How do I get the values of pixels in a certain area?

Question

so since I am relatively new to programming I would need a little help with that problem. I am using SimpleCV with Python 2.7 on a Windows Computer. What I am trying to do is to get a (selfwritten) program to tell me the values of the pixels along a preset line, the most important thing here would be the color of each pixel.

I don't really know where to start since I only found examples where it was asked for the values of a single pixel.

What would probably also be important to know is that I don't want to do that with a picture but with a live video made with a webcame and the preset line will be the radius of an object I will track with the webcame.

So to sum it up: I want to track an object with my webcam and need a program to tell me the color (in numbers, so for example "255" for white) of each pixel along he radius line of the tracked object.

This is a prewritten code I am currently using for obejct tracking:

print __doc__

import SimpleCV

display = SimpleCV.Display()
cam = SimpleCV.Camera()
normaldisplay = True

while display.isNotDone():

    if display.mouseRight:
         normaldisplay = not(normaldisplay)
         print "Display Mode:", "Normal" if normaldisplay else "Segmented" 

img = cam.getImage().flipHorizontal()
dist = img.colorDistance(SimpleCV.Color.BLACK).dilate(2)
segmented = dist.stretch(200,255)
blobs = segmented.findBlobs()
if blobs:
    circles = blobs.filter([b.isCircle(0.2) for b in blobs])
    if circles:
        img.drawCircle((circles[-1].x, circles[-1].y), circles[-1].radius(),SimpleCV.Color.BLUE,3)

if normaldisplay:
    img.show()
else:
    segmented.show()

here is a snapshot of the tracked object

I need the pixels color along the radius because I want to know how the light intensity decreases going from the center to the rim.

Does anybody maybe have an idea how to approach this problem? Thank you!

Can you share some sample input snapshots, Then it would be a lot easier to get the solution. — ZdaR, Aug 03 '16 at 11:09

score 0 · Answer 1 · edited May 23 '17 at 12:08

Let's say you have the center (x0, y0). You want to iterate over the pixels on the radius going from (x0, y0) to (x1, y1) which is a point on the circle.

In C++, you would be able to use the LineIterator for that purpose. However, I didn't find it for python...

So you can either implement your own LineIterator in Python (or take the one made by a nice StackOverFlow contributor) or you could resort to a hideous hack if performance is not a critical issue in your application:

Create a black binary mask the size of your image
Draw your line on this mask
Find all non-black pixels using NumPy nonzero function.
You've got the pixels of your line (though they are not ordered)
Order them according to their distance from the center.

I reckon that just taking the line iterator I linked to on another StackOverflow question is still the easiest way for you.

Thanks for that explanation it helps me alot! – Jennan Aug 04 '16 at 10:52 — Jennan, Aug 04 '16 at 10:52

How do I get the values of pixels in a certain area?

1 Answers1