Create a matrix in MATLAB with a specific order from left to right and top to bottom

1
votes

I'm trying to create a matrix in MATLAB.

The matrix goes from 255 to 0 from left to right as well as from top to bottom. That is, the top left corner is the only place the matrix has a value of 255, and the farthest right column as well as the very bottom row have values of 0.

I'm trying to do this without using any loops, any ideas?

3
matlabmatrix
Well I was able to create two other matrices that wipe smoothly from either side, here's an example: ramp = uint8(0:255); redChannel = repmat(ramp', [1, 256]); I was thinking perhaps using the distance from the top left corner to each individual element, dividing by the distance from top left to bottom right and multiplying by the distance to the element, but I don't think you can implement that without any loops. – Will

3 Answers

1
votes

Here is a solution using bsxfun and min:

a = 255:-1:0;
result = bsxfun(@min, a, a.');

As of MATLAB r2016b you can write:

result = min(a, a.');

For a maximum value of 3 this output will be generated:

result =

   3   2   1   0
   2   2   1   0
   1   1   1   0
   0   0   0   0
0
votes

Based on the answer found here: circle with gradient gray scale

One possible solution would be:

N = 255; %// this decides the size of image
[X,Y] = meshgrid(1:-1/N:0, 1:-1/N:0) ;
out= X + Y-1;
out(out<0)=0;
out=uint8(out*255);
0
votes

You can do this in one operation using hankel

M = hankel(255:-1:0, zeros(1, 256));

Output on a smaller example:

n = 3;  % Top left value
M = hankel(n:-1:0, zeros(1, n+1))

>> M = [ 3     2     1     0
         2     1     0     0
         1     0     0     0
         0     0     0     0 ]

Another useful function for constructions like this is toeplitz. The same result can be generated:

M = fliplr(toeplitz(zeros(1, 256), 0:255));