Jan van der Vegt - 5 months ago 21

Python Question

I have used convolution2d to generate some statistics on conditions of local patterns. To be complete, I'm working with images and the value 0.5 is my 'gray-screen', I cannot use masks before this unfortunately (dependence on some other packages). I want to add new objects to my image, but it should overlap at least 75% of non-gray-screen. Let's assume the new object is square, I mask the image on gray-screen versus the rest, do a 2-d convolution with a n by n matrix filled with 1s so I can get the sum of the number of gray-scale pixels in that patch. This all works, so I have a matrix with suitable places to place my new object. How do I efficiently pick a random one from this matrix?

Here is a small example with a 5x5 image and a 2x2 convolution matrix, where I want a random coordinate in my last matrix with a 1 (because there is at most 1 0.5 in that patch)

Image:

`1 0.5 0.5 0 1`

0.5 0.5 0 1 1

0.5 0.5 1 1 0.5

0.5 1 0 0 1

1 1 0 0 1

Convolution matrix:

`1 1`

1 1

Convoluted image:

`3 3 1 0`

4 2 0 1

3 1 0 1

1 0 0 0

Conditioned on <= 1:

`0 0 1 1`

0 0 1 1

0 1 1 1

1 1 1 1

How do I get a uniformly distributed coordinate of the 1s efficiently?

Answer

`np.where`

and `np.random.randint`

should do the trick :

```
#we grab the indexes of the ones
x,y = np.where(convoluted_image <=1)
#we chose one index randomly
i = np.random.randint(len(x))
random_pos = [x[i],y[i]]
```