Decrease polygon-gamma

[matthijsmelissen]

Decrease polygon-gamma and polygon-pattern-gamma to 0.3.
This resolves #672.

[imagico]

I am somewhat confused - on #672 you wrote:

The artefacts are still visible (but less noticeable) with polygon-gamma 0.2

As said before decreasing the gamma does not really fix the underlying problem, it just hides it. The low gamma has a visible effect emphasizing the colored area relative to the background. This creates an effective bias on pixels partially covered by a single polygon. You can see this in the following comparison (gamma 0.6 vs. gamma 0.3 on the large polygons)

for reference here with gamma 1 for all polygons (which by design of the rendering engine is biased against the colored area for all pixels partially covered by more than one polygon):

I am not saying this should not be done but it should be kept in mind this is a workaround for a design issue in the renderer which has negative side effects and is not an actual solution.

[gravitystorm]

I'm with @imagico on this one - we also need a proper solution for this. Simply reducing the gamma all the time is just papering over the cracks.

[pnorman]

To actually fix this isn't it an upstream Mapnik issue? I don't see it as a data issue as splitting one polygon into two polygons that share a common edge is valid in OSM, and in fact in most datasets I have seen.

@springmeyer, any thoughts?

[dieterdreist]

2014-11-17 6:40 GMT+01:00 Paul Norman [email protected]:

I don't see it as a data issue as splitting one polygon into two polygons
that share a common edge is valid in OSM, and in fact in most datasets I
have seen.

it depends on the tagging. Splitting "one thing" into arbitrary "parts of
that thing" doesn't sound like a good idea (or would at least require to
adjust tagging accordingly), but if the only tags are attributes (e.g. this
area has a surface made of x, this area is used for y (where y isn't an
entity but e.g. an activity), here grows foo, ...) you can split them
without problems.

One might still argue that showing the joint isn't a bug but a feature, but
more likely it should be visible only in cases where the borders are shown
on purpose (i.e. by defining an outline style).