Looking to identify a football from an image

I currently have a still image of a football about to be kicked. I would like to identify the ball and it's center.

Answer 1

[Kevin Holly]

RGB = imread('YourImage.png');

% Convert RGB image to lab space
I = rgb2lab(RGB);

% Apply thresholds
BW = (I(:,:,1) >= 14.5 ) & (I(:,:,1) <= 89.77) & ...
    (I(:,:,2) >= 0.12) & (I(:,:,2) <= 12.5) & ...
    (I(:,:,3) >= 3.8) & (I(:,:,3) <= 33.0);

% Open mask
se = strel('disk', 13,0);
BW = imopen(BW, se);

% Filter out smaller objects
BW=bwareafilt(BW,[50000 Inf]);

% Find Centroid and Area of object
rp = regionprops(BW,"Centroid","Area")

rp = struct with fields:
        Area: 58864
    Centroid: [644.7752 215.7900]

% Display masked image for verfication purposes
maskedRGBImage = RGB;
maskedRGBImage(repmat(~BW,[1 1 3])) = 0;

imshow(maskedRGBImage)

% Add marker on centroid
hold on
scatter(rp.Centroid(1),rp.Centroid(2),'b','filled')

[Michael Sposato]

I also need this to work for other images as well. What is there to change to do this? I attached another sample image as well.

Also how is the centroid calculated? Is it by the extremes or by matlab calculating the area of the ellipse?

Answer 2

[Image Analyst]

You can use transfer learning to identify footballs in a variety of images with a variety of colors, orientations, sizes, etc. See this answer:

https://www.mathworks.com/matlabcentral/answers/508234-can-the-googlenet-on-transfer-learning-in-10-lines-of-matlab-code#answer_418228

[Vicente]

Hi @Image Analyst, im working on a colour analysis project, and i have been reading a lot of your comments about colour science in this place.

Can I take advantage of this conversation to ask you some questions about this?

I found in some of your comments your PPT post where you explain a process of correction in RGB and calibration to LAB. I am trying to do the latter. I tried to build a calibration to transform photos from SRGB to Lab. To do this, I built a least squares regression to obtain a transformation equation, using the RGB values obtained from a color checker in my images and its reference Lab values.

My question is: for colour analysis applications where I need to extract reliable Lab or RGB values, what’s the recommended photo format and processing pipeline? Should I use RAW/Bayer images and linearize them, or is a high-quality PNG good enough?

[Image Analyst]

What I do is to set the gamma of my camera to 1 so that there is no nonlinear transform applied to the captured image. If you don't have a machine vision camera where you have that kind of control, then I need to know what you want to do. If you're just doing segmentation (like finding areas/regions) then it may not be necessary to do all the operations that you need to do if you were wanting color measurement traceable to a spectrophotometer. If you do want to do accurate color measurement that will give you LAB values very close to what you'd get from a spectrophotometer, then you really need a good machine vision or scientific/industrial camera where you can do white balancing and specify the gamma, and turn the automatic gain control and automatic exposure control off -- it must be fully manual because you don't want the camera doing any funny unknown things in an attempt to be helpful. If you are stuck with a camera where the PNG or TIFF or JPG image has a gamma, then you can still use the ColorChecker chart to develop a transform. The transform will attempt to "undo" the gamma and linearize it. If you use a higher order transform than the 3x3 matrix that MATLAB uses

https://www.mathworks.com/help/images/end-to-end-implementation-of-digital-camera-processing-pipeline.html?searchHighlight=camera+bayer&s_tid=srchtitle_support_results_4_camera+bayer#CameraPipelineExample-2

then you could do a better job than doing it the way MATLAB shows you. However if you want to do it all with built-in MATLAB functions (using their 3x3 matrices) then you can but it won't be quite as accurate as doing it yourself manually with a higher order transform (like I do). So if you do it yourself, again, I'd use a machine vision camera that you can set gamma=1 and turn off all automatic adjustments, and but if you can't then the higher order transform will somewhat "undo" the gamma. Or you could linearize it first with that link and then do the custom higher order transform. Though it won't be as close to a spectrophotometer it may be good enough for whatever you want to do, which I don't know what that is (saying you want to measure color is not a "use case" and is very vague).