How to apply a colour gradient for faces using mapshow command

I am trying to draw various sqaures in a plot. each sqaure is supposed to display a unique value depending on its weight. Can any only help to explain how to assinga color gradient similar to jet for this case?

[Usman Shafique]

xbox = [0 0 8.46 8.46 0 ;0 0 8.46666667 8.466667 0 ;0 0 8.46666667 8.466667 0 ; 0 0 8.46666667 8.466667 0 ]

ybox = [ 0 5 5 0 0 ;5 10 10 5 5 ;10 15 15 10 10 ;15 20 20 15 15]

weight = [1,5,6,8

mapshow(xbox,ybox,'DisplayType','polygon','LineStyle','none')

[Usman Shafique]

I want to create a colour gradient based on weight values and change the color of the polygon

Answer 1

[Omega]

Hi Usman,

I understand that you would like to create polygons on a plot, with each polygon being coloured according to its weight, and obtaining these colours from a predefined colour scheme.

To apply a colour gradient to the faces of polygons created with the “mapshow” command in MATLAB based on weight values, you can use the “FaceColor” property of the polygons. Here's how you can do it:

% Define your data 
xbox = [0 0 8.46 8.46; 0 0 8.46666667 8.466667; 0 0 8.46666667 8.466667; 0 0 8.46666667 8.466667]; 
ybox = [0 5 5 0; 5 10 10 5; 10 15 15 10; 15 20 20 15]; 
weight = [1, 5, 6, 8]; 

% Define a colormap (you can change 'jet' to any other colormap like 'parula') 
colormap('jet'); 
cmap = jet; 

% Create a color scale based on the weight values 
clim([min(weight), max(weight)]); 

% Display polygons using mapshow and set the FaceColor based on weight 
for i = 1:length(xbox) 
    color_value = weight(i); % Get the weight value for the current polygon 
    
    % Normalize color_value to the range [0, 1] 
    normalized_color = (color_value - min(weight)) / (max(weight) - min(weight)); 
    
    % Map the normalized_color to a color from the colormap 
    color_index = round(normalized_color * (size(cmap, 1) - 1)) + 1; 
    polygon_color = cmap(color_index, :); 
    
    % Create a polygon object 
    polygon = mapshow(xbox(i, :), ybox(i, :), 'DisplayType', 'polygon', 'LineStyle', 'none'); 
    
    % Set the FaceColor based on the mapped color 
    set(polygon, 'FaceColor', polygon_color); 
    
    hold on; % Keep the current axes for multiple polygons 
end 

% Set the color axis label 
colorbar; 
title('Polygon Color Gradient Based on Weight'); 
xlabel('X-axis'); 
ylabel('Y-axis'); 

To know more you can refer to the following documentation links:

• mapshow: https://www.mathworks.com/help/map/ref/mapshow.html
• FaceColor: https://www.mathworks.com/help/matlab/ref/matlab.graphics.primitive.patch-properties.html#buduav0-1_sep_shared-FaceColor

I hope it helps to resolve your query!