Fred,
You can do what you want with CImg. While it does have good documentation,
there are still some functionality that isn't fully explained. You want to
make use of the permute_axes member function which allows you to specify a new
ordering of pixels. CImg normal axis ordering is xyzv (where by convention v
is the color channel), so to switch to a 3 byte pixel interleaved image use:
Img = img.permute_axes("vxyz");
Then no need to write any code yourself to write the png, render to the
Image32, convert to CImg then you can use CImg to write to png using CImg's
ImageMagick functionality. If you have problems you may have to explicitly
define the ImageMagick path before including CImg.h, here's what the code would
look like:
#define cimg_imagemagick_path "imagemagickpath"
#include "CImg.h"
// Mapnik code and rendering to an Image32 here
CImg<long> img(img32,width,height);
img.permute_axes("vxyz").save_graphicsmagick_external("filename.png");
Good luck!
I think you'll fine CImg an extremely useful package, it can do pretty much
anything you can think of and if you have problems use it's forums. David is
quite responsive and there's been a few occasions where I've asked about some
functionality which he then added to the library within a few days!
matt
-----Original Message-----
From: Frederik Ramm [mailto:[EMAIL PROTECTED]
Sent: Friday, June 13, 2008 8:26 PM
To: Matt Hanson
Cc: [email protected]
Subject: Re: [Mapnik-users] Mapnik reprojection - how?
Hi,
> > Fred, Image32 does appear to be a pretty basic type
> > (yes it's a mapnik class). If you're using C++ at all (or have
> > the option to) I'd highly suggest you take a look at CImg...
>
> I'm using C++ and tend to stick what I already know (which is
> GD and ImageMagick) but will check out CImg as well.
Ok here's an update on what I've achieved so far.
I have successfully used the cairo_renderer to draw onto a cairo surface
which I subsequently rendered onto a new surface with scaling applied.
This gives the desired result, with good quality, but unfortunately it
is very slow (full execution time for rendering a 2° by 2° OSM styled
map onto an 800x600 canvas - 15 seconds with cairo compared to 7 seconds
with AGG).
Actually applying the scaling to the cairo context used internally by
the cairo_renderer, instead of scaling the resulting surface like I do,
gains you a second but still the performance makes this unsuitable for
my task.
So I went back to the idea of using the original AGG render result and
somehow scale that bitmap before returning it. Unfortunately, neither GD
nor ImageMagick nor CImg have a convenient method to work with the kind
of interleaved data store that backs the Image32 type (GD stores an
array of interleaved three-byte pixels and the alpha value is elsewhere;
ImageMagick has 64bit pixels; CImg uses a non-interlaved four byte
store, while Image32 is an interleaved four byte store). Somewhat
exasperated, I did the ugliest thing and had Mapnik write the image to
disk for me, then called ImageMagick externally to scale the PNG file,
then read it back in - which adds an extra half second to processing but
at least works for testing.
Since I don't need top-notch image quality for this corner case where
the WMS client requests non-matching width/height, I will perhaps try
the homebrew way of simply duplicating or dropping full lines in the PNG
output to achieve the desired height. I have to roll my own PNG output
from Image32 anyway because (unlike cairo which supports callback
output) the existing Mapnik code will only create PNG files on disk and
not in memory, where I need them if I want to return them to the user.
If anyone in the know reads this and has some insight to share, do speak up.
Bye
Frederik
--
Frederik Ramm ## eMail [EMAIL PROTECTED] ## N49°00'09" E008°23'33"
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