Opened 11 years ago

Closed 6 years ago

#745 closed defect (wontfix)

Layer transitions on highways (and other ways?) cause visible "seams"

Reported by: dermotm@… Owned by: osm@…
Priority: minor Milestone:
Component: osmarender Version:
Keywords: Layer Seam Bridge Cc:


The behaviour is particularly noticeable on bridges. The seams were not present on earlier versions of osmarender. Although a minor bug, this behaviour is already prompting at least one mapper to adopt workarounds involving non-standard use of the layer attribute, with annoying results.

Change History (6)

comment:1 Changed 9 years ago by Vid the Kid

I think I know how to fix this, though I'm not comfortable writing a patch myself. I was looking at the code for the smart-linecaps feature, and I noticed that, when a connecting path on a lower level is detected, the linecaps for the end-segment are set to "butt". Since this occurs on both the casing and fill, and because of how antialiasation works in nearly all SVG renderers, this causes some of the casing to show through in the "crack" between fill segments. There are two solutions:

1) When a connecting path on a lower layer is detected, set the casing linecaps to "butt" but set the highway fill linecaps to "round".


2) Render map tiles without antialiasing. Seam-free antialiasing can be achieved by rendering (without SVG antialiasing) at 4x final size, then resampling resulting raster image down to final size. (This will result in improvements to appearance for any situation where two edges are incident or nearly incident, such as thin casings or adjacent landuse areas.)

comment:2 Changed 9 years ago by Vid the Kid

Oh, I just thought of a simplified version of solution 1:

On the "middle" portion of a path that uses smart-linecaps, set the linecaps to "round" instead of "butt". Assuming the middle casing doesn't render over the end-segment fills, that should work great.

comment:3 Changed 9 years ago by Jochen Topf

Owner: changed from Jochen Topf to osm@…

comment:4 Changed 9 years ago by spaetz

Can you point to an area where that happens? I am still not clear what the exact symptom is.

comment:5 in reply to:  4 Changed 9 years ago by mackerski

Replying to spaetz:

Can you point to an area where that happens? I am still not clear what the exact symptom is.

Look at either end of the "Bowstring Bridge" You'll see a faint line drawn across the roadway.

comment:6 Changed 6 years ago by iandees

Resolution: wontfix
Status: newclosed

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