How exactly do height (land) and depth (sea) contours work?

Looking at the tools they are just smooth polys with white fills for the land (which lightens the underlying) and dark fills for the sea (which darkens the underlying). They are set to Alpha Transparency, which I'm pretty certain is responsible for the lightening/darkening. Is this the case? If so, what exactly is happening at the pixel level? I've read a bit about alpha channels, but I'm still not grasping it.

Thanks in advance!

Best Answer

  • MonsenMonsen 🖼️ 43 images Mapmaker Administrator
    Accepted Answer

    These tools use a raster image for their fill, which is just a 1-pixel image with either a black and white pixel. This image is set up with built-in transparency. You'll find these fills in the fills list as Solid 10, Solid 20, ... Solid 90 and Solid White 10, ... Solid White 90, where the number indicates the opacity (10 is almost fully transparent, 90 is almost fully opaque).

    When overlaid other elements in the map, the pixels below are mixed with that fill according to the transparency level of the fill, thereby serving to lighten or darken whatever they overlay.



  • Thanks, Monsen. So it's kind of like an effect without needing to turn on effects?

  • MonsenMonsen 🖼️ 43 images Mapmaker Administrator

    Kind of. You could do the same with a solid color and the transparency effect.

    Transparency in images is a common thing though, and is used in a lot of the CC3+ symbols. The advantage to have it baked into the image instead of using an effect is that you can control the transparency for each pixel of the image, while the transparency effect applies equal to the entire sheet.

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