[mapserver-users] jagged edges on images

[chris_faulkner at anytimenow.com]

HI 

How do you store your imagery ? Presumably, you will need to have many SHAPEPATH definitions in map files and tileindex shape files around the place. I'd be interested to know how you deal with this area. 

Chris

> Chris -
>  
> I deal with multi-terabyte data sets under MapServer already, and while
> there is a price for the resampled images, it does only have to be paid
> once.  Even an extensive set of overviews (2x, 4x, 8x, 16x, etc.)
> increases the data storage by about a third (the 2x resample is
> obviously by far the largest).  The additional processing to produce
> high-quality resampled images is pretty substantial relative to the rest
> of what MapServer does.
>  
> However, if you do want to go that route, the best solution right now
> would be to have MapServer generate the "native resolution" output image
> - perhaps of a larger size than the output image if needed - and then
> write an image post-processor that does something like bicubic
> resampling in RGB space to the desired output size and then send it
> along to the client.  High-quality resampling is somewhat compute- and
> memory-intensive, but if your output map images are modest sizes it
> might not be a big deal.
>  
>      - Ed
> 
>  -----Original Message----- 
>  From: chris_faulkner at anytimenow.com
> [mailto:chris_faulkner at anytimenow.com] 
>  Sent: Fri 6/14/2002 12:00 PM 
>  To: Ed McNierney 
>  Cc: mapserver-users at lists.gis.umn.edu; chris_faulkner at anytimenow.com 
>  Subject: RE: [mapserver-users] jagged edges on images
>  
>  
> 
>  Hi
>  
>  Sorry for the incorrect terminology.
>  
>  I wouldn't want to store multiple copies of the data because our image
> database is measured in terrabytes. The cost of the disk is not
> necessarily of prime importance (although top-end robust, reliable, raid
> is not cheap when you buy that sort of quantity). The management issues
> of all those files would come into play, though. I'd rather wait another
> fraction of a second for it to do the resampling every time, especially
> when the image is being served over the web and the slowest part of the
> process is much longer than that and is out of our control - the
> download ro the user's device.
>  
>  
>  Chris
>  
>  > Chris -
>  >
>  > In the example you sent, it's not downsampling, it's upsampling - the
>  > output image is at a higher resolution than the input image.  While
>  > there are alternatives, and it would be nice for MapServer to support
>  > more resampling options, doing that well really needs to wait for the
>  > 24-bit output support to be available.
>  >
>  > Why do you not want to store multiple copies of the data?  With
> multiple
>  > copies you greatly reduce the amount of resampling that needs to be
>  > done, and I'm a big fan of precomputing everything you possibly can.
>  >
>  >  - Ed
>  >
>  >
>  > -----Original Message-----
>  > From: chris_faulkner at anytimenow.com
>  > [mailto:chris_faulkner at anytimenow.com]
>  > Sent: Friday, June 14, 2002 3:41 AM
>  > To: Ed McNierney
>  > Cc: chris_faulkner at anytimenow.com; mapserver-users at lists.gis.umn.edu
>  > Subject: RE: [mapserver-users] jagged edges on images
>  >
>  >
>  > Hi
>  >
>  > Yes - when I make sure that the input image resolution is the same as
>  > output,  the image looks OK.
>  >
>  > However, as my last email says, I do want to be able to  change scale
>  > without storing multiple copies of the data so it would be nice if
> the
>  > down-sampling operation could support an option of a higher quality
>  > alogorithm. IN this case, the output image might need to increase in
>  > depth, though - maybe MapServer wouldn't support that.
>  >
>  > Chris
>  >
>  >
>  > > Chris -
>  > >
>  > > This looks like an artifact of image scaling in the output image.
>  > > MapServer prudently uses a fast algorithm for scaling source input
>  > > images to produce the output result.  If you ask for an output
> image
>  > > whose pixel resolution (that is, meters per pixel, for example) is
>  > > different than the source image resolution, the source image has to
> be
>  > > scaled.  Your sample image looks like an output image with a pixel
>  > > resolution slightly smaller than the input image.  For example,
> let's
>  > > assume your source image is at a resolution of 2 meters per pixel.
>  > Then
>  > > let's assume you asked for a 400 x 400 pixel output image from
>  > MapServer
>  > > with a spatial extent of 600 meters by 600 meters.  That means you
>  > want
>  > > the output pixels to be 1.5 meters per pixel.
>  > >
>  > > Since the output is "zoomed in" a bit, MapServer is being asked to
>  > > generate 400 output pixels (in each dimension) out of 300 input
> pixels
>  >
>  > > (600 meters at 2 meters per pixel).  It will do that by duplicating
>  > > every third input pixel in the output, so if you had a row of
> pixels
>  > > with these values:
>  > >
>  > > 0 1 2 3 4 5
>  > >
>  > > you would get an output image with these values:
>  > >
>  > > 0 1 2 2 3 4 5 5
>  > >
>  > > Image then, that you had two single-pixel vertical lines in the
> source
>  >
>  > > image.  In the output, the one at pixel position 2 would end up
> being
>  > > two pixels wide, while the identical input line at pixel position 4
>  > > would still be only one pixel wide in the output.  This will look
>  > > strange.
>  > >
>  > > In your specific example, you appear to have antialiased text in
> the
>  > > input image.  The intermediate-shaded pixels along the edge of the
>  > > text are sometimes replicated, giving the odd effect you see in the
>  > > words "Briar Patch".
>  > >
>  > > For optimum image quality without artifacts, keep the output image
>  > > resolution the same as the source resolution, or a multiple of 2x
> the
>  > > source resolution.  If you're going to be doing that often,
> consider
>  > > using GDAL and create overview images (preprocessed subsamples of
> the
>  > > source image) to avoid having to resample the source image over and
>  > > over again to serve map requests.
>  > >
>  > >  - Ed
>  > >
>  > > Ed McNierney
>  > > President and Chief Mapmaker
>  > > TopoZone.com / Maps a la carte, Inc.
>  > > 73 Princeton Street, Suite 305
>  > > North Chelmsford, MA  01863
>  > > ed at topozone.com
>  > > (978) 251-4242
>  > >
>  > > -----Original Message-----
>  > > From: chris_faulkner at anytimenow.com
>  > > [mailto:chris_faulkner at anytimenow.com]
>  > > Sent: Wednesday, June 12, 2002 6:14 PM
>  > > To: mapserver-users at lists.gis.umn.edu
>  > > Subject: [mapserver-users] jagged edges on images
>  > >
>  > >
>  > > Hello again
>  > >
>  > > I've been looking at some imagery being generated via PHP
> Mapscript.
>  > > On my imagery , jagged effects appear along some of the edges. The
>  > > jagged version from MapServer is attached. Look along the road
>  > > boundaries and the word "Briar Patch". Anyone know why this happens
> ?
>  > > These effects don't appear when I view and zoom in and out of the
>  > > source with ImageMagick. Here is the tiffinfo output. Note that I
>  > > started with tiled Tifs and converted to stripped - I have played
> with
>  >
>  > > different sized strips.
>  > >
>  > > TIFF Directory at offset 0x1255d6
>  > >   Image Width: 3500 Image Length: 3500
>  > >   Bits/Sample: 8
>  > >   Compression Scheme: PackBits
>  > >   Photometric Interpretation: palette color (RGB from colormap)
>  > >   FillOrder: msb-to-lsb
>  > >   Document Name: "Standard Input"
>  > >   Image Description: "converted PNM file"
>  > >   Orientation: row 0 top, col 0 lhs
>  > >   Samples/Pixel: 1
>  > >   Rows/Strip: 2
>  > >   Planar Configuration: single image plane
>  > >   Color Map: (present)
>  > >
>  > > If anyone needs it, I can send the original one but I didn't want
> to
>  > > waste too much bandwidth !
>  > >
>  > > Thanks
>  > >
>  > >
>  > > Chris
>  > >
>  > >
>  >
>  >
>  
> 
> 
>