Subject: [Thuban-list] displaying raster is very slow

From: Moritz Lennert <[email-masked]>
Date: Wed, 29 Jun 2005 16:05:20 +0200

Me again,

I am using Thuban CVS from June 21 on a debian testing centrino
machine with 512MB RAM and Pentium M 1.6GHz.

I have a 41MB Tif which I want to display (Corinne Landcover at 250m
resolution for all of Europe). It takes a long time (ca. 30s) to
load and display the file. Even when I just hide the layer and then
redisplay it, it takes ca. 15s to show up.

As a comparison, QGIS shows the map in 1 second.

Why is Thuban so slow ?

Moritz

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From: Bernhard Reiter <[email-masked]>
Date: Wed, 29 Jun 2005 16:22:19 +0200

Hi Moritz,

> Why is Thuban so slow ?

My wild guess is the raster projection support
and the lack of caching.

	Bernhard

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From: Jan-Oliver Wagner <[email-masked]>
Date: Wed, 29 Jun 2005 23:15:52 +0200

> As a comparison, QGIS shows the map in 1 second.

in fact Thuban has so far not undergone performance tuning
for raster. There is probably lots of opportunities
to make it run faster.

IIRC Jonathan did something in this area, but it might
only work with wxWidget 2.5/2.6 ... ?

	Jan
-- 
Jan-Oliver Wagner               http://intevation.de/~jan/
Intevation GmbH                      http://intevation.de/
Kolab Konsortium               http://kolab-konsortium.de/
FreeGIS                                http://freegis.org/

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From: Jonathan Coles <[email-masked]>
Date: Wed, 29 Jun 2005 20:08:35 -0400

> only work with wxWidget 2.5/2.6 ... ?

The work that I did earlier in the year was about supporting different
kinds of transparency. Alpha blending requires wxWidgets >= 2.5.3.
There is some more limited transparency support that uses a bitmap to
represent which pixels should be on or off. That works under ealier
versions of wxWidgets as well as later versions.

Off the top of my head, I can think of three areas that might be
affecting the speed:

1) The warping of the source image to fit the projection. This code is
essentially just calls to the GDAL library, which is seperate from
Thuban.

2) If any form of transparency is enabled, especially the bitmapping form, then
this could be taking a while for a large image.

3) The image drawing code of wxWidgets, but I've been drawing large
images (i.e. those that cover a large number of physical pixels) and
haven't noticed this to be a problem.

The easiest thing to try is to turn off transparency on the image
layer. If that improves performance then that's the source of the
problem. Using alpha blending may also improve the speed over bitmap
transparency because a lot of tedious computation isn't necessary.
Remember, however, that alpha blending is only supported under
wxWidgets >= 2.5.3.

Hope that helps,

--jonathan

-- 
=====================================================================
Jonathan Coles                               http://www.jpcoles.com
[email-masked]
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