find new coordinates after joining the images

Question

I have 25 images with the coordinates (x,y) and images are of size 128*128 . SO I am joining the images and new the image size would be 640 * 640 . I want to find out the new coordinates of (x,y)

I tried something like

#w,h of the new image i.e 640 , 640
#imw,imh of the old image i.e 128*128
#x,y old coordinates
  
newx = int(( w * x )/ imw)
newy = int(( h * y )/ imh)

Answer 1

Small image (i, j) to large image:

X = 128 i + x, 
Y = 128 j + y

Large image to small:

i = X / 128,
j = Y / 128,
x = X mod 128,
y = Y mod 128

(with integer division).