How do I extract the coordinates of a colored arrow on an otherwise black and white image?

I have a black and white image (not grayscale) with a green arrow on it. I need to automatically get the coordinates of this line without using the impixel() function.

Answer 1

[Walter Roberson]

mask = YourImage(:,:,2) >= 200 & YourImage(:,:,1) <= 20;   %Green presence but not much red, must be green arrow
[r, c] = find(mask);

[Coder Bee]

Thank you. But how do I use this to find the starting and ending coordinate of the line while retaining the background? Or find the starting and ending coordinates on this and then restoring the background?

[Walter Roberson]

Is it a straight line? What style of arrow head does it have? Determining the direction an arrow points is not always easy, and it might not be possible to uniquely define a starting point for an arrow.

Have you considered using regionprops() to analyze the arrow? For example you might be able to find its orientation and the extrema might be of use.

[Image Analyst]

You forgot to post your image. Why make it more difficult for us to answer you? Post your image so we don't have to guess.

And why would finding starting and ending coordinates do anything at all with the background? Why would it have to be "restored"?

[Coder Bee]

I am not allowed to share the image. But it is for medical imaging and thus retaining the background is vital. Also I dont need to determine the orientation of the arrow. Just need the coordinates of the line to calculate the mid-point of the arrow. Right now i am doing that using impixel(). But I wanted to find an automatic way to do it. To answer your previous question - yes it is a straight line with an arrow head pointing to the right. Thank you.

[Image Analyst]

Well you could use image segmentation. Walter showed one way. So that's about all we can say unless you give an example image. Maybe you can mock one up in Photoshop that is generic (without personally identifiable information, PII) but gets your point across.

[Walter Roberson]

Look at http://www.mathworks.com/help/matlab/ref/annotationarrow-properties.html#property_headstyle and tell us which of the head styles the arrow has.

[Coder Bee]

Sure, I will try to send you a mock image. Meanwhile, the head style of the arrow is plain.

[Coder Bee]

Found this image off google.

Consider getting the coordinates of the green arrow on images 'a' and 'g' in the below image.

[Walter Roberson]

In the particular case of exacty one horizontal arrow where you do not care about the direction, then

mask = YourImage(:,:,2) >= 200 & YourImage(:,:,1) <= 20;   %Green presence but not much red, must be green arrow

regionstats = regionprops(mask, 'Centroid');
horz_center = regionstats.Centroid(1);

[Image Analyst]

Define coordinates. Coordinates of what? The center of the arrow? The tail? The head? Every single green pixel in the arrows? And why??? Why do you care about where some arrow from someone else's image is?

And do you want the coordinates with respect to the large composite image, or to the smaller sub-images? for example, the bottom arrow may be at line 600 with respect the the entire big image, but at line 140 with respect to the small subimage.

[Walter Roberson]

Above the user indicated "Just need the coordinates of the line to calculate the mid-point of the arrow"