# How to find out Actual coordinate(pixel values) of chunk relative to the actual image?

I have an image of let say 500 X 500 px, I created tiles/chunks of size (10, 10). Now I want to find out actual chunk coordinate relative to the Original 500X500 px image.

Ex I have first chunk of size (10, 10), the coordinates in this will be liks this

``````[0, 0][0, 1]..........[0, 10]
[10, 0][10, 1]........[10, 10]
``````

Now for the each coordinate(x, y), How to find out the Actual coordinates in Original Image?

• By 'Actual coordinates' you mean coordinates in the Coordinate Reference System of your original image? How did you read your original image? Do you have a geotransform? Or an Affine transform? – Loïc Dutrieux Jan 12 '17 at 11:00
• Yes for your first question. I didn't read my first image because actual size is really large, so here I have mention a example of 500X500. Actually image size is 100000 X 100000 and I created 5000X5000 chunks of my actual image now for each chunk (x, y) pixels i want to find out the value coorespoding to Origin 100000 image. – Anup Panwar Jan 12 '17 at 11:15
• Is that make sense ? – Anup Panwar Jan 12 '17 at 11:17

I will structure my answer in three parts:

1) Why I am answering this : I was myself looking to find the corresponding pixel of my cropped image.... and looking for the solution I landed on your question, only to realize that it is still unsolved! Hence I am sharing my solution after some brainstorming! Hope it helps !

2) Mistake in question : In the example you stated the chunk size = (10,10) according to this the coordinates will be 0 to 9 on both x and y.. per say x=10 or y=10 will not exist.

3) Solution : Let's say the pixel in main image is X=24,Y=27 i.e. (24,27) = A

Now by simple maths or else Arithmetic Progression we know that it will be part of sub image of size (10,10) starting from X=20, Y=20 i.e. (20,20)=S and ending at X=29, Y=29 i.e. (29,29)=E.

Now in this sub image, coordinates are :

[0, 0][0, 1]..........[0, 9]

[9, 0][9, 1]........[9, 9]

Now in this sub-image we want to find the original-image pixel for x=4, y=7 i.e. (4,7)=a

Thus to go from a(x)(or a(y)) to A(X) (or A(Y)), the relation is as follows:

A(X) = S(X) + a(x) =>> A(X) = 20 + 4 =24

A(Y) = S(Y) + a(y) =>> A(Y) = 20 + 7 =27

Thus , a ---> A is (4,7) ----> (24,27)

The inverse is also true!

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All the best!!!

• try checking out the code here in my thesis and the attached documents... check the linked data.. for me it works as i am using matplot lib and opencv in conjunction for extraction ... the two libraries read the image in opposite directions.. researchgate.net/publication/… – Akanksha Rastogi Jul 4 at 15:07