[mapserver-users] jagged edges on images

[Ed McNierney]

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