ECW Pyramid

[Rahkonen Jukka]

Hi,
 
To my understandment and experience it is unnecessary to have anything like ECW pyramids if you mean by that the same thing as with Geotiffs, for example.  That is, downsampled versions of individual image files.  ECW has a wavelet based internal system for getting the same effect and it works fine.
 
Another thing is that if you are looking your site through tile index and you have zoomed out very far then pyramid layers (or overviews or whatever they are called) do not help very much.  In this case MapServer has to open a bunch of physical files from the file system, perhaps tens of image files, and that will ineviatably slow down the response time.  What will help in this case is a separate, radically downsampled image that covers large area of your imagery.  For example, we have often sites which are 50 km by 50 km in size and they hold 100 aerial images with 0.5 metre pixel size.  Zooming to whole site through tile index means that all the 100 files must be opened and it is for sure always slow with any file format and whether we have fine internal pyramids or not.  Pyramids do help a bit but the key to the speed is to avoin opening so many files from disk.  What we use to do is to create a quick look image with something like 10 metre pixel size and use that until the user has zoomed in so close that the resolution is not good enough.  At that moment only 1-4 original images must be opened through tileindex and that goes fast.  
 
A simple way to create a quick look image is to define Geotiff outputformat in the mapfile and ask MapServer to send the image with whole site extents with some reasonable width and height.
 
-Jukka Rahkonen-
 
________________________________

Lähettäjä: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] Puolesta José Ramón López
Lähetetty: 12. joulukuuta 2007 14:09
Vastaanottaja: MAPSERVER-USERS at LISTS.UMN.EDU
Aihe: [UMN_MAPSERVER-USERS] ECW Pyramid



	Hi List.
	
	I have populated a raster layer using a Tileindex made from ECW Files. This layer is visible 1:1000 scale with MAXSCALE parameter.
	As is not possible to create ECW pyramids. Whe have got another raster layer resampling these ECW files. 
	We would like to populate a single raster layer that uses one tile index created from the resampled ECW, visible at 1:10000 scale (and smaller), and another tileindex from the original ECW files visible at bigger scales. 
	Is it possible? If not, please tell me how could I do something similar.
	
	Tnaks.
	

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