Bug report #1963

"Ghost Lines" when using Anti-aliasing, polygons with shared boundaries, and no outline/QT::NoPen

Added by springmeyer - about 14 years ago. Updated about 5 years ago.

Category:Map Canvas
Affected QGIS version:master Regression?:No
Operating System:All Easy fix?:No
Pull Request or Patch supplied:No Resolution:wontfix
Crashes QGIS or corrupts data:No Copied to github as #:12023


QT's anti-aliasing output exhibits an inherent limitation of AA, where polygon edges are drawn/feathered twice and the background color "bleeds" through what are supposed to be tightly shared edges of polygons.

I wonder if some of the new render hints in QT4.4 > might help work around this issue:


See also: http://trac.mapnik.org/ticket/428, where I am working on a workaround using AGG that may apply to the QT renderer, given an exposed API.

antialiasing_artifacts_with_nopen.png - Example of rendering glitch were seamless (no lines) are expected between polygons with shared boundaries (54 KB) springmeyer -, 2009-10-01 11:47 AM

testlinux.pdf (330 KB) Henrik Uggla, 2013-02-26 01:24 AM


#1 Updated by Paolo Cavallini almost 14 years ago

Is this a QGIS, or a Qt bug?

#2 Updated by springmeyer - almost 14 years ago

QT limitation which may benefit from workarounds in QGIS. I think this is an important ticket to have open for other users to see, or potentially as an FAQ only if we don't find an easy solution.

#3 Updated by Paolo Cavallini about 13 years ago

Is this still true for current Qt and QGIS versions? Please confirm.

#4 Updated by John Tull about 13 years ago

This problem still exists in trunk with the most recent Qt release.

#5 Updated by Tim Sutton over 12 years ago


I did a little test using both:


In src/gui/qgsmapcanvasmap.cpp for AntiAlias flags. Both choices did not resolve the issue for me. Using:

QGIS commit:e93c1841 (SVN r15735) Trunk
Qt 4.7

#6 Updated by springmeyer - over 12 years ago

We solved this sufficiently in Mapnik within the AGG renderer by allowing the user to control the AA gamma. A gamma of 0 is aliased while 1 is fully anti-aliased and setting gamma to around .6-.7 is able to remove the faint lines while keeping an AA-ish look with much more definition to edges (like coastlines) than can be achieved by the only other known workaround: overpainting with a thin line of the same color as the polygon fill. In short, we solved this by reducing the aggressiveness of the AA algorithm so that polygons are slightly dilated rather are fully AA or aliased.

So, I assume that QT will expose somewhere an equivalent gamma setting (or partial AA ability) - as QT's renderer is originally based on AGG (http://labs.qt.nokia.com/2009/12/18/qt-graphics-and-performance-the-raster-engine/)

#7 Updated by Tim Sutton over 12 years ago

  • Status changed from Open to Closed
  • Resolution set to wontfix

Reading the the referenced article it seems they only were inspired by AGG and didnt directly use any source code from AGG in their implementation. It seems like Qt4 rendering engine does not expose any agg-like gamma options and there isnt really any reasonable work around we can come up with for this. I am going to close this ticket since the only way to resolve this issue currently is to disable AA rendering it would seem and there isnt anything else we can do to fix it.

#8 Updated by springmeyer - over 12 years ago


#9 Updated by Henrik Uggla over 10 years ago

  • Assignee deleted (nobody -)
  • Crashes QGIS or corrupts data set to No
  • Affected QGIS version set to master
  • Target version changed from Version 1.7.0 to Version 2.0.0
  • Status changed from Closed to Reopened
  • File testlinux.pdf added

Turning off "Make lines appear less jagged..." removes the thin lines on the screen but they still show when printing (see attached pdf). I can find no option for disabling anti-aliasing in the composer. Applies to both Windows7 and ubuntugis.

Note regarding attached pdf: The thin lines are displayed differently in different pdf-viewers.

#10 Updated by Jürgen Fischer over 9 years ago

  • Target version changed from Version 2.0.0 to Future Release - Lower Priority

#11 Updated by Johannes Kroeger over 6 years ago

I realised that #7241 and #10166 were duplicates of this and closed them accordingly.

Has anything happened in the past years or maybe, hopefully, in QT5 that allows this to be fixed? It would highly improve cartographic quality in some use cases.

If not, a "use fill color for stroke" checkbox would be nice, if that indeed is a workaround for this (as suggested in the duplicates). It could be a useful addition for other use cases as well maybe?

#12 Updated by Giovanni Manghi over 6 years ago

  • Pull Request or Patch supplied set to No
  • Regression? set to No
  • Easy fix? set to No

#13 Updated by Johannes Kroeger about 5 years ago

A workaround to this is:

Set the Stroke width to Hairline
Set the Stroke color to @symbol_color using Data defined override

I forgot who shared this, either Nathan or Nyall. :)

#14 Updated by Giovanni Manghi about 5 years ago

  • Status changed from Reopened to Closed
  • Description updated (diff)

Johannes Kroeger wrote:

A workaround to this is:

Set the Stroke width to Hairline
Set the Stroke color to @symbol_color using Data defined override

I forgot who shared this, either Nathan or Nyall. :)

is this in the docs? Can't find it.

#15 Updated by Johannes Kroeger about 5 years ago

I don't think so, it was either on IRC or Twitter.

This issue remains, please keep it opened. The workaround is just that, a workaround!

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