"Monitorize" an Image

Matlab 2013, 150 bytes

Here is my attempt in Matlab. Definitely won't be the shortest code, but it's a start.

Warning, this overwrites the original image, so make a copy first.

Golfed Version

function  mi(f,n)
o=imread(f);
s=size(o);
imwrite([imresize(o(:,1:((s(2)-n)/2),:),[s(1),s(2)/2]) imresize(o(:,((s(2)+n)/2):end,:),[s(1),s(2)/2])], f);
end

---

Ungolfed Code, with improvements for odd image sizes and odd number of columns

function  monitorizeImage( filename, num_columns )

orig = imread(filename);
orig_size = size(orig);

f = factor(orig_size(2));
origsize_iseven = f(1)==2;

f = factor(num_columns);
num_columns_iseven = f(1)==2;

odd_even_size_mismatch = xor(origsize_iseven,num_columns_iseven);

img_resized = imresize(orig,[orig_size(1) orig_size(2)+odd_even_size_mismatch]);

leftimg = img_resized(:,1:((orig_size(2)+odd_even_size_mismatch-num_columns)/2),:);
leftimg = imresize(leftimg,[orig_size(1),floor(orig_size(2)/2)]);
rightimg = img_resized(:,((orig_size(2)-odd_even_size_mismatch+num_columns)/2):end,:);
rightimg = imresize(rightimg,[orig_size(1),floor(orig_size(2)/2)]);

monitorized_image = [leftimg rightimg];
monitorized_image = imresize(monitorized_image,[orig_size(1),orig_size(2)+ ~origsize_iseven]);

[~, ~, ext] = fileparts(filename); 

imwrite(monitorized_image,strcat(filename(1:end-length(ext)),'_',num2str(num_columns),ext));

end

Wolfram Language, 134, 127, 119 111 bytes

f[i_,c_]:=(d=ImageDimensions@i;ImageAssemble[ImageTake[i,a=All,#]&/@{{0,e=-#&@@d/2-c/2},{-e,a}}]~ImageResize~d)

Creates a function f that takes an image as the first input (as a symbol in Mathematica or the Wolfram Cloud), and an integer as the second input.

Ungolfed:

f[image_,columns_]:=(  (*Define Function*)
    d=ImageDimensions[image];  (*Get image dimensions*)
    e=d[[1]]/2+columns/2;  (*Add half the image width to half the number of removed columns*)
    ImageResize[ImageAssemble[Map[ImageTake[i,All,#]&,{{0,-e},{e,All}}]],d]  (*Map the function onto a list with the desired column ranges and merge and scale the resulting image*)
)

Technically, it won't work properly if either of the image dimensions exceed 362,880 pixels, but I assume that's okay, since that is way outside the scope of the problem (and some computers). Fixed!

PHP, 206 bytes

($c=imagecopyresized)($t=imagecreatetruecolor($w=imagesx($s=imagecreatefrompng($argv[1])),$h=imagesy($s)),$s,0,0,0,0,$v=$w/2,$h,$x=$v-$argv[2]/2,$h);$c($t,$s,$v,0,$w-$x,0,$v,$h,$x,$h);imagepng($t,$argv[3]);

takes three command line arguments: source file name, number of lines to crop and target filename. Run with -r.

You may want to use imagecopyresampled instead of imagecopyresized (+2 bytes) for a better result.

ungolfed

$s=imagecreatefrompng($argv[1]);    # load source image
$w=imagesx($s);$h=imagesy($s);      # get image dimensions
$t=imagecreatetruecolor($w,$h);     # create target image
$v=$w/2;                            # $v = half width
$x=$v-$argv[2]/2;                   # $x = width of remaining halves
                                    # resize and copy halves:
imagecopyresized($t,$s, 0,0,    0,0,$v,$h,$x,$h);
imagecopyresized($t,$s,$v,0,$w-$x,0,$v,$h,$x,$h);
imagepng($t,$argv[3]);              # save target image

I could save 9 more bytes by sending the PNG result to STDOUT ... but what for?