in reply to Images built on the fly do not display

So you're saving an image to a file which then you expect the webserver to pass on to the browser... You could have a problem with webserver side caching. The fact that the image filename is always the same, could have a lot to do with it.

You could try to make sure the image name is always different. Use a counter to increment a number, and use that as part of an image name. To delete old files, you could use a cron job; or you can have the parent do that, while it's waiting for the child to finish. You'll never have too many old files, that way. I hope. :-)

But my preference would be to invoke the image creating script directly from a separate CGI script (or equivalent), and send the image data directly to the browser instead of saving it to a file first. You do need to provide the appropriate content-type header, as others already have suggested, and then simply print out the image data. The parameters you now use on the command line, can be passed as part of the image URL, after a question mark, using the familiar GET URL syntax.

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Re: Re: Images built on the fly do not display
by Anonymous Monk on Jan 08, 2003 at 11:41 UTC
    The image is saved to file and although this snippet doesn't show it, the image filename was manually changed each time the script was run, so the caching problem shouldn't really apply.

    We did try to give the give the process ID as the filename.

    Yes, I can see how it would be a good idea to send the image directly to the browser but I am curious to know why the current effort doesn't work Here is the full code: 

    #!/usr/bin/perl -w

$|=1;
use strict;
use CGI::Carp "fatalsToBrowser";
use CGI ":all";
use DBI;

print header,start_html(-title=>'Bright', -bgcolor=>'#BDBDB0');

print h1(font{-face=>'Futura Lt BT'}, "Genome view");
print font{-size=>'3', -color=>'#800000', -face=>'Futura Lt BT'};


my($tth,$sth,$dbh,@position,$start,$finish,@row,$i,$child_pid,@accessi
+on,$SQLstr,$imagename);



if(!param)
{
    $dbh = DBI->connect("DBI:mysql:database=dicty;host=localhost", "**
+**", "****");
    $sth=$dbh->prepare("select min(fstart),max(fstop) from fdata");
    $sth->execute;

    while(@row=$sth->fetchrow_array)
    {
        $position[0]=$row[0];
        $position[1]=$row[1];
    }

    $dbh->disconnect;
    print start_form;
    print textfield(-name=>'start',-value=>$position[0],-size=>15),br,
     textfield(-name=>'finish',-value=>$position[1],-size=>15);
    print submit(-name=>'submit'),end_form;
}

if(param("submit"))
{
    $imagename=$$;

    $start=param("start");
    $finish=param("finish");
    

    if(!defined($child_pid=fork()))
    {
        die "cannot fork $!";
    }
    elsif($child_pid==0)
    {
        
        my $query= "/usr/apache/cgi-bin/order/ldasdump.pl -png -d dict
+y chr5.1:$start,$finish --user root --pass mysqladmin > /usr/apache/h
+tdocs/imagename.png";
        exec($query);
        
    }
    else
    {
        
        waitpid($child_pid,0);
        
        print img({src=>"../../imagename.png",align=>'LEFT'});
    
    }

}

    [download]
[reply]
[d/l]
      I am curious to know why the current effort doesn't work

      The problems start with this line   print header,start_html(-title=>'Bright', -bgcolor=>'#BDBDB0'); which sends a MIME type of "text/html" back to the browser, then starts an HTML page. At this point, it's a bit late to start printing image bits and expect the browser to display an image.

      Read the description of header() in the CGI.pm POD, and note how to override the MIME type (aka Content-type). You want to send "image/png".

      I should have given up and gone to bed hours ago.

      Is it possible that there's something in $start or $finish that is fouling up the shell meta-character expansion that happens when you use the single-argument form of exec(), preventing the command from generating an image?

      Try using the list form of exec(), which doesn't trigger shell meta-character expansion.

[reply]
[d/l]
[select]
        Nonono, you've missed how it is supposed to work. This script invokes an external script to produce an image and save it to a temporary file. This script sends out an HTML which contains a link to this temporary file as an URL for an IMG tag. The browser is supposed to pick up the HTML and load the image in it.

        To the OP: perhaps the time difference between the saving of the file and sending the URL to the browser is too short. Perhaps the server hasn't finished saving when the webserver tries to access the file. I don't know, it's just a wild guess, really.

[reply]
        mmm, i don't think there is something wrong with $start or $finish because an image is being generated.

        We can display the previously generated images, just never the one that is produced in the current run

[reply]

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