creating channels in an array

I saw a code from rgbe bayer conversion program as follows:

% subsample HDR image to get RGGB Bayer pattern 
  I(1:2:end, 1:2:end) = HDR(1:2:end, 1:2:end, 1); % R
  I(1:2:end, 2:2:end) = HDR(1:2:end, 2:2:end, 2); % G odd
  I(2:2:end, 1:2:end) = HDR(2:2:end, 1:2:end, 2); % G even
  I(2:2:end, 2:2:end) = HDR(2:2:end, 2:2:end, 3); % B

To understand this concept i tried this code with simple arrays in command window as follows:

>> x=[1 2 3 4 5;6 7 8 9 10;11 12 13 14 15;16 17 18 19 20]

x =

1 2 3 4 5

6 7 8 9 10

11 12 13 14 15

16 17 18 19 20

>> h=[20 19 18 17 16; 15 14 13 12 11; 10 9 8 7 6; 5 4 3 2 1]

h =

20 19 18 17 16

15 14 13 12 11

10 9 8 7 6

5 4 3 2 1

>> x(1:2:end,1:2:end)=h(1:2:end,1:2:end,1)

x =

20 2 18 4 16

6 7 8 9 10

10 12 8 14 6

16 17 18 19 20

>> x(1:2:end,2:2:end)=h(1:2:end,2:2:end,2)

error: h(_,_,2): but h has size 4x5

What does this error mean?

Why did it happen?

Answer 1

[Stephen23]

"What does this error mean?"

You are trying to access array elements that do not exist!

You defined h to have size 4x5(x1x1x1...):

h = [4x5x1] % you defined |h| to have this size
h(...,..,2) % what the indexing refers to: note that 2>1 

The RHS indexing refers to a 3D array, but your example h array only has size 1 along the third dimension. So when the indexing refers to the 2nd "page", it throws an error because your array only has one "page":

https://www.mathworks.com/help/matlab/math/multidimensional-arrays.html

This third dimension is used for storing image RGB data:

https://www.mathworks.com/help/matlab/creating_plots/image-types.html

If you want your example to work you will need to create an h array that has atleast size 3 along the third dimension (i.e. atleast 3 pages), e.g.:

>> h = permute(reshape(60:-1:1,5,4,3),[2,1,3])
h(:,:,1) =
   60   59   58   57   56
   55   54   53   52   51
   50   49   48   47   46
   45   44   43   42   41
h(:,:,2) =
   40   39   38   37   36
   35   34   33   32   31
   30   29   28   27   26
   25   24   23   22   21
h(:,:,3) =
   20   19   18   17   16
   15   14   13   12   11
   10    9    8    7    6
    5    4    3    2    1

[Pravitha A]

Thankyou