Asked by Sara Foster
on 7 Sep 2019

I have a function which calculates square distance between pixels, however it wants to calculate the distance between 2 pixels which has a 1x1x3 array.

For example, inputs P and Q both have 1 row, 1 column and 3 colour layers, and single output is squared distance.

I found this function on matlab forums which calculates squared distance and it gives correct results: https://ww2.mathworks.cn/matlabcentral/answers/479129-how-can-i-have-multiple-arrays-as-an-input-for-my-function

function [SquaredDistance] = PixelDistance(P,Q)

P1 = P(1); P2 = P(2); P3 = P(3);

Q1 = Q(1); Q2 = Q(2); Q3 = Q(3);

SquaredDistance = (P1-Q1)^2 + (P2-Q2)^2 + (P3-Q3)^2;

end

However, if I have a table (as an example) like this: (where :,:,1 is red values :,:,2 is green values etc)

Colours(:,:,1) =

143

156

231

Colours(:,:,2) =

37

139

142

Colours(:,:,3) =

30

22

65

How do we calculate squared value using input commands such as P = Colours(1,1,:) and Q = Colours(3,1,:) using the code?

Answer by Jos (10584)
on 7 Sep 2019

Accepted Answer

The function you found is rather badly coded ...

Another expression for "calculating a distance" is "taking the norm". Matlab has a function for that, called NORM :-) When you have two points in 3D (colour, RGB) space, P and Q, represented as vectors with three values, you can calculated the distance by looking at the vector between P and Q:

P = [10 ; 100 ; 10], Q = [30 ; 40 ; 50] % example

norm(P-Q) % which is the same as

sqrt(sum((P-Q).^2))

In your examples you have a your 3D points (vectors) stacked up in a single array Colours. It looks like Colours is a 3-by-1-by-3 array. You can get rid of the obfusciating second dimension using squeeze

Colours = randi(100,[3 1 3]) % example of your data ??

C = squeeze(Colours) % a 3-by-3 array: each column is a point

norm((C(1,:)-C(2,:)))

Jos (10584)
on 7 Sep 2019

You're welcome.

RANDI is a function that returns an array of a specific size filled with random numbers. The first argument is the upper limit, the other arguments are size argument.

You can easily find out what a function does using matlab's excellent help and documentation system. That is how still, after many many years, learn about (new) functions and how to use them. It is also shows that you should always write a decent help section when writing your own function to make it useful for others :-)

>> help randi

Sara Foster
on 7 Sep 2019

Jos (10584)
on 7 Sep 2019

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Answer by Sara Foster
on 7 Sep 2019

Jos (10584)
on 8 Sep 2019

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