# Co-ordinate system followed in cv2.warpAffine()

I was trying to implement a naive version of the warpAffine function to understand how it works. This is my implementation

```
def custom_warpaffine(image, matrix, shape):
'''
clone of cv2.warpAffine()
Follows nearest-neighbour interpolation
@Params:
image - numpy array - 1 channel, source image
matrix - numpy array of size (2, 3), affine transform matrix
shape - int tuple, shape of source image
@returns:
output - numpy array - 1 channel, image after affine transform
'''
output = np.zeros_like(image, dtype=np.float32)
for x in range(shape[0]):
for y in range(shape[1]):
transformed_x = int(matrix[0,0]*x + matrix[0,1]*y + matrix[0,2])
transformed_y = int(matrix[1,0]*x + matrix[1,1]*y + matrix[1,2])
if transformed_x >=shape[0] or transformed_y >=shape[1]:
pass
else:
output[x, y] = image[transformed_x, transformed_y]
return output
```

I followed the mapping given in the [documentation](https://docs.opencv.or..., but my output is different from that of the inbuilt function.

my affine transform matrix is [[1, 0, 100], [0, 1, 50]],

i.e dst[x, y] = scr[x+100, y+50]

but the output was different.

The output matched the inbuilt cv2.warpAffine(.., flags=cv2.WARP_INVERSE_MAP,..) function output only when the mapping was changed from

`output[x, y] = image[transformed_x, transformed_y]`

to

`output[y, x] = image[transformed_y, transformed_x]`

I am confused as to why this is the case. Thanks for any help!

Have a look at the indexing in Numpy. In short, it is

`img[row, col]`

or`img[y,x]`

.Afaik, here x is rows and y is column.