# Finding nearest non-zero pixel

I've got a binary image `noObjectMask`

(`CV_8UC1`

) and a given point `objectCenter`

(`cv::Point`

). If the `objectCenter`

is a zer-value pixel, I need to find the nearest non-zero pixel starting from the given point.

The number of non-zero points in the whole image can be large (even up to 50%), so calculating distances for each point returned from `cv::findNonZero`

seems to be non-optimal. As the highest probability is that the pixel will be in the close neighborhood, I currently use:

```
# my prototype script in Python, but the final version will be implemented in C++
if noObjectMask[objectCenter[1],objectCenter[0]] == 0:
# if the objectCenter is zero-value pixel, subtract sequentially its neighborhood ROIs
# increasing its size (r), until the ROI contains at least one non-zero pixel
for r in range(noObjectMask.shape[1]/2):
rectL = objectCenter[1]-r-1
rectR = objectCenter[1]+r
rectT = objectCenter[0]-r-1
rectB = objectCenter[0]+r
# Pythonic way of subtracting ROI: noObjectMask(cv::Rect(...))
rect = noObjectMask[rectL:rectR, rectT:rectB]
if cv2.countNonZero(rect)>0: break
nonZeroNeighbours = cv2.findNonZero(rect)
# calculating the distances between objectCenter and each of nonZeroNeighbours
# and choosing the closest one
```

This works okay, as in my images the non-zero pixels are typically in the closest neighborhood (`r`

<=10px), but the processing time increases dramatically with the distance of the closest pixel. Each repetition of `countNonZero`

repeats counting of the previous pixels. This could be improved by incrementing the radius `r`

by more than one, but this still looks a bit clumsy to me.

How to improve the procedure? And ideas? -Thanks!

what about cv::distanceTransform

Okay, this will show me how far I am from the closest non-zero point, and the local gradient will let me estimate the direction of closest point, right? Seems a good idea...