artifacts drawing un-textured semi-transparent spherical regions

[alex-w]

Original report by Emmanuel: https://bugs.launchpad.net/stellarium/+bug/1174892

This bug report is about artifacts that appear when drawing un-textured semi-transparent polygons using the stellarium C++ API.

Artifact 1:
Drawing a semi-transparent polygon near the horizon with atmospheric refraction on.
=> The color of the drawn polygon is not uniform, the polygon seems to have areas drawn twice and hence more opaque. These areas have the shape of elongated triangles that touch the horizon.

Artifact 2:
Drawing a large semi-transparent polygon.
=> The color of the drawn polygon is not uniform, the polygon seems to have areas drawn twice and hence more opaque. These areas have the shape of lines or elongated triangles located near the boundary of the polygon.

Artifact 3:
Drawing a large semi-transparent polygon and looking opposite to it.
=> The polygon fills the entire screen when looking in the opposite direction to where it is located and when part of its boundary is visible in the viewport. The part of the polygon that is actually visible is drawn twice, and the complementary of the polygon that should not be visible is drawn once.

These artifacts are illustred in the attached picture and can be obtained with the following code.
Artifact 1:

renderer->setGlobalColor(0, 1, 0, 0.5);
StelProjectorP prj = core->getProjection(StelCore::FrameAltAz);
SphericalCap(Vec3d(1, 0, 0), 0.99).drawFill(renderer, SphericalRegion::DrawParams(prj.data()));

Artifacts 2 and 3:

renderer->setGlobalColor(0, 1, 0, 0.5);
StelProjectorP prj = core->getProjection(StelCore::FrameAltAz);
SphericalCap(Vec3d(1, 0, 0), 0.5).drawFill(renderer, SphericalRegion::DrawParams(prj.data()));

[alex-w]