I don't fully understand Chroma Sub-sampling 4:2:0 ?

i want to implement chroma subsampling 4:2:0 by a fcator of 2, horizontally and vertically, where we calculate the average chrominance every four pixels, but i don't fully understand how to write a code for it?

i wrote a function that concerts an rgb image to ntsc (YIQ color model) using the transforamtion matrix, then i resized the image to a multiple of 4 to make things simple, i already know how to implement 4:2:2 and 4:1:1 but 4:2:0 i can't figure out.

[Hussam Noah]

Dear Ahmed, to do 4:2:0 subsampling use 4:2:2 and then to the new array take the average of 2 row elements

e.g.

1 2 3 4

4 3 2 1

1 1 1 1

3 3 3 3

|

\/

4:2:2

|

\/

1 3

4 2

1 1

3 3

|

\/

taking average

|

\/

(1+4)/2 (3+2)/2 = 2.5 2.5

(1+3)/2 (1+3)/2 = 2 2

you can do the average by traversing the array rows and jump 2 rows at a time doing the following

(thisRow + bellowRow) /2

example

(Row1 + Row2) / 2

then

(Row3+ Row4) /2

and so on

Goodluck with your assignment ;)