I have came across the very-very-same problem.
In short:
- Willing to have original css in an "internal" dir (Resources/assets/css/a.css)
- Willing to have the images in the "public" dir (Resources/public/images/devil.png)
- Willing that twig takes that css, recompiles it into web/css/a.css and make it point the image in https://waybackassets.bk21.net/bundles/mynicebundle/images/devil.png
I have made a test with ALL possible (sane) combinations of the following:
- @notation, relative notation
- parse with cssrewrite, without it
- css img background vs direct tag src= to the very same image than css
- css parsed with assetic and also without parsing with assetic direct output
- And all this multiplied by trying a "public dir" (as Resources/public/css) with the css and a "private" dir (as Resources/assets/css).
This gave me a total of 14 combinations on the same twig, and this route was launched from
- "/app_dev.php/"
- "/app.php/"
- and "/"
thus givin 14 x 3 = 42 tests.
Additionally all this has been tested working in a subdirectory so no way to fool by giving absolute URLs because they would simply don't work.
The tests were two unnamed images and then divs named from 'a' to 'f' for the css built FROM the public folder and named 'g to 'l' for the ones built from the internal path.
I observed the following:
Only 3 of the 14 tests were shown adequately on the three urls. And NONE was from the "internal" folder (Resources/assets). It was a pre-requisite to have the spare css PUBLIC and then build with assetic FROM there.
These are the results:
Result launched with /app_dev.php/

Result launched with /app.php/

Result launched with /

So... ONLY
- The second image
- Div B
- Div C
are the allowed syntaxes.
Here there is the TWIG code:
<html>
<head>
{% stylesheets 'bundles/commondirty/css_original/container.css' filter="cssrewrite" %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{# First Row: ABCDEF #}
<link href="{{ '../bundles/commondirty/css_original/a.css' }}" rel="stylesheet" type="text/css" />
<link href="{{ asset( 'bundles/commondirty/css_original/b.css' ) }}" rel="stylesheet" type="text/css" />
{% stylesheets 'bundles/commondirty/css_original/c.css' filter="cssrewrite" %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{% stylesheets 'bundles/commondirty/css_original/d.css' %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{% stylesheets '@CommonDirtyBundle/Resources/public/css_original/e.css' filter="cssrewrite" %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{% stylesheets '@CommonDirtyBundle/Resources/public/css_original/f.css' %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{# First Row: GHIJKL #}
<link href="{{ '../../src/Common/DirtyBundle/Resources/assets/css/g.css' }}" rel="stylesheet" type="text/css" />
<link href="{{ asset( '../src/Common/DirtyBundle/Resources/assets/css/h.css' ) }}" rel="stylesheet" type="text/css" />
{% stylesheets '../src/Common/DirtyBundle/Resources/assets/css/i.css' filter="cssrewrite" %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{% stylesheets '../src/Common/DirtyBundle/Resources/assets/css/j.css' %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{% stylesheets '@CommonDirtyBundle/Resources/assets/css/k.css' filter="cssrewrite" %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
{% stylesheets '@CommonDirtyBundle/Resources/assets/css/l.css' %}
<link href="{{ asset_url }}" rel="stylesheet" type="text/css" />
{% endstylesheets %}
</head>
<body>
<div class="container">
<p>
<img alt="Devil" src="../bundles/commondirty/images/devil.png">
<img alt="Devil" src="{{ asset('bundles/commondirty/images/devil.png') }}">
</p>
<p>
<div class="a">
A
</div>
<div class="b">
B
</div>
<div class="c">
C
</div>
<div class="d">
D
</div>
<div class="e">
E
</div>
<div class="f">
F
</div>
</p>
<p>
<div class="g">
G
</div>
<div class="h">
H
</div>
<div class="i">
I
</div>
<div class="j">
J
</div>
<div class="k">
K
</div>
<div class="l">
L
</div>
</p>
</div>
</body>
</html>
The container.css:
div.container
{
border: 1px solid red;
padding: 0px;
}
div.container img, div.container div
{
border: 1px solid green;
padding: 5px;
margin: 5px;
width: 64px;
height: 64px;
display: inline-block;
vertical-align: top;
}
And a.css, b.css, c.css, etc: all identical, just changing the color and the css selector.
.a
{
background: red url('../images/devil.png');
}
The "directories" structure is:
Directories

All this came because I did not want the individual original files exposed to the public, specially if I wanted to play with "less" filter or "sass" or similar... I did not want my "originals" published, only the compiled one.
But there are good news. If you don't want to have the "spare css" in the public directories... install them not with --symlink but really making a copy. Once "assetic" has built the compound css, you can DELETE the original css from the filesystem, and leave the images:
Compilation process

Note I do this for the --env=prod
environment.
Just a few final thoughts:
This desired behaviour can be achieved by having in the git or mercurial the images in "public" directory and the "css" in the "assets" directory. Ie: instead of having them in "public" as shown in the directories, imagine a, b, c... residing in the "assets" instead of "public", than have your installer/deployer (probably a bash script) to put the css temporarily inside the "public" dir before assets:install is executed, then assets:install, then assetic:dump, and then automating the removal of css from the public dir after assetic:dump has been executed. This would achive EXACTLY the behaviour desired in the question.
Another (unknown if possible) solution would be to explore if "assets:install" can only take "public" as the source or could also take "assets" as a source to publish. That would help when installed with the --symlink option when developing.
Additionally, if we are going to script the removal from the "public" dir, then, the need of storing them in a separate dir ("assets") disappears. They can live inside "public" in our version-control system as there will be dropped upon deploy to the public. This allows also for the --symlink usage.
BUT ANYWAY, CAUTION NOW: As now the originals are not there anymore (rm -Rf), there are only 2 solutions, not 3. The working div "B" does not work anymore as it was an asset() call assuming there was the original asset. Only "C" (the compiled one) will work.
So... there is ONLY a FINAL WINNER: Div "C" allows EXACTLY what it was asked in the topic: To be comppiled, respect the path to the images and do not expose the original source to the public.
Winner is C

Arrgg after spending a lot of time preparing images, I now cannot post images beause I'm new to stackoverflow and I still don't reach the minimum reputation. I've to post them as if they were links. If someone with enough reputation wants to take them and bring them inline, please do. Will be easier to read.
app_dev.php
? – apfelbox Mar 1 '12 at 7:40