Hillshade WMS rendering no data and black values discrimination

[micpilon]

Hillshade layer rendered with the default built-in style. All cells of value 0 gets rendered as transparent pixel in the resulting png file.
The desired output would be a rendering on which we could preserve black value and still get transparent background where cell value is nodata.

It seems that this bug is associated with the way renderPng assigns transparency to some bits on the 8-bit value range, including 0 (black).
See: https://github.com/locationtech/geotrellis/blob/master/raster/src/main/scala/geotrellis/raster/render/PngRenderMethods.scala#L56
https://github.com/geotrellis/geotrellis-server/blob/develop/ogc/src/main/scala/geotrellis/server/ogc/style/ColorRampStyle.scala

[pomadchin]

I hope that this PR locationtech/geotrellis#3278 would cover this issue; I'll check it ASAP.

[pomadchin]

It looks like it can be a configuration issue with the color ramp application. Try the clamp-with-color = true setting in the layer.styles definitions!

[jfbourgon]

Indeed, it seems it was a configuration issue with the colorrampconf style.

Here are conf snippet and related output that confirm this.

No Clamp

{
                name = "greyscale-noclamp"
                title = "Greyscale Hillshade / Relief ombré en tons de gris (no clamp)"
                type = "colorrampconf"
                colors = ${color-ramps.greyscale}
                stops = 255
                legends = [
                    {
                        format = "image/png"
                        width = 36
                        height = 36
                        online-resource = {
                            type = "simple"
                            href = "https://datacube.services.geo.ca/canada-icon.png"
                        }
                    }
                ]
             }

Black values are rendered as transparent pixels and we can see the world imagery underneath.

Clamp

{
                name = "greyscale-clamp"
                title = "Greyscale Hillshade / Relief ombré en tons de gris (clamp)"
                type = "colorrampconf"
                colors = ${color-ramps.greyscale}
                stops = 255
                clamp-with-color = true
                legends = [
                    {
                        format = "image/png"
                        width = 36
                        height = 36
                        online-resource = {
                            type = "simple"
                            href = "https://datacube.services.geo.ca/canada-icon.png"
                        }
                    }
                ]
             }

Black values are rendered as opaque color as expected.

Thanks for letting us know about the clamp-with-color style property.