Cartodb: line-offset makes the tile borders visible

[AbelVM]

Context

Trying to style a layer using line-offset and the tiles borders become visible as a side effect

Steps to Reproduce

1. Create a map from a polygon dataset or from scratch with an empty layer, and then draw a polygon by hand
2. Style the layer with an extra border with Npx offset as

#layer {
  polygon-fill: #374c70;
  polygon-opacity: 0;
  line-width: 2;
  line-color:red;
  line-opacity: 1;
}
#layer::2 {
  polygon-fill: #374c70;
  polygon-opacity: 0;
  line-width: 2;
  line-color: blue;
  line-opacity: 1;
  line-offset:-3;
}

Current Result

red lines: actual polygon border
blue lines: polygon border with offset

• If offset is < 0: the tiles that intersect my polygon has visible borders inside the polygon
  

• If offset = 0: Behavior as expected
  

• if offset > 0: the tiles that intersect my polygon has visible borders outside the polygon
  

Expected result

Just no tiles borders :)

Browser and version

Any

.carto file

test_offset.carto.zip

Additional info

Please add any information of interest here below

[makella]

Sorry I never posted, but found a temporary workaround haven't tested lately, but you can see here

[rafatower]

It can be reproduced easily in local, with and without metatiling. I could not confirm it yet, but I'm pretty sure the bug is within mapnik, most likely around these lines:

https://github.com/mapnik/mapnik/pull/2955/files

(offset converter, line symbolizer)

Aside, as @makella pointed out, there's a workaround to it, that I reproduce:

#layer {

  // Use just one layer w/ 2 styles
  // this line-width below prevents the grid effect
  line-width: 0;

  ::top {
    polygon-fill: #374c70;
    polygon-opacity: 0;
    line-width: 2;
    line-color:red;
    line-opacity: 1;
  }
  ::2 {
    polygon-fill: #374c70;
    polygon-opacity: 0;
    line-width: 2;
    line-color: blue;
    line-opacity: 1;
    line-offset: -3;
  }
}

I created a PR reverting the suspect change: https://github.com/CartoDB/mapnik/pull/59

but did not have time to test it. Mind that this is just a hail mary pass in an attempt of quickly fixing it...

[rafatower]

Indeed my intuition was correct, reverting that change fixes the bug:

• mapnik v3.0.15-carto branch:

• mapnik revert_offset_converter_fix branch:

(the slight change in style is just to avoid caching between tests)

but it breaks other things (see the polygon vertex and the many visual tests broken).

Fixing this would require some c++ & mapnik expertise, finesse keeping the good parts and amending the bad ones, and some extra test for it.

[Algunenano]

I'm having a look into into this. It can be solved by disabling line-clip in the second layer:

#layer {
  polygon-fill: #374c70;
  polygon-opacity: 0;
  line-width: 2;
  line-color:red;
  line-opacity: 1;
}
#layer::2 {
  polygon-fill: #374c70;
  polygon-opacity: 0;
  line-width: 2;
  line-color: blue;
  line-opacity: 1;
  line-offset:-3;
  line-clip: false; //<<<<<<<<<<
}

[Algunenano]

I have it partially fixed (it was an issue in where the clipping was being done):

But I've noticed that the clipping has some issues (you can see some lines disappearing now) so I'm looking into this with a minimal test case to have it shipped together:

Left = no clipping, Right = with clipping

[Algunenano]

The issue is found in how the offsetted points are calculated. After some changes I see both improvements and deterioration in the visual tests, so this is really tricky to get it right:

I'll see if I can find a better compromise since the issue with the test from my previous message is that after clipping the points are too close (closer than the static threshold) so it decides to skip them. I'll look into making that threshold more dynamic (based on the properties of the CartoCSS) or increase the distance to the clipping grid.

[Algunenano]

Should be fixed with https://github.com/CartoDB/mapnik/pull/63

Since it requires review, performance regression tests and multiple intermediate releases it might take a while to get it into production. I'll let you know once it's ready.

[Algunenano]

Moving this out of RT. It'll be deployed with the next Mapnik release

[ramiroaznar]

Fixed in https://github.com/CartoDB/cartodb/pull/14124

[Jesus89]

@ramiroaznar I'm not sure that this is related to #14124.

I think it's related to mapnik (engine).

[Algunenano]

Yeah, it's not related to #14124. I'm keeping this open until the fix is released and installed in production.

[ramiroaznar]

Sorry! Yes, I see it is not the same problem.

[Algunenano]

Yeah, it's not related to #14124. I'm keeping this open until the fix is released and installed in production.

Well, it's only taken me ~13~ 15 months but it's now in production.

[makella]

woot!!

[ramiroaznar]

:100: