Generating HTML Canvas image data server-side?

Question

The title of this question may be slightly misleading, but I\'m not sure what the best title would be (since I can\'t guess at a solution yet).

Basically the system I am

Answer 1

I used phantomJs (like node.js but different) serverside to run exactly the same code as client side, and get the same result. all you need is one single exe-file (like a webkit stand alone web brower)

The following program (in Perl, but should be feasible to translate to you favourite language) takes some data, inserts into a web-page (could be ajax'ed) and either sends that web page to the client, or stores it as a temporary file, and starts PhantomJs on the same page. Then ask PhantomJs to generate a jpg, that is then picked up (and in this case sendt to the client).

#!/usr/bin/perl

use strict;
use File::Temp;
$|=1;
#this script returns a graph, either as html +js web page to render client side,
#or renders the same page server side, and returns the jpg image.

#files needed:
#.\phantom_srv_client.pl  #this script
#.\phantomjs.exe          #the webkit runtime stand alone file, from http://phantomjs.org/
#.\Scripts\excanvas.min.js #canvas simulator for IE8-
#.\Scripts\jquery.min.js   #jQuery as we know it
#.\Scripts\jquery.jqplot.min.js #graph library on top of jQuery from http://www.jqplot.com/ (Flot or any can be used)


#do we want client side rendering (html + js), or server side rendering (jpg)
#jpg seems to render nicer than png on some pages?
use CGI;
my $show_as_jpg = CGI::param("jpg");

#path to javascript libraries (jQuery etc). 
#Must be absolute file location for server rendering, relative for web
use FindBin;
my $script_path = $show_as_jpg 
    ? $FindBin::Bin."/Scripts" 
    : './Scripts';


#data to send to graph (two sets)
my $data = [[2,5,4], [6,4,5]];

#use json to get this as a javascript text
my $json_data;
eval {require JSON; $json_data=JSON::to_json($data)};
#in case JSON is not installed, get the json/javascript data manually (just for demo)
$json_data ||= "[[2,5,4], [6,4,9]]"; #here 9 at the end to see a difference

#The following is the web page that renders the graph, client or server side 
#(links to scripts must be abolute to work serverside, as temp-files may go anywhere, $script_path keeps track of that)
#$json_data is the Perl data structure converted to JSON (aka javascript, but not)
my $graph_html =qq|



    
    
    

    
    

    

    View as jpg


|;


#this is the javascript that tells PhantomJs what to do (ie open a doc and render it to bitmap)
my $phantom_doc_js =qq|
    var system = require('system');
    //read from commandline which files to open, and write to
    var open_doc = system.args[1];
    var return_doc = system.args[2];
    var page = require('webpage').create();
    page.open(open_doc, function () {
        page.render(return_doc);
        phantom.exit();
    });
|;

#see if we shall render this page serverside
if ($show_as_jpg) {
    #get temporary filenames with related file handlers
    #where to put phantomjs script (generic so could be a static file)
    my ($phantom_doc_filehandler, $phantom_doc_filename) = File::Temp::tempfile(  SUFFIX => '.js', TMPDIR => 1);
    #where to put web page with data to render and ref to javascripts etc
    my ($phantom_graph_filehandler, $phantom_graph_filename) = File::Temp::tempfile(SUFFIX => '.html', TMPDIR => 1);
    #also get a filename with no handler, so phantomjs can return the jpg file. Extention must be .jpg!
    my (undef, $image_filename) = File::Temp::tempfile( SUFFIX => '.jpg',TMPDIR => 1, OPEN => 0);

    #store file content and close files
    print $phantom_doc_filehandler $phantom_doc_js; close $phantom_doc_filehandler;
    print $phantom_graph_filehandler $graph_html;   close $phantom_graph_filehandler;

    #now call PhantomJs with filenames to read from and write to.
    #Next version should support piping, which would simplify a lot

    #use absolute path to phantomjs.exe in case web-server does not use current path
    system($FindBin::Bin.'\\phantomjs', $phantom_doc_filename, $phantom_graph_filename, $image_filename) == 0 
        or die "system failed: $?";

    #read the entire image file
    my $img = slurp_file($image_filename);
    print "Content-Type: image/jpeg\nPragma: no-cache\n\n".$img;

    #The temp files are no more needed
    unlink $phantom_doc_filename, $phantom_graph_filename, $image_filename;

} else { # just render client side
    print "Content-Type: text/html\nPragma: no-cache\n\n".$graph_html;
}

#slurp is not always std perl   
sub slurp_file{
  my $filename = shift;
  my $string;
  local $/ = undef;
  open FILE, $filename or die "Couldn't open file: $!";
  binmode FILE;
  $string = ;
  close FILE;
  return $string;
}