# How to find all the points of contact of all the circles of an image using opencv

Here is the image,I want to find all contact points in this image,I know the center and radius of each circle.In my mind, the euclidean distance maybe is not a efficient method to calculate the center-to-center distance.So,My question is how to find all the contact point of this image in a efficient way.Thanks. Circles' contact points will be placed on the line from center1 to center2. So for example if both circles have the samze size you would choose `intersection = center1 + 1/2*(center2-center1)` But since your circles aren't of same size you could choose something like:

``````compute dirVector = center2-center1
compute d = |dirVector|
if d < radius1 + radius2 + threshold: consider circles to intersect so now compute contact point
gap = d - radius1 - radius2 // remark: here gap might become < 0 which is ok
```
```

I've tested this approach with primitive C++ code and I'm getting this result: green circles are your original image, red circles are my extracted circles (not perfect but simple), pink dots are `contact points` for some contact-distance-threshold (20 pixel)

and this is the result for a much lower contact-distance-threshold (5) (but keep in mind that my extracted circle radius is a bit lower than yours): To make this more efficient (because you would have to compare each pair of circles, no matter how far away they are => `O(n²)` ) you should use some kind of binary space partitioning like:

KD-Tree

or similar

or try Spatial Hashing, but those are better suited for sparse areas.

In general you might want to search for collision detection optimizations to speed things up (there you'll find more information about binary space partitioning and spatial hashing).

Since your space is very filled you might consider a simple grid, too (which is similar to spatial hashing in this case):

``````//choose size of each grid cell to be the maximum found radius of all your circles.
gridCellSize = max diameter + contact-threshold-distance // radius was wrong I think
for each circle
indexX = round(center.x/gridCellSize)
indexY = round(center.y/gridCellSize)
grid[x,y].push_back(current circle)

for each circle in a grid-cell:
compare this circle to each other circle in the cell
compare this circle to each circle in all surrounding cells
ignore all circles in all other cells
```
```

for my previous example I've extracted a maxium radius of 44 pixel (yours might be something like 47 pixel, depending on the line size you used for drawing the circles.

So a grid like this is computed: For example in grid-cell position (1,1) if grid is 0-indexed, there are 2 circles located. Each of them should be compared to all circles in the same cell and each circle in surrounding cells. So those 2 circles will be compared to only 12 circles instead of against `n-1` circles.

Or graphically marked examples: All circles in a red cell have to be compared against each circle in the surrounding blue cells in addition to the other circles in the red cell (none in this example): 