tangram slope shaders

slope.yaml

import:
  - https://www.nextzen.org/carto/walkabout-style/8/walkabout-style.zip
  - https://www.nextzen.org/carto/walkabout-style/8/themes/label-10.zip
  - https://www.nextzen.org/carto/walkabout-style/8/themes/walkabout-road-shields-usa.zip
  - https://www.nextzen.org/carto/walkabout-style/8/themes/walkabout-road-shields-international.zip

styles:
    slope:
        base: lines
        raster: custom
        texcoords: true
        shaders:
            extensions: OES_standard_derivatives
            blocks:
                color: |
                    vec2 path = vec2(dFdy(v_texcoord.x), dFdx(v_texcoord.x));
                    path = normalize(path);
                    vec3 terrain = (sampleRaster(0).rgb - .5) * 2.;
                    float steepness = clamp(abs(dot(path, terrain.xy)) * 8., -1., 1.);
                    color.rgb = vec3(1., 1. - steepness, 1. - steepness);

layers:
    roads:
        draw:
            lines:
                style: slope