ploting a 2D matrix and adjusting axes

Hello, I have the following code:

M = zeros(50)
for x = [20,30]
  M(x,:) = 1
end
for y = [13,25,37]
  for x = [1:20, 30:50]
    M(x,y) = 1
  end
end
imagesc(M)

when i get the plot, i see the y-axis is in reverse order i-e starting from 50 at the bottom. In my overall simulation this affects my results. You can see the attached picture .Figure 1 is what i want and figure 2 is what this code gives me. I need to know how can i get.

Answer 1

[Sohaib Bin Altaf]

This thing worked for me:

clc

clear all

close all

M = zeros(50);

% draw lines

for y = [20,30]

M(:,y) = 1;

end

for x = [13,25,37]

for y = [1:20, 30:50]

M(x,y) = 1;

end

end

p1 = [12,48];

p2 = [39,5];

% get direction

d = p1-p2;

d = d/max(abs(d)); % limit to 1 px step size

steps = 0:1:max(abs(p2-p1));

for i=length(p1):-1:1

p_line(:,i) = round(p2(i) + steps.*d(i));

end

idx = sub2ind(size(M), p_line(:,1), p_line(:,2));

walls = sum(M(idx));

M(idx) = 2;

M = flipud(M)

M = rot90(M,-1)

imagesc(M)

set(gca,'YDir','normal')

set(gca,'XDir','normal')

fprintf('You passed %i walls', walls)

Answer 2

[dpb]

hAx=gca;             % handle to the axes
hAx.YDir='normal';   % set Y axis direction--imagesc uses 'reverse' Y

See link to Axes Properties at

doc axes

or the higher level info under Graphics/Formatting and Annotation/Axes Appearance

ADDENDUM

xlim([0 50]), ylim([0 50])

to get the 0 origin to show, maybe??? Your array is 1,50 each direction which are the coordinates imagesc uses; by default it pads a fraction on either direction.

Without more explanation of just what it is you're really after, we're just guessing what the actual problem you perceive is...

[Sohaib Bin Altaf]

it does not help. the axis remains the same.

[Sohaib Bin Altaf]

"axis xy" inverts the y-axis as i require but the coordinate system still does not represents the value the way i need.

[dpb]

I forgot; imagesc uses YDir='reverse' by default; it's

hAx.YDir='normal';

or, as you discovered, that's what axis xy will do as well.

I fail to see how either of these two don't leave you with precisely the axis orientation desired; what do you need that isn't ok.

[Sohaib Bin Altaf]

"axis xy" command inverts the y-axis as i require but the coordinate system still does not represents the value the way i need. when i use "surf" instead of imagesc the axis are fine but the coordinate system plots it differently, like (x,y) is plotted as (y,x).

Even if i don't plot the matrix into figure, still the results are similar as if i plot an image.

[Sohaib Bin Altaf]

M = zeros(50);
% draw lines
for x = [20,30]
  M(x,:) = 1;
end
for y = [13,25,37]
  for x = [1:20, 30:50]
    M(x,y) = 1;
  end
end
p1 = [15,25];
p2 = [33,45];
% get direction d = p1-p2;
d = d/max(abs(d)); % limit to 1 px step size
steps = 0:1:max(abs(p2-p1));
for i=length(p1):-1:1
  p_line(:,i) = round(p2(i) + steps.*d(i));
end
idx = sub2ind(size(M), p_line(:,1), p_line(:,2));
walls = sum(M(idx));
M(idx) = 2;
imagesc(M)
fprintf('You passed %i walls', walls)

This is the code, in case you are curious to know what it is, i need to know the number of walls that comes between two points along the shortest distance. But when i see the matrix plot the coordinate system is different from how i want it to be.

[Sohaib Bin Altaf]

you can see in the attached image i have entered as [15,10] but it is plotted as [10,15]. Please check the figure attached to this comment

[dpb]

That's the documented behavior of imagesc -- from the doc:

"imagesc(C) displays the data in array C as an image ... Each element of C specifies the color for 1 pixel of the image. The resulting image is an m-by-n grid of pixels where m is the number of columns and n is the number of rows in C. The row and column indices of the elements determine the centers of the corresponding pixels."

To use imagesc in Cartesian coordinates with the origin at lower left you need to transpose the C array as well as set the Y-axis 'YDir' property to 'normal'.

I don't know the basis for this convention, specifically.

[Sohaib Bin Altaf]

thanks a lot for your time. I appreciate it.