raster : bad quality in output

[Ed McNierney]

Didrik -

Thanks!  Congratulations on getting the RESOLUTION calculation right on the first try.

        - Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com 

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On Behalf Of Didrik Pinte
Sent: Friday, April 08, 2005 4:20 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] raster : bad quality in output

Le jeudi 07 avril 2005 à 12:00 -0400, Ed McNierney a écrit :
> Didrik -
> 
> What I meant was, "are all the pixels in the relevant portion of the input image used exactly once to produce one pixel in the output image"?  That is, is the output image a rectangular piece of the original input image at the same size, or is it "zoomed in" or "zoomed out"?
> 
> Unfortunately, scale ratios are meaningless for digital images without also specifying the output resolution in pixels per physical unit (inch, cm, etc.).  It is very likely that the scale ratios of your source images refer to a different resolution than the default 72 pixels per inch that MapServer uses.
> 
> Can you find out what the intended output resolution of your rasters is?  For example, if I tell you that a raster is at 1/50000 scale, I am saying that one cm on the image equals 5 km on the ground.  But that obviously depends on the physical resolution at which the image is viewed - if I am viewing it at 50 pixels per cm, then 50 pixels in the image equals 5 km on the gorund.  But if I'm viewing it at 100 pixels per cm, then 100 pixels in the image equals 5 km on the ground.  You need to know the intended viewing resolution in order to answer this question.
> 
> If the rasters are scanned images of printed maps, this value would normally be the scan resolution.
> 
>      - Ed

Great, I received the information about the "intented viewing resolution".

It was 32 points per cm, giving 81.28 dpi ! 

I've set the RESOLUTION parameter to 81.28 and it renders perfectly when using scale near the raster scale !

Thank you very much for the information (and the explanation too).

Didrik