Extracting long lines from images with short lines as noise?

Question

I have working on a project to extract out the long lines from the image given below. I have no idea how to remove rest of the short lines and small dots from the image. Please give me some hints. My interest lines are both side long lines and middle ones.

After applying RANSAC and some thresh holding operation i get output like below

Answer 1

Providing you have the image processing toolbox, you can follow the suggestions of @rayryeng and @Shai and do something like this:

Morphological filtering:

BW1=bwareaopen(YourOriginalBWImage, 100);

Filtered image:

Hough transform (adapted from this):

[H,theta,rho] = hough(BW1);
P = houghpeaks(H,7,'threshold',ceil(0.3*max(H(:))));
lines = houghlines(BW1,theta,rho,P,'FillGap',5,'MinLength',3);
figure, imshow(YourOriginalBWImage), hold on
for k = 1:length(lines)
   xy = [lines(k).point1; lines(k).point2];
   plot(xy(:,1),xy(:,2),'LineWidth',2,'Color','green');
end

Result:

Answer 2

For detecting straight lines, better use the well known method of Hough transform.

img = imread('https://i.stack.imgur.com/V6bDn.png');
bw = rgb2gray( imcrop( bw, [85 35 390 290] ) ) > 128; % crop and threshold image
[H,T,R] = hough(bw,'RhoResolution',0.5,'ThetaResolution',0.5);
P  = houghpeaks(H,5,'threshold',ceil(0.3*max(H(:))),'NHoodSize',[31, 11]);
lines = houghlines(bw,T,R,P,'FillGap',25,'MinLength',30);
% display the result
figure;imshow(bw);hold on;
for k = 1:length(lines), 
    xy = [lines(k).point1; lines(k).point2];
    plot(xy(:,1),xy(:,2),'LineWidth',2,'Color','green');
end

You'll end up with something like