Reading pixels of an image

Hello, How does MATLAB read the pixels in an image? Is it row wise or column wise? Suppose I have a 5x6 image and I wanted to read 21st pixel, what value is returned? Is it the value at (4,3) or (1,5)?

Answer 1

[jonas]

Indexing works by going through the matrix column by column, so that would be (1,5) in your example. Don't be afraid to test things in MATLAB.

A =

     1     2
     3     4

>> A(3)

ans =

     2

You can also test it by writing out the values of all indices with this syntax

>> A(:)

ans =

     1
     3
     2
     4

As you can see, the two columns are stacked on top of eachother.

[Deep P]

Thank You. That's right, but if I am reading any image into a variable and want to know a particular pixel value, can I still use the same synatx? Like A(x), where x is the pixel number.

[Deep P]

syntax*

[Image Analyst]

I guess you haven't seen my answer below yet.

[Deep P]

Yes I did, what if it is a RGB image? Image with 3 dimensions, how are pixel values read?

[Image Analyst]

See comment on my answer below.

Answer 2

[Image Analyst]

It's the 21st element which you can see by looking at m(21). If you use (row, column) indexing element 21 is the same as (1, 5). See this little illustration:

m = reshape(1:30, 5, 6) % Sample data

% Show the value of the 21st element using linear indexing:
m(21)

% Show the value of the (4, 3) element using (row, column) indexing:
m(4, 3)

% Show the value of the (1, 5) element using (row, column) indexing:
m(1, 5)

What you'll see:

m =
     1     6    11    16    21    26
     2     7    12    17    22    27
     3     8    13    18    23    28
     4     9    14    19    24    29
     5    10    15    20    25    30
ans =
    21
ans =
    14
ans =
    21

[Deep P]

Thank You

[Image Analyst]

Indexes cycle fastest from left to right. For a color image, the left most index (rows) goes first/fastest, then you move over a column and go down columns. This is all for the red channel (3rd index = 1). Then it repeats all that for the green channel (index 3 = 2), and finally for the blue channel (3rd index = 3).

[Deep P]

Hello, Thanks for explaining. Is there any way to do indexing row wise in MATLAB?

[Image Analyst]

If you change only the first index, you will change the row while remaining in the same column.

Answer 3

[Abdul Rehman]

Basically MATLAB Read image in form of Matrix,

A=[22 33 55 66 77 ; 33 55 77 88 99 ; 4 2 66 8 9];

like this, when you try in matlab Random 5*6 Matrix and perform operation.it's return the pixel value at that location in Matrix. For grayscale image it's range is (0-255).

 if true
    A=[22 33 44 1 2 4 ; 2 3 55 77 99 2 ;2 44 5 77 84 2; 33 4 55 2 1 4 ; 33 55 66 77 88 32];
    A(4,3);
    A(1,5); 
end

Answer is:

Hopefully you get it.