3645 -61.6661

Big sigh of relief!