why is the coordinate of impixel function opposite to a image matrix?

It's a just somple question.

a = imread('abc.tif');

a(100, 200, :) is different from impixel(a, 100, 200).

Of course, I know that the column and row are exchanged in impixel(a, c, r).

Is there any special reason why the coordinate is opposite to the matrix index?

Answer 1

[Swathik Kurella Janardhan]

There is no special reason. In a(r,c,:) refers to indexing in a matrix whereas in impixel(a, c, r), c and r are equivalent to x, y coordinates of a plane.

Is there a specific reason for your question? Please explain.

[YoungTae Kwak]

Actually, I just wanted to know why the row and column in order are changed in impixel(a, c, r). But if there is no special reason, I will just follow the instruction.

Thank you for your answering, Swathik!

Answer 2

[Image Analyst]

Some functions and operations in MATLAB take coordinates/indexes in row, column order, like indexing locations in an array, etc. Others take coordinates/indexes in x,y order, like plot(), etc. impixel() has other ways you can pass in input arguments and they've chosen to use the x,y form, not the row, column form. So if you're going to use x,y you need to pass in column, row since column is the x coordinate, and row is the y coordinate. Actually I think more or even most functions take and return x,y rather than row, column.

Mixing these two up is probably one of the most common beginner mistakes. I can't tell you how many times we see questions here asking why theirArray(x, y) is giving the wrong values as compared to looking in the variable editor. The answer of course is that they are indexing an array and should have row as the first item, so it should be theirArray(y,x) and not theirArray(x,y) like beginners often think.