Florian Pellegrin - 6 months ago 21

Python Question

Newbie here. =) I tried to reverse an image but there's an error and I don't know why :/

**The Error:**

`Traceback (most recent call last):`

File "C:/Users/Florian/Documents/ISN/S10/défi11.py", line 10, in <module>

im.putpixel((x,600-y),(p[0],p[1],p[2]))

File "C:\Python27\lib\site-packages\PIL\Image.py", line 1267, in putpixel

return self.im.putpixel(xy, value)

IndexError: image index out of range

`# -*- coding: cp1252 -*-`

from PIL import Image

im=Image.open("H:\Belem.png")

L,H=im.size

for y in range(H):

for x in range(L):

p=im.getpixel((x,y))

im.putpixel((x,600-y),(p[0],p[1],p[2]))

im.save("H:\defi11.png")

Answer

I you mean to flip the image vertically, then you should do this:

```
for y in range(H/2):
for x in range(L):
p1=im.getpixel((x,y))
p2=im.getpixel((x,H-1-y))
im.putpixel((x,H-1-y),p1)
im.putpixel((x,y),p2)
```

This avoid overwriting pixels you will need later. It only loops over the first half of the lines, and exchanges them with the other half of the lines. Another approach would be to create a different output image with the same shape to write to:

```
im2 = im.copy()
for y in range(H):
for x in range(L):
p=im.getpixel((x,y))
im2.putpixel((x,H-1-y),p1)
im2.save("flipped.png")
```

This has the same effect as the version above, but uses more memory.

I guess the `600`

in your example is a hardcoded version of `H`

, but you have to subtract one extra from that (like I do above) in order to take into account that the indices go from `0`

to `H-1`

, not from `1`

to `H`

. On the first loop of your program `y`

is zero, so `600-y`

is `600`

. If `600`

is the height of the image, then you are going one beyond the last index (`600-1`

), and hence triggering an `IndexError`

exception.

If you have `numpy`

installed, then a faster and simpler way to achieve the same thing is:

```
import numpy as np, PIL
original=PIL.Image.open("original.png")
arr = np.array(im)
flipped = PIL.Image.fromarray(arr[::-1])
flipped.save("flipped.png")
```

The `numpy`

format also makes it easy to perform other operations like doing maths on the pixels.