QGIS » QGIS Application

Dataset available at:
http://kosatka.fns.uniba.sk/~1balazovic/qgis/oceansea.tar.gz

I note that the polygon in question appears to cross the +/-180 degree line or perhaps is discontinuous in some way. This is clearly seen by doing the following:

• load the oceansea_af.shp file
• zoom out slightly
• use the Select Features tool to select the rightmost region
• note how the leftmost region with the missing shading is highlighted too, indicating they are the same feature

Note: the problem goes away if one zooms in enough or turns on anti-aliasing (Settings:Options:Rendering).

Hi,

I have the same problem (or a similar one), see
attached screenshot. This shows a SHAPE file
with white spreckle. The same happens on
another installation of a person I know (different
Linux version). I am using
qtcore-4.1.0-2 on Redhat Enterprise 4.

Markus

Hi again,

I darkly remember to have seen these problems in
a very early version of QGIS (0.5 or earlier) and
I think that I had submitted a bug report into
the sourceforge bugtracker in those days.

Is it possible that islands are causing the problem?

Markus

The white speckle is caused by holes (or islands - depends on yor point of view) in polygons in combination with the way that qgis draws such holes. This seems to have become worse in Qt4 over Qt3. IF anti-aliasing is turned on, polygons render correctly.

The problem with qgis not filling some polygons correctly happens even on polygons without holes in them.

"IF anti-aliasing is turned on, polygons render correctly." - how to do that? For example in KDE?

Markus

There is an anti-aliasing toggle in qgis, available in the Settings:Options:Rendering dialog box tab. Note that this makes drawing much slower.

The comments about white speckle in polygons may be caused by this Qt bug (http://www.trolltech.com/developer/task-tracker/index_html?method=entry&id=107816), which is scheduled to be fixed in Qt 4.2.0.

This is a bug in Qt, the status of which can be followed at http://www.trolltech.com/developer/task-tracker/index_html?method=entry&id=118762

No info yet on when this will be fixed, but since it's out of our control, I've removed the 0.8 milestone from this ticket.

Some images showing the problem are available in ticket #173.

In the latest preview release, this problem can NOT be solved by turning on anti-aliasing, at least with the data set I tested. My comments intially reported in ticket #173 still stand. See the attached badpolys_aa.gif.

My, it's even worse with anti-aliasing on:)

As far as I could tell, the only solution while Qt still has the bug is to move qgis back to drawing to a QPixmap - which will prevent anti-aliasing and transparency. It may well be a serious enough problem to do that, I think, especially when there isn't a user accessible workaround for all cases where it occurs.

SVN 6abc64b5 (SVN r5568) has a possible work-around to the problem. There is now a toggle in the options dialog box that lets the user swap between using a QImage or a QPixmap for rendering the map. QImage is the default one, and was the only option before this commit.

Note that using QPixmap disables the anti-aliasing because a QPixmap renderer doesn't support anti-aliasing.

On my system this fixes the problem with drawing the shapefiles at http://kosatka.fns.uniba.sk/~1balazovic/qgis/oceansea.tar.gz, and also removes the white speckle discussed above. Others have reported that it crashes their X server...

More examples of this problem are in ticket #293

Using Qt 4.2.0 seems to resolve the problem reported above where the option available since 6abc64b5 (SVN r5568) causes the X server. The original problem still exists. The tracker item now stats that a fix for this will be in Qt 4.3.0

Since this is a Qt issue and can't be resolved until Qt 4.3.0, I have moved this ticket to the 0.9 Milestone.

Hi, I am getting the bug same here, but only on Windows.
(0.8 works on the Mac, 0.8 untested on Linux)

MacOSX + 0.7.4 had a similar problem (mentioned in the OSX release notes) for polygons with more than 10,000 verticies.

MacOSX + 0.8pre2 -- it works. (!)

Debian/Sarge + 0.7.4 - it works.

Windows + 0.7.4 -- it works.
Windows + 0.8pre2, 0.8pre3_Tim_10Dec2006 -- I see the bug.
(I'll try to attach a screenshot, very close to Alaska img)

I will explore anti-aliasing and Qoptions and get back to you.

Vector file is the new zealand coastline, saved as a number of clean ed polygons (no islands), saved as a shapefile (exported from GRASS).

Hamish

Hi, tested Anti-aliasing and "Fix incorrectly filled polygon" options on Windows (0.8p3_Tim_10Dec2006 build)

Zooming way in fixed it

Turning on Anti-aliasing fixed it

Turning on "Fix incorrectly filled polygon" in options/rendering menu did not fix it (!!!)

I can make the shapefile available to individual devels for testing purposes, but not public. Email me if you want it (leave in the "_nospam").

This bug is important for us: I will not be able to deploy 0.8 on our PCs with this bug, I will have to keep them at 0.7.4. That's ok, as I'd rather the students use the Macs anyway ;)

Hamish

ps- user friendly but vague option descriptions are a total pain. Can the "technical version" of the option description appear in the tool tip? [anti-alias, Qpixmap|Qimage]

Still happens with the 0.8.0 release (binary from qgis.org).

I find if I use a lower resolution vector polygon (NOAA GSHHS coastline land shapefile) the filling works ok (turn of border) but the border doesn't. Using a higher resolution coastline both border and filling (turn border pattern to none) show the bug.

Playing with rendering options in the setup menu had no effect.

Hamish

This bug is supposedly fixed in Qt 4.3.0, and the situation is improved, but non-anti-aliased shapefiles still draw incorrectly :)

The 'fix problems with incorrectly filled polygons' works well for me now (Linux).

Reports from other OS's welcome...

Still broken on Windows, but seems to work elsewhere. Changing Platform to that.

Hamish

I have just installed qgis_setup0.9.1.18_12_2007.exe on a Windows XP machine and it is still broken WRT filling polygons with many thousands of vertices.

(same as last year's qgis-pre3cvs10dec2006_huge_polygon_err.png attached to this report)

The same shapefile + QGIS 0.7.4 on the same machine renders correctly. (but I blew that installation away to install 0.9.1)

Hamish

Hi,

I have just installed the new 0.10.0 for WinXP and it renders well!
i.e. the problem seen in the attached screenshot qgis-pre3cvs10dec2006_huge_polygon_err.png is gone.

Before closing the report it might be nice to have someone test with the alaska and world shapefile datasets, and take a stab at documenting what changed to fix this.

Hamish

Under WINE 1.0.0 configured to emulate Win XP, on amd64 Debian testing, I can't reproduce this error with Shapefile data using Marco's "QGIS 0.11.0 for Windows pre-release testing installer" and the QGIS Alaska sample dataset, in either QGIS renderinf mode.

Milestone Version 1.0.0 deleted