artifacts at tiled image borders

[Frank Warmerdam]

On 7/27/05, Dylan Beaudette <dylan at iici.no-ip.org> wrote:
> Greetings,
> 
> I have been doing some research on better ways to store my aerial imagery, so
> that I can keep a higher resolution version of my source files. Until now I
> have been downsampling USGS DOQQ images from 1 meter resolution to 10 meters,
> saving the result as a geoTiff. With overviews, these file are about 1.5 mb
> per quarter quad. A shapefile tile index is used to render the images in
> Mapserver.
> 
> Last night I re-created a section of the tiled dataset in JPEG format at 3
> meter resolution. I am much happier with the resulting increase in detail,
> however I am now noticing some strange artifacts showing up at the "seams" of
> the tile index.
> 
> here is an example of what I am talking about:
> http://169.237.35.250/~dylan/temp/mapserver/bad_edges.jpg
> 
> the source data do not appear to have these type of "rough edges":
> http://169.237.35.250/~dylan/temp/mapserver/original_clean_edges.jpg
> 
> I am using the OFFSITE operator in my map file to ignore pixels with an RGB
> value of '0 0 0'.
> 
> Does anyone have any idea what could be causing these rough looking seams at
> the tileindex boundaries?

Dylan,

The problem is that jpeg is a lossy format, and along the boundaries
areas that should be black are in fact just very slightly non-black and
so are not recognised as offsite. 

This will occur to varying degrees with any lossy format like jpeg, 
ecw, jpeg2000 and mrsid. 

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent