"Patches"" of color rather than shades.

[greybeard1]

When there is little light (dawn, dusk, rain, etc.), I'm getting landscapes (especially sky, clouds and sea) divided in areas of slightly different colour (or light), each with its own "border", instead of a continuous gradient.
In the past, I saw this defect when trying to render a higher colour depth image with a lower colour depth display.
Here follows related section of my conf.ini file; I would ask an help to solve this issue.

[Render_OpenGL]
TexQual=3
TexMipFilter=1
TexCompress=2
TexFlags.UseDither=0
TexFlags.UseAlpha=0
TexFlags.UseIndex=0
TexFlags.PolygonStipple=0
TexFlags.UseClampedSprites=0
TexFlags.DrawLandByTriangles=1
TexFlags.UseVertexArrays=1
TexFlags.DisableAPIExtensions=0
TexFlags.ARBMultitextureExt=1
TexFlags.TexEnvCombineExt=1
TexFlags.SecondaryColorExt=1
TexFlags.VertexArrayExt=1
TexFlags.ClipHintExt=1
TexFlags.UsePaletteExt=1
TexFlags.TexAnisotropicExt=0
TexFlags.TexCompressARBExt=1

TexFlags.TexEnvCombine4NV=1
TexFlags.TexEnvCombineDot3=1
TexFlags.DepthClampNV=1
TexFlags.SeparateSpecular=1
TexFlags.TextureShaderNV=1

HardwareShaders=1

Shadows=2
Specular=2
SpecularLight=2
DiffuseLight=2
DynamicalLights=1
MeshDetail=2
VisibilityDistance=3

Sky=1
Forest=2
LandShading=3
LandDetails=2

LandGeom=2
TexLarge=1
TexLandQual=3
TexLandLarge=1

VideoSetupId=17
Water=4
Effects=1
ForceShaders1x=0

PolygonOffsetFactor=-0.15
PolygonOffsetUnits=-3.0