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PC Plus HelpDesk - issue 227

This month, Paul Grosse gives you more insight into some of the topics dealt with in HelpDesk and HelpDesk Extra

From the pages of HelpDesk, we look at:

  • Apache Fancy Indexing;
  • Arbitrary angle motion blur; and,
  • JPEG compression.

From HelpDesk Extra, we look at Server-Side Includes (SSIs):

  • Configuration file;
  • Code in html files; and,
  • Code to run.
Paul Grosse

HelpDesk

Apache Fancy Indexing

Usually, if a file called index.html exists in a directory, that file will be sent to the client by the server when it gets a request for the default file in a directory so, when asking for http://www.foo.com/, the file http://www.foo.com/index.html will be sent. The default file does not have to be index.html, this can be changed if you want using the DirectoryIndex directive. In httpd.conf, you should find the line

DirectoryIndex index.html

so, if you want to use home.htm as your default file, just change this to...

DirectoryIndex home.htm

If, however the default file is not there and Fancy Indexing is turned on, you will get a page that Apache has created dynamically for you.

To turn it on, you need a module loaded, so you need a like that looks like...

LoadModule autoindex_module /usr/lib/apache/mod_autoindex.so

and also a line like

AddModule mod_autoindex.c

These should be in your default Apache htpd.conf file and you just need to make sure that they are there if you cannot get it working.

With the module loaded and added, that page looks like the one below...

If you click the column links, the page is re-sent using the column you clicked on to order the page contents - clicking again reverses that order.

Whist this is good enough for most purposes, you can add a personalised header and footer to the page. Again, this is done by adding files to the directory. To get the header, you add a file called 'HEADER' and to get the footer, you add a file called 'README' as follows...

If you want to change the name of those files, you can find the lines in the httpd.conf file that read...
ReadmeName README
HeaderName HEADER

and replace the names of the file with whatever you want. The content of the header and footer files can be just plain text. If it is only text, it will appear at the top or bottom of the page as though it was between <PRE> and </PRE> tags.

You are not limited to using plain text though. If you want, you can put html in there to do whatever you want. However, remember that you don't need to add the <HTML> <HEAD> <TITLE> </TITLE> </HEAD> <BODY> tags or, in the footer (README file), the </BODY> </HTML> tags. Remember though, that if you do use html tags, any plain text will just be mangled in the way that web pages mangle plain text - ie, that all new lines are ignored and spaces are condensed down to a single space so if you have some formatted text such as that ascii art picture you've been working on for days or as in 'VIDEO' in the example above, this will just become meaningless unless you do bracket it with <PRE> and </PRE> tags.

VVV VVV IIIIIII DDD and so on.

If there are files in the directory that you want to hide from the index, you can hide them by using the IndexIgnore directive inside a </Directory> in the httpd.conf like so

IndexIgnore HEADER README .htaccess *~

In this way, you can refine the list so that people can only see what you want them to.

If you have run a web server for a while, you will notice that some search engine spiders will try to get a fancy index by supplying the server with a request for the directory with the conditions appended as though they had already got a fancy index - in a directory that has an index.html file. ie...

http://myserver.com/abcdef/?M=A

On Apache, this gets them nowhere.

You can find out more about Fancy Indexing from http://httpd.apache.org/docs/mod/mod_autoindex.html


Arbitrary angle motion blur

If you blur a normal photograph, such as the one of the Roborovski hamster on the right, you can blur it at any angle you want. and it will look all right (assuming that blur is the effect you want).
Like, so. This is because there are different sized areas of different densities and the viewer concentrates on details (s)he recognises.
However, if you want to blur something that only has small details and you want to use an angle that is close to a multiple of 45 degrees, you have something of a problem. Take the image on the right...
If we give this a blur of 16 pixels, on an angle of 4 degrees and then bring the density range up so that we can see what we are looking at (as I will do with all of these as the dots will get lost if I don't), we see that there are distinct steps in the blur trace.
We can get around this by scaling up the image by, say, a factor of four and using no interpolation method so that the pixels end up as squares like so...
Next, we apply the blur (remembering to multiply the distance by the same factor that we blew up the image by), like so...

You can see that the lines are still stepped over the pixels but how they render themselves in the final image depends upon how you scale it down again...

Using no interpolation, we get the same image that we would have got if we hadn't bothered to do the scaling at all...
Using linear interpolation, we get better results - on the right, the full image and on the left, a blow-up of it.
Finally, this is with cubic interpolation which is the better of the three interpolation methods that are on offer with the Gimp 2.0.

If your graphics package cannot do arbitrary blur angles, you can blow up the image, then rotate it, blur it at 0 degrees, rotate it back again and then shrink it back down again. Again, the reason for blowing it up is so that any cross-pixel steps are obscured.

You might notice in the blown up images above, that there are a number of distinct steps in the density of the blurred dots.. If you want this to be smoother, you can blow it up by a larger factor but remember that in your strive for perfection, you might be chasing detail that nobody is going to look for.


JPEG compression

Many cameras come with jpeg compressed files (.jpg) being the only option for the image files they produce. Whilst it is commendable that they conserve memory use by using a compressed image file format, jpeg does leave a number of artefacts in the reproduced image.

One of the predominant artefacts is the way that the compression puts everything into blocks. This is noticed if you are doing anything that cuts down on the number of colours (such as posterisation) or enhances any edges (edge detect ends up with loads of edges that run in a square fashion in addition to the edge lines that you are actually interested in.

One advantage about jpeg is that you can control the level of compression so that you can balance image file size against image quality.

  Quality Size
(/kB)
Saved
Image
Enhanced
Portion
This is the photograph of the Horse Chestnut in all of its autumnal glory saved as a JPEG image file with least compression.

If you want to see the .png file that these images came from, just click here.

Even with this amount of compression (taking the original 396kB and compressing it to 51.5kB), you can still notice some block-like structures in the enhanced slice on the right.

1.00 51.5
0.80 gives a fairly good pay-off of image size for quality if you are going to use the image on the web.

In the enhanced slice, you can now see, more plainly, square features creeping into the image.

0.80 14.9
At 0.60, the image is down to 10kB and whilst looking all right, the enhanced version is showing distinct blocks now. 0.60 10.0
At 0.40, the plain image is starting to show some signs of blocks. 0.40 7.7
By 0.20, the image is not really good enough to show on the web.

If you had trouble seeing where the artefacts were on the enhanced version, you can now see where to look from this one and when you look at the images above, they will be clearer.

0.20 4.9
This is for the morbidly curious. You can see how JPEG creates blocks of plain colour and blocks that have gradients in them.

In the enhanced image, you can see a new design for that tiling project for the kitchen.

 

0.00 0.9

All of these images were produced on the GIMP version 2.0. The enhanced versions were created using
Filters> Enhance> Sharpen... with the sharpness set to 90.


HelpDesk Extra

Server-Side Includes

Server-Side Includes (SSIs) allow you to have dynamic content in your web pages without having to go to the length of producing pages that are made up entirely of program-generated html code. Whilst CGI (Common Gateway Interface) scripts are an effective way of processing forms and other user input, they are something of a sledgehammer of a way of doing things. For SSIs, a walnut sized problem, you do not need such a sledgehammer.

In effect, SSIs allow you to have a normal html file but include within it, a reference that the server will see and then process. In the final output, the reference is replaced completely so the user will not be aware of the existence of SSI code in the original html file.

To get the server to read through the file (parse it), we need to let it know that it needs parsing. There are two ways of doing this:

  1. Use a special file extension such as .shtml; or,
  2. Set a meta-data bit on the file.

Apache supports both of these. It is better to use the normal file extensions because:

  1. The end user will not be aware that they are getting the opportunity to run a program on your server; and,
  2. Any file that you decide to add SSI code to will not need to have its file extension changed (easy) or have every page that refers to it changed so that the new reference has the new extension (difficult).

To get Apache to use normal (.html) file extensions, you need to set the executable bit on the file so that the file is executable. Unfortunately, Windows doesn't recognise files by their content, only by their extension so you cannot do this on Windows.

Linux, *BSD et al

However, on Linux (et al), you can modify the x-bit simply by

right-clicking on the file and selecting properties...
... then, making sure that the exec bit is checked.

Windows

On Windows, you can only change the file extension to .shtml

Configuration file

General

In order to make SSIs work, you need to have directives in the httpd.conf file that reads...

LoadModule includes_module /usr/lib/apache/mod_include.so

.. and also a line like ...

AddModule mod_include.c

.shtml (Windows, Linux, *BSD, UNIX...)

To get Apache to read .shtml files, you need to have a couple of lines in httpd.conf as follows...

AddType text/html .shtml

... which adds .shtml to the list of file extensions that are to be considered fair game for the server, and...

AddHandler server-parsed .shtml

... which tells Apache that .shtml files need to be scanned through for SSI code instead of just being copied to the client's browser.

x-bit (Linux, *BSD, UNIX...)

To get Apache to search through normal .html files that have the exec bit turned on, you need a line in the httpd.conf file that looks like...

<IfModule mod_include.c>
  XBitHack on
</IfModule>

Apache will now parse .html files with the x-bit on.

Just before we leave the httpd.conf file, there are still one or two things we can do with it that are related to SSIs.

We can add a line that look at the browser and, dependent upon the type of browser, set various variables. This is done by having a line that looks like...

BrowserMatchNoCase MSIE IE

...which looks for the string 'MSIE' in the browser information that the browser sends Apache. As the name suggests, it is case-independent. If it finds the string, it sets the variable 'IE'. You can, of course, look for other sub-strings such as Gecko, Firefox, Links and so on and have any number of variables. We can use these variables in the SSI code as follows...


Code in html files

Now that we can get Apache to read our .html files for code, we need to put some code in there.

With SSIs we can do many things:

  • select static text from within the page - no external references
  • add static text from other files - insert the contents of another file
  • add dynamic text from local variables
  • add dynamic text from a script.

All of this is achieved without having anything special on the URI (the browser address line) other than the page name that is being requested.

Dynamic Text - Local Variables

So, as an example, to add the date, we need to use the DATE_LOCAL variable within our html file. This is done like so...

<!--#echo var="DATE_LOCAL" -->

The <--#command other necessary bits --> part is replaced so remember to include spaces where necessary.

Select Static Text From Within Page

To select text from within a page, you need to have some way of knowing what is gong on (otherwise you may as well just use static text). So, if in your httpd.conf file, you have set a variable for MSIE (which you have called 'IE') and one for NetScape (which you have called NETSC), you can now use them for selecting code within your page. Thus...

<p>
<!--#if expr="${IE}" -->
  You are using Internet Explorer.
  The United States Computer Emergency
  Readiness Team (US-CERT) suggests that
  you use a different browser.<br><br>
  Also, there are some issues with
  standards compliance with this browser
  so you might have a degraded experience
  of these pages as a result of this
  non-compliance.
<!--#elif expr="${NETSC}" -->
  You are using Netscape.
  Some of the functionality of these
  pages will be lost as a result of
  standards non-compliance.
<!--#else -->
  Have a nice Day
<!--#endif -->
</p>

You can, of course, include anything you want including html tags that refer to images and so on.

Add Static Text From Other Files

To add the contents of another file, you just need to make sure that the context of that file is correct (if you are adding something to the middle of a table, such as a menu, you need to have your html tags sorted out).

If we have a partial file called specialmenu.txt and we wanted to include it, we would need the line...

<!--#include virtual="specialmenu.txt" -->

which simply replaces the text between the <> (as well as the < and the >) with the contents of the file specialmenu.txt.

The contents of the menu file would be along the lines of...

<center>
<a href="index.html">Home</a><br>
<a href="blog.html">WebLog</a><br>
<a href="dls.html">Downloads</a><br>
<a href="links.html">Links</a><br>
</center>

.. which, whilst making it easy to update the menu for all of the pages, would have the disadvantage of giving a live link to the page that the user was already looking at.

Add Dynamic Text From A Script

To add dynamic text from a script (instead of having a file with the text already in it, we run a program that generates the text according to a set of conditions) such as if we wanted to have a program that generates a menu that has a current-page-link that is not live, we need to use a line like the following in our parsed html page...

<!--#exec cgi="cgi-bin/menus/ms1.cgi" -->

... where cgi-bin/menus/ is where ms1.cgi is stored. As for the contents of such as script...


Code to run

Once we have our page looking for a script to run, we need to justify the requirement. So, let's look at two scripts, one for a counter and the other for a dynamic version of the menu in the previous section. You can find these files in Windows readable format (ie, crlf) in the files directory or by clicking here.

Counter

#!/usr/bin/perl -w
print "Content-type: text/plain", "\n\n";
$counter_file = "counter1.txt";
open(FILE, ">>$counter_file");
close(FILE);
chmod(0777,"counter1.txt");
open (FILE, "<" . $counter_file) ||
  die "Can not read from the counter file.\n";
flock (FILE, 2);
$number_accesses = <FILE>;
flock (FILE, 8);
close (FILE);
open (FILE, ">" . $counter_file) ||
  die "Can not write to the counter file.\n";
flock (FILE, 2);
$number_accesses++;
print FILE $number_accesses;
flock (FILE, 8);
close (FILE);
1 while $number_accesses =~ s/^(\d+)(\d\d\d)/$1,$2/;
print $number_accesses;
exit (0);

Basically, it lets Apache know what it is up to with the content type and then two blank lines, then it finds out that the file is there, locks it, reads it, increments it, saves it, unlocks it and then the last few lines format the value into comma-separated thousands and then prints it.

Dynamic Menu

This is similar although we look at an environment variable passed to it by Apache - DOCUMENT_NAME

I've highlighted the first part in Blue so that it is more visible

#!/usr/bin/perl -w
print "Content-type: text\/plain", "\n\n";
my $thispage = $ENV{DOCUMENT_NAME};
print "<center>\n";
if ($thispage eq "index.html") {
  print <b>Home</b>";
} else {
  print "<a href=\"index.html\"
    ><b>Home</b></a>";
};
print " <br>\n";
if ($thispage eq "blog.html") {
  print <b>WebLog</b>";
} else {
  print "<a href=\"blog.html\"
    ><b>WebLog</b></a>";
};
print " <br>\n";
if ($thispage eq "dls.html") {
  print <b>Downloads</b>";
} else {
  print "<a href=\"dls.html\"
    ><b>Downloads</b></a>";
};
print " <br\n";
if ($thispage eq "links.html") {
  print <b>Links</b>";
} else {
  print "<a href=\"links.html\"
    ><b>Links</b></a>";
};
print "</center>\n"

You can see that it compares the name of the page that the output of this program will become and if it is the same, just outputs a plain piece of text (albeit bold). If it is not, then it uses an anchor href to generate a link to that page.

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