Error 500 after rewrite url to load image - regex

Question

I have a subdomain pics.mysite.com that i use to load images, so the src of an image would be something like http://pics.mysite.com/image.png
When image is loaded the url is rewrited to another folder inside (pics_files) where are the files like this:
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ pics_files/$1 [L,QSA]
Works fine, but now i'm having a problem.
If the image doesn't exists it creates an Internal Server Error - 500, but i would like to be able to load an "error" image, like errorImg.png, but so far i can't.
How can i rewrite the url to that errorImg.png if image do not exists?

Answer 1

Replace your code with this:
Options +FollowSymLinks -MultiViews
# Turn mod_rewrite on
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^pics\.mysite\.com$ [NC]
RewriteRule ^$ /errorImg.png [L]
# if valid image file then load it from /pics_files/ directory
RewriteCond %{DOCUMENT_ROOT}/pics_files/$1 -f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /pics_files/$1 [L]
# if not-existing image file then load /pics_files/errorImg.png
RewriteCond %{DOCUMENT_ROOT}/pics_files/$1 !-f
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /errorImg.png [L]

Answer 2

This is because you are capturing the entire URI and routing it into the pics_files directory. If the image doesn't exist, the !-f condition passes and the URI is rewritten again, so you end up with:
/pics_files/pics_files/image.png
which doesn't exist, so the URI is rewritten again:
/pics_files/pics_files/pics_files/image.png
which doesn't exist, so the URI is rewritten again, etc, etc.
Instead of checking if the URI isn't a file or directory as a condition, check that the target image actualy exists:
RewriteEngine On
RewriteBase /
RewriteCond %{DOCUMENT_ROOT}/pics_files%{REQUEST_URI} -f
RewriteRule ^ /pics_files%{REQUEST_URI} [L]