How to get requested not found images url in the redirected page in htaccess Laravel

My functionality is to check whether the requested image URL exists or not. If the image exists then generate a thumbnail and return that path, if not exists then return no-image image path.

I am trying to identify in .htaccess like

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule \.(jpe?g|png)$ /generate-image [R,L=301]

After that in the generate-image route, I am trying to get that requested image URL

Route::get('/generate-image', function () {
    echo $_SERVER['REQUEST_URI'];
   // $request->fullUrl();
});

Here I am getting REQUEST_URI is generate-image instead of the original requested image URL. How to get that original requested image URL in the redirected page