Unable to create custom colormap from RGB data?

Still bogged down in the difference between an array and a matrix...

Everything was working well until I stumbled upon the following error message, at the time of attempting to create the colormap :

Colormap must be an Mx3 array of type single or double with values in the range [0,1] or an Mx3

array of type uint8.

Here's my code :

% Generate three vectors 
 Axe_A_B = transpose(linspace(-128, 128, 21));
 Axe_La = transpose(linspace(39, 50, 21));
 Axe_Lb = transpose(linspace(15,100, 21));
 
 Lab_Axe_AB = [Axe_La, Axe_A_B, Axe_A_B]; % 21 x 3 double
 RGB_Axe_AB = lab2rgb(Lab_Axe_AB)
 
 % negatives values?
 [row,col] = size(RGB_Axe_AB);
 
 for ROW = 1:row     
     if RGB_Axe_AB(ROW,1) < 0
        RGB_Axe_AB(ROW,1) = 0;
     end
     if RGB_Axe_AB(ROW,2) < 0
        RGB_Axe_AB(ROW,2) = 0;
     end
     if RGB_Axe_AB(ROW,3) < 0
        RGB_Axe_AB(ROW,3) = 0;
     end
 end


 RGB_Axe_AB_ColorMap = colormap(RGB_Axe_AB)

I'm going in circle...

Documentation states :

% Colormap must have 3 columns: [R,G,B]

% defining a three-column matrix of values

Here's the content of my RGB_Axe_AB 'variable' :

RGB_Axe_AB =

0 0.5400 1.2184

0 0.5321 1.1330

0 0.5240 1.0486

0 0.5152 0.9653

0 0.5058 0.8831

0 0.4952 0.8020

0 0.4833 0.7220

0 0.4695 0.6432

0 0.4536 0.5654

0.2388 0.4349 0.4886

0.4126 0.4126 0.4126

0.5271 0.3857 0.3369

0.6198 0.3526 0.2607

0.7011 0.3106 0.1812

0.7756 0.2540 0.0886

0.8459 0.1660 0

0.9139 0 0

0.9810 0 0

1.0478 0 0

1.1142 0 0

1.1804 0 0

I did not bother to create a 256 size variable at this point. ...

Answer 1

[Roger Breton]

Oopsy, doopsy! I just realize there are values > 1.0 in those columns! Oh boy! Egg on my face...

Here's the revised code :

for ROW = 1:row     
     if RGB_Axe_AB(ROW,1) < 0
        RGB_Axe_AB(ROW,1) = 0;
     end
     if RGB_Axe_AB(ROW,1) > 1
        RGB_Axe_AB(ROW,1) = 1;
     end
     if RGB_Axe_AB(ROW,2) < 0
        RGB_Axe_AB(ROW,2) = 0;
     end
      if RGB_Axe_AB(ROW,2) > 1
        RGB_Axe_AB(ROW,2) = 1;
     end
     if RGB_Axe_AB(ROW,3) < 0
        RGB_Axe_AB(ROW,3) = 0;
     end
     if RGB_Axe_AB(ROW,3) > 1
        RGB_Axe_AB(ROW,3) = 1;
     end
 end

Naturally, everything works now. No more errors....

Is there a way to do this process more efficiently? Without resorting to a loop?

Thank you in advance for your help and patience :-)

[Roger Breton]

Making progress... For those who are interested. The code is not elegant but it's the result I want. Right now, all I care to achieve is a colormap that goes from CIE Lab = 'the a* axis', from a* = -128 to a +127. This is my code :

% Generate three vectors 
 Axe_A_B = transpose(linspace(-128, 128, 21));
 Axe_La = transpose(linspace(39, 50, 21));
 Axe_Lb_Zero = transpose(linspace(0, 0, 21));
 
 Axe_Lb = transpose(linspace(15,100, 21));
 
 Lab_Axe_AB = [Axe_La, Axe_A_B, Axe_Lb_Zero]; % 21 x 3 double
 RGB_Axe_AB = lab2rgb(Lab_Axe_AB)
 
 % negatives values?
 [row,col] = size(RGB_Axe_AB);
 
 for ROW = 1:row     
     if RGB_Axe_AB(ROW,1) < 0
        RGB_Axe_AB(ROW,1) = 0;
     end
     if RGB_Axe_AB(ROW,1) > 1
        RGB_Axe_AB(ROW,1) = 1;
     end
     if RGB_Axe_AB(ROW,2) < 0
        RGB_Axe_AB(ROW,2) = 0;
     end
      if RGB_Axe_AB(ROW,2) > 1
        RGB_Axe_AB(ROW,2) = 1;
     end
     if RGB_Axe_AB(ROW,3) < 0
        RGB_Axe_AB(ROW,3) = 0;
     end
     if RGB_Axe_AB(ROW,3) > 1
        RGB_Axe_AB(ROW,3) = 1;
     end
 end


 RGB_Axe_AB_ColorMap = colormap(RGB_Axe_AB)
 
 n=21
 colormap(RGB_Axe_AB_ColorMap(21))
 pcolor([1:n+1;1:n+1])
 

And this is the image -- what a beauty! :-)

Needless to say, there's still a ton of work to do. This is very encouraging. Thank you, Mr. Matlab :-)

I owe you a beer :-)

[Roger Breton]

Making progress, using this code :

% Generate three vectors 
 Axe_A_B = transpose(linspace(-128, 128, 21));
 Axe_La = transpose(linspace(39, 50, 21));
 Axe_Lb_Zero = transpose(linspace(0, 0, 21));
 
 Axe_Lb = transpose(linspace(15,100, 21));
 
 Lab_Axe_AB = [Axe_La, Axe_A_B, Axe_Lb_Zero]; % 21 x 3 double
 RGB_Axe_AB = lab2rgb(Lab_Axe_AB)
 
 % negatives values?
 [row,col] = size(RGB_Axe_AB);
 
 for ROW = 1:row     
     if RGB_Axe_AB(ROW,1) < 0
        RGB_Axe_AB(ROW,1) = 0;
     end
     if RGB_Axe_AB(ROW,1) > 1
        RGB_Axe_AB(ROW,1) = 1;
     end
     if RGB_Axe_AB(ROW,2) < 0
        RGB_Axe_AB(ROW,2) = 0;
     end
      if RGB_Axe_AB(ROW,2) > 1
        RGB_Axe_AB(ROW,2) = 1;
     end
     if RGB_Axe_AB(ROW,3) < 0
        RGB_Axe_AB(ROW,3) = 0;
     end
     if RGB_Axe_AB(ROW,3) > 1
        RGB_Axe_AB(ROW,3) = 1;
     end
 end


 RGB_Axe_AB_ColorMap = colormap(RGB_Axe_AB)

 Xcoord = linspace(-128, 128, 21);
 Ycoord = linspace(0, 0, 21);
 plot(Xcoord,Ycoord, 'LineWidth', 2, 'MarkerSize', 10);

 axis([-128 128 -128 128]);
 xticks([-128 -96 -64 -32 0 32 64 96 128]);
 yticks([-128 -96 -64 -32 0 32 64 96 128]);  


xs = linspace(-128, 128, 21);
hold on;

colorList = RGB_Axe_AB_ColorMap;

xAxisWidth = 10;

%  k = -128 : 6.0952 : 128

xRef=-128

for k = 1 : 21
    thisColor = colorList(k, :);
    line([xRef, xRef + 12.1905], [0, 0], 'Color', thisColor, 'LineWidth', xAxisWidth)
    xRef = xRef + 12.1905
end

And this is the result :

Almost feel like putting my feet up, now :-)

Thanks everyone for your kind and patient help!

[Roger Breton]

BTW, I like the 'interactive" way of working with Matlab.

[Roger Breton]

Is it ever complicated (read 'abstract') to work in 3D!

Got this encouraging result :

Using this humble code :

% Generate three vectors

Axe_A_B = transpose(linspace(-128, 128, 21));

Axe_La = transpose(linspace(39, 50, 21)); % Green to Magenta

Axe_Lb_Zero = transpose(linspace(0, 0, 21));

Axe_Lb = transpose(linspace(15,100, 21)); % Blue to Yellow

Lab_Axe_AB = [Axe_La, Axe_A_B, Axe_Lb_Zero]; % 21 x 3 double

RGB_Axe_AB = lab2rgb(Lab_Axe_AB) % Green to Magenta

Lab_Axe_BA = [Axe_La, Axe_Lb_Zero, Axe_A_B]; % 21 x 3 double

RGB_Axe_BA = lab2rgb(Lab_Axe_BA) % Yellow to Blue

% negatives values?

[row,col] = size(RGB_Axe_AB);

% Process Green/Magenta axis first

for ROW = 1:row

if RGB_Axe_AB(ROW,1) < 0

RGB_Axe_AB(ROW,1) = 0;

end

if RGB_Axe_AB(ROW,1) > 1

RGB_Axe_AB(ROW,1) = 1;

end

if RGB_Axe_AB(ROW,2) < 0

RGB_Axe_AB(ROW,2) = 0;

end

if RGB_Axe_AB(ROW,2) > 1

RGB_Axe_AB(ROW,2) = 1;

end

if RGB_Axe_AB(ROW,3) < 0

RGB_Axe_AB(ROW,3) = 0;

end

if RGB_Axe_AB(ROW,3) > 1

RGB_Axe_AB(ROW,3) = 1;

end

end

[row,col] = size(RGB_Axe_BA);

% Process Yellow/Blue axis second

for ROW = 1:row

if RGB_Axe_BA(ROW,1) < 0

RGB_Axe_BA(ROW,1) = 0;

end

if RGB_Axe_BA(ROW,1) > 1

RGB_Axe_BA(ROW,1) = 1;

end

if RGB_Axe_BA(ROW,2) < 0

RGB_Axe_BA(ROW,2) = 0;

end

if RGB_Axe_BA(ROW,2) > 1

RGB_Axe_BA(ROW,2) = 1;

end

% fprintf('RGB_Axe_BA(ROW,3) Avant = %4.2f\n', RGB_Axe_BA(ROW,3))

if RGB_Axe_BA(ROW,3) < 0

RGB_Axe_BA(ROW,3) = 0;

end

if RGB_Axe_BA(ROW,3) > 1

RGB_Axe_BA(ROW,3) = 1;

end

% fprintf('RGB_Axe_BA(ROW,3) Après = %4.2f\n', RGB_Axe_BA(ROW,3))

end

RGB_Axe_AB_ColorMap = colormap(RGB_Axe_AB) % Green to Magenta colormap

RGB_Axe_BA_ColorMap = colormap(RGB_Axe_BA) % Yellow to Blue colormap

Xcoord = linspace(-128, 128, 21); % 1 x 21

Ycoord = linspace(0, 0, 21); % 1 x 21

Zcoord = linspace(0,100, 21);

Line3D = plot3(Xcoord,Ycoord, Zcoord); grid on; hold on;

set(Line3D, 'Visible', 'off');

xlabel('X axis a* (-128 +128)');

ylabel('Y axis b* (-128 +128)');

zlabel('Z axis L* (0 100)');

axis([-128 128 -128 128 0 100]);

xticks([-128 -96 -64 -32 0 32 64 96 128]);

yticks([-128 -96 -64 -32 0 32 64 96 128]);

zticks([0 10 20 30 40 50 60 70 80 90 100]);

% plot3(-128,-128, 0, 'o')

% plot3(-96,-128, 0, 'o')

xs = linspace(-128, 128, 21);

hold on;

colorListAB = RGB_Axe_AB_ColorMap; % Green to Magenta ColorList

colorListBA = RGB_Axe_BA_ColorMap; % Blue to Yellow ColorList

xAxisWidth = 5;

xRef=-128

yRef=-128

% line([-128, 128], [-128, -128], [0,0], 'Color', thisColor, 'LineWidth', xAxisWidth)

for k = 1 : 21

thisColor = colorListAB(k, :);

line([xRef, xRef + 12.1905], [yRef, yRef], [0, 0], 'Color', thisColor, 'LineWidth', xAxisWidth)

xRef = xRef + 12.1905

end

% plot3(-128,128, 0, 'o')

% plot3(-128,96, 0, 'o')

% line([-128, -128], [128, 96], [0,0], 'Color', thisColor, 'LineWidth', xAxisWidth)

xRef=-128

yRef=128

for k = 1 : 21

thisColor = colorListBA(k, :);

line([xRef, xRef], [yRef, yRef - 12.1905], [0, 0], 'Color', thisColor, 'LineWidth', xAxisWidth)

yRef = yRef - 12.1905

end

I had to resort to plotting individual points in order to visualize the required coordinates.

There are still points which puzzles me but I know I'm in the right direction.

Thank you all for your help and patience.

[Roger Breton]

"One more thing" as Steve Jobs used to say... My axes were "reversed"? So, I thought I was going to crawl under untangling the correct logic but I only changed a few lines and here is the correct result :

I had to change these lines of code :

% Generate three vectors

Axe_A_B = transpose(linspace(-128, 128, 21));

Axe_B_A = transpose(linspace(128, -128, 21));

Axe_La = transpose(linspace(39, 50, 21)); % Green to Magenta

Axe_Lb_Zero = transpose(linspace(0, 0, 21));

Axe_Lb = transpose(linspace(15,100, 21)); % Blue to Yellow

Lab_Axe_AB = [Axe_La, Axe_A_B, Axe_Lb_Zero]; % 21 x 3 double

RGB_Axe_AB = lab2rgb(Lab_Axe_AB) % Green to Magenta

Lab_Axe_BA = [Axe_La, Axe_Lb_Zero, Axe_B_A]; % 21 x 3 double

RGB_Axe_BA = lab2rgb(Lab_Axe_BA) % Yellow to Blue

Still many, many, many details to work on but, I'm so please with this result, so far.

[Roger Breton]

Rome was not built in one day... I spotted an error in the appearance of yellow? This is the corrected code :

% Generate three vectors

Axe_A_B = transpose(linspace(-128, 128, 21));

Axe_B_A = transpose(linspace(-128, 128, 21));

Axe_La = transpose(linspace(39, 50, 21)); % Green to Magenta

Axe_Lb_Zero = transpose(linspace(0, 0, 21));

Axe_Lb = transpose(linspace(15,100, 21)); % Blue to Yellow

Lab_Axe_AB = [Axe_La, Axe_A_B, Axe_Lb_Zero]; % 21 x 3 double

RGB_Axe_AB = lab2rgb(Lab_Axe_AB) % Green to Magenta

Lab_Axe_BA = [Axe_Lb, Axe_Lb_Zero, Axe_B_A]; % 21 x 3 double

Flipped = flipud(Lab_Axe_BA);

% RGB_Axe_BA = lab2rgb(Lab_Axe_BA) % Yellow to Blue

RGB_Axe_BA = lab2rgb(Flipped) % Yellow to Blue

Had to flip the array upside down, to get the correct Lab (RGB) values to appear at the correct (b* = +128) end of the scale. Silly me. Now this is better :