[Interest] Zooming with QGraphicsView/QGraphicsItem

Brad Pepers bpepers at me.com
Thu Aug 8 19:49:24 CEST 2019


A shapefile is a bunch of geometry with attributes. Your coastline is likely just a simple line with each point so easy to convert into a line for drawing. The gdal library can be used to read shapefiles as well as a ton of other vector file formats. Might also be other simpler C++ libraries for shapefile reading. That should get you the points and now draw it as a line and hopefully much better results!

You might need to care about the coordinate reference systems of what your map is using vs the shapefiles but gdal can also help with that. Can both tell you if the shapefile has a CRS and allow you to convert the points from one CRS to another. 

--
Brad


> On Aug 8, 2019, at 9:55 AM, Israel Brewster <ijbrewster at alaska.edu> wrote:
> 
> I do have .shp files for the coastlines - perhaps I could convert those to something I could use? If I can get vector data, how would I go about utilizing it?
> 
> I played around a bit with loading different image files, but so far the performance has been unacceptable - the entire app locks up for several seconds when displaying the higher resolution images. I think Vector would be a better way to go, if I can figure out how. Thanks!
> ---
> Israel Brewster
> Software Engineer
> Alaska Volcano Observatory 
> Geophysical Institute - UAF 
> 2156 Koyukuk Drive 
> Fairbanks AK 99775-7320
> Work: 907-474-5172
> cell:  907-328-9145
> 
>> On Aug 7, 2019, at 10:12 PM, Brad Pepers <bpepers at me.com> wrote:
>> 
>> In the end the answer is to get the coast as vector data instead of a raster. The raster is going to be at a fixed map scale and any scale above or below is going to extrapolate or interpolate the results eventually ending in poor results because there just isn’t the information needed or too much information is being lost. Having different resolutions should help and you can keep multiple levels and switch between them at appropriate times (LOD). Those are your best bets if you can’t get the actual vector data!
>> 
>> --
>> Brad
>> 
>> 
>>> On Aug 7, 2019, at 12:01 PM, Israel Brewster <ijbrewster at alaska.edu> wrote:
>>> 
>>> I have a QGraphicsScene/QGraphicsView that I am using to display a map (and some data). The map is in the mercator projection, with the x and y coordinates of the scene corresponding to map coordinates, so the background images are rather large. Images are loaded into a QGraphicsObject which is then added to the scene. The object types are defined by a third-party library, so I don’t have the option of using different objects, such as QGraphicsPixmapObject or something.
>>> 
>>> This works fine when I’m zoomed in to the map. Coastlines are nice and sharp, with no issues:
>>> <Screen Shot 2019-08-07 at 9.50.10 AM.png>
>>> 
>>> However, when I zoom out, the coastline becomes quite pixelated, and difficult to see in places:
>>> <Screen Shot 2019-08-07 at 9.50.28 AM.png>
>>> 
>>> How can I fix this? My first thought was to change the size of the background image so it could scale better, but that didn’t seem to help. In retrospect, this makes sense: at the end of the day, the background image has to be the correct size for the mercator projection, so while I can certainly change the pixel density, the overall size, and thus how much scaling is needed to display a given area, is fixed.
>>> 
>>> ---
>>> Israel Brewster
>>> Software Engineer
>>> Alaska Volcano Observatory 
>>> Geophysical Institute - UAF 
>>> 2156 Koyukuk Drive 
>>> Fairbanks AK 99775-7320
>>> Work: 907-474-5172
>>> cell:  907-328-9145
>>> 
>>> _______________________________________________
>>> Interest mailing list
>>> Interest at qt-project.org
>>> https://lists.qt-project.org/listinfo/interest
> 
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