[mapserver-users] Output image resolution - explanation needed

[Rahkonen Jukka]

Hi,
 
What are the dimensions of the image you have captured in real world utits?  Your original aerial images have half a metre pixel size.  It mean that at 300 dpi one original image pixel corresponds with one pixel at paper if square inch on your printout is presenting exactly 150 by 150 meters in nature.  That is the maximum quality you can achieve.  If square inch on paper equals to 75 by 75 meters the maximum quolity is achieved already at 150 dpi. Multiplicating original image pixels for printing cannot improve the quality.  But if you have zoomed out so far that creating output image means downsampling the original imagery you should see the differense.  So, if your 2150x1111 low resolution image is covering a considerably larger area than 1075x555 meters in real world it should look better when downloaded at 300 dpi with 4300x2222 pixels. 
 
-Jukka Rahkonen-
 
 
________________________________

Lähettäjä: mapserver-users-bounces at lists.osgeo.org [mailto:mapserver-users-bounces at lists.osgeo.org] Puolesta Dejan.Gambin at pula.hr
Lähetetty: 31. heinäkuuta 2008 9:32
Vastaanottaja: mapserver-users at lists.osgeo.org
Aihe: Re: [mapserver-users] Output image resolution - explanation needed




	Thanks Steve 
	
	I still (unfortunatelly) don't understand why I am getting the "same quality" image on the paper with the first and second print (the second image has twice more pixels). Is it because of the input image itself? To be specific - the raster input is aerial image that gdalinfor reports as: 
	
	Driver: GTiff/GeoTIFF 
	Files: 5C14-05-DOF.tif 
	Size is 4500, 6000 
	Coordinate System is `' 
	Origin = (5419000.154658853100000,4964998.794652804700000) 
	Pixel Size = (0.499829146050620,-0.499829146050620) 
	Corner Coordinates: 
	Upper Left  ( 5419000.155, 4964998.795) 
	Lower Left  ( 5419000.155, 4961999.820) 
	Upper Right ( 5421249.386, 4964998.795) 
	Lower Right ( 5421249.386, 4961999.820) 
	Center      ( 5420124.770, 4963499.307) 
	Band 1 Block=256x256 Type=Byte, ColorInterp=Red 
	  Overviews: 2250x3000, 1125x1500, 563x750, 282x375 
	Band 2 Block=256x256 Type=Byte, ColorInterp=Green 
	  Overviews: 2250x3000, 1125x1500, 563x750, 282x375 
	Band 3 Block=256x256 Type=Byte, ColorInterp=Blue 
	  Overviews: 2250x3000, 1125x1500, 563x750, 282x375 
	
	regards, dejan 
	
	"Steve Lime" <Steve.Lime at dnr.state.mn.us> wrote on 30.07.2008 21:13:55:
	
	> Resolution in MapServer-speak is referring to the resolution of the 
	> output device a map
	> will be displayed on. The value is used for scale computations and 
	> nothing else. It does not
	> affect the size (in pixels) of MapServer. Scale values do impact the
	> size at which features
	> and text are rendered IF scaling (via SYMBOLSCALE) is turned on. 
	> Higher quality output is
	> usually a matter of having more pixels to print, however you usually
	> want larger features
	> too which is where the resolution parameter combined with scaling can help.
	> 
	> Steve
	> 
	> >>> On 7/30/2008 at 2:21 AM, in message
	> <OF8A10F31F.0E3F1C94-ONC1257496.0026AB23-C1257496.00286464 at pula.hr>,
	> <Dejan.Gambin at pula.hr> wrote:
	> > Hi,
	> > 
	> > I am trying to download a high resolution image and print it on a paper. I 
	> > am using p.mapper and its download function that enables downloading the 
	> > map image on different resolutions. 
	> > 
	> > I have turned on my raster layer (ortophoto image). I have zoomed in to an 
	> > area of interest. I have downloaded the map twice - with a resolution of 
	> > 150 and 300. The image is shown i a new page/tab (firefox). I right click 
	> > on the image and save it as a png (also tried with a jpeg) file. Now, the 
	> > first file is 2150x1111 pixels, 120 dpi vertical and horizontal resolution 
	> > (right click on a file, properties, summary, what does it mean?), the 
	> > second file is 4300x2222 pixels, 120 dpi vertical and horizontal 
	> > resolution. Seems ok. The second image is "doubled" in pixel size.
	> > 
	> > Now, when I try to print this image on A4 paper, using 300dpi, the images 
	> > are the same, there is no "better quality" in second image.
	> > 
	> > I have just finshed reading the Mapserver thread on "Change image 
	> > resolution in mapfile". Several limitations are mentioned there - max. 
	> > image size supported by Mapserver, "resolution" value/tag embedded in 
	> > output format, etc. Can it be related to my problem? Or is it maybe that 
	> > software I am using to print this image doesn't "use" this extra pixels? 
	> > Or I am just missing some basic stuff I need to know :-(
	> > 
	> > 
	> > thanks very much on any explanation
	> > 
	> > regards, dejan
	> 
	

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