[Qgis-user] aerial photo issues

[Brent Wood]

Yep.... DPI is pretty much a printer (or screen) issue... so you want to have an image of high res that prints at that resolution - DPI will vary depending on page (screen)  size & pixel size as well as image resolution...  so just deal with the image, retain the high resolution & print it as well as you can. If you need better quality hard copy, try a better printer or printing/graphics agency


Brent


________________________________
 From: Geoff Hay <geoffrey.hay at otago.ac.nz>
To: Brent Wood <pcreso at pcreso.com>; "qgis-user at lists.osgeo.org" <qgis-user at lists.osgeo.org> 
Sent: Sunday, April 6, 2014 9:34 AM
Subject: RE: [Qgis-user] aerial photo issues
 


Right. The myth of DPI is the problem. So what I need is to simply save the image exactly as it was but with the vector information overlaid - no compression, no resampling, no resizing. 

________________________________
 
From: Brent Wood [pcreso at pcreso.com]
Sent: Saturday, 5 April 2014 3:52 p.m.
To: Geoff Hay; qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] aerial photo issues


If OSGeo4Win does mostly what you want, you can try GraphicsMagick to convert images to various formats...

Also Open Source, but command line based...

it can't get much easier than "gm convert image.png image.jpg" and there are lots of options to play with if necessary...


http://www.graphicsmagick.org/convert.html

or the less stable but perhaps more adventurous ImageMagick...




________________________________
 From: Geoff Hay <geoffrey.hay at otago.ac.nz>
To: "qgis-user at lists.osgeo.org" <qgis-user at lists.osgeo.org> 
Sent: Saturday, April 5, 2014 3:11 PM
Subject: [Qgis-user] aerial photo issues



 
Hi
I'm new to QGIS. I am working with aerial photos and overlaying polygons and labelling. QGIS works nicely for these tasks. I want to print these maps at a high resolution but have encountered a number of problems. I'd like some advice.
 
The original georeferenced JPG files are 72 dpi and quite large (for example 9000 x 8000 pixels and 57Mb). The objective is to export as JPG at 300 dpi so that the vector layers and labelling are high quality and the original image quality is maintained when printed at full size. This seems to be no problem in ArcMap but I have so far been unable to achieve it in QGIS. Is this possible in QGIS?
 
The default JPG exporter in Composer has a size limitation so it is unable to export the required size and resolution when I calculate and set a custom (large) paper size. When I scale the image the default producer also seems to be compressing the output (from 57Mb down to around 8Mb) which is ruining the print quality . I've read that  GDAL has a JPEG2000 driver which could possibly do the job but I have not been able to figure out how to make composer use this for exporting images.
 
I've tried exporting to PNG in composer and this seems to work without issues however the requirement is for JPG images - I wonder if it is reasonable practice to do a conversion later to get a JPG file?
 
I've tried the OSGeo4W command line snapshot tool which seems to produce high res images but is limited to PNG output - is there any way to change this to JPG? 
 
I've tried exporting to TIFF however this didn't work and resulted in an 8Kb file. Perhaps the original image needs to be converted to a TIFF file first?
 
 
 
Other comments:
In composer, it would be nice to be able to set the extent of the composition to the actual extent of the base image rather than the QGIS canvas extent - I have to manually input the image extents. Actually it would be nice to have some mechanism that sets up the extents and paper size based on supplied dpi and the original image dimensions. perhaps this is not a common use case.
 
TIA
geoff
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