MATLAB: “cleaning” a horizontal line removal code for object detection.

Question

I have a code that removes horizontal lines in an image but the problem is that it removes some of the lines that are needed for object detection. Any ide on how to make my output "cleaner"?

here is my code:

A=imread('sheetmusic.jpg');
B=rgb2gray(A);
C=double(B);
for i=1:size(C,1)-2
    for j=1:size(C,2)-2
        %Sobel mask for x-direction:
        Gx=( ( 2*C(i+2,j+1)+C(i+2,j)+C(i+2,j+2) ) - ( 2*C(i,j+1)+C(i,j)+C(i,j+2) ) );
        %Sobel mask for y-direction:
        %Gy=((2*C(i+1,j+2)+C(i,j+2)+C(i+2,j+2))-(2*C(i+1,j)+C(i,j)+C(i+2,j)));
      
        %The gradient of the image
        if(Gx~=0)
            B(i,j)=abs(Gx);
        end
      
    end
end
figure,imshow(B); title('Sobel gradient');

and here is my input image:

and here is my output:

Answer 1

Hi Janrex,

For a nice clean image like this, there's a very simple solution. Just compute a row sum; rows with staff lines have a lower sum:

tmp=sum(C,2);
figure;plot(tmp)

Now set those rows to white:

ind=find(tmp<1E5)
C(ind,:)=255;
figure;imagesc(C);axis image;

(The downsampled image above doesn't look great; looks better for real)

If the white lines through the notes cause issues, you could do some morphology to close them (dilate then erode).