MATLAB Answers

Function calculating distance between pixels in 1x1x3 arrays

3 views (last 30 days)
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?

Accepted Answer

Jos (10584)
Jos (10584) on 7 Sep 2019
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,:)))
  4 Comments
Jos (10584)
Jos (10584) on 7 Sep 2019
In this code, Matlab expects Colours to be a variable in your workspace. I think you would benefit greatly from looking at an introduction course, like Matlab Onramp

Sign in to comment.

More Answers (1)

Sara Foster
Sara Foster on 7 Sep 2019
Okay thank you again, and another problem I'm finding is (C(1,:)-C(2,:)) only subtracts row 2 from row 1, how do we make it so that our input commands determine which row it needs to be subtracted from e.g. Q = Colours(3,1,:) and P = Colours(1,1,:), we are required to subtract row 3 from row 2 using P,Q command.
  1 Comment
Jos (10584)
Jos (10584) on 8 Sep 2019
I do not get this question, sorry. Also, do not pose a new question as an answer here but open a new thread, or put it as a comment/follow-up on a given answer, to avoid confusion :-)

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!