Change way_pixels calculation to use scale_denominator

[jeisenbe]

Changes proposed in this pull request:

• Way_pixels will now be calculated from way_area divided by the square of scale_denominator and the standard pixel size

Explanation:

• Currently, the value of the column way_pixels is calculated from the way_area (which represents the approximate area of a polygon in square meters, but by Mercator projection) by dividing by pixel_width * pixel_height.
• However, this causes the calculation to change when the resolution is changed, for example, when exporting at 2x resolution in Kosmtik, or when rendering "retina" resolution tiles.
• Using the scale_denominator instead of the actual pixel height/width results in a consistent rendering at different rendering resolutions.

Test rendering with links to the example places:

Southern Australia
9/-32.7988/137.0764
Screenshot at standard resolution - unchanged before and after PR
z8

z9

Export at 2x resolution
z8 2x Before

z8 2x After

[jeisenbe]

In #3648 we discussed the best way to show this code. @imagico had suggested basing it on the ppi (pixels per inch) of the standard resolution, which is approximately 90.71, hence:

way_area/NULLIF(POW(!scale_denominator!*0.001*25.4/90.71, 2),0)

I think the current code, way_area/NULLIF(POW(!scale_denominator!*0.001*0.28*, 2),0) is somewhat simpler. This is based on the 0.28mm standard pixel size that Mapnik assumes.

However, I'm happy to change this.

[imagico]

Looks good but i have not tested it so far.

@hholzgra and @woodpeck might be able to comment if this works well practically.

[imagico]

Tested and works fine, thanks.

Would still be good to get some feedback if this is useful in practical for-print rendering

[matthijsmelissen]

Good to have this finally fixed!

[jeisenbe]

Thanks! I use Kosmtik’s export function for printing, so it will help me, at least. I hope it will help with generating PDFs and SVG files for printing too. (BTW, does anyone know a good way to create PDFs for printing?)

…

On Sun, Feb 3, 2019 at 5:02 AM Matthijs Melissen ***@***.***> wrote: Good to have this finally fixed! — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3657 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AoxshOCuH-KOzB6hMIsVpOoNQGrB11Zsks5vJe7fgaJpZM4aUjko> .

[Adamant36]

Have you looked into MyOSMatic? It creates printable maps and I think it supports PDF. If so, maybe you could email the person that created for ideas.

[jeisenbe]

That’s true, I’ve used their service before, but I assume it runs on Unix? I think @imagico should suggest that they add his alt-colors style to the options at https://maposmatic.osm-baustelle.de/. That way we could make test renderings of any part of the world at mid and high zoom levels. It’s also a pretty nice looking style for printing.

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On Sun, Feb 3, 2019 at 1:04 PM Adamant36 ***@***.***> wrote: Have you looked into MyOSMatic <https://maposmatic.osm-baustelle.de/>? It creates printable maps and I think it supports PDF. If so, maybe you could email the person that created for ideas. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3657 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AoxshDvZMo-OdU85grzIZMkcNc6Ry8T3ks5vJl_hgaJpZM4aUjko> .

[sommerluk]

As it seems it's difficult to use way_pixels on vector tiles, I've played a bit with the MSS code. It is possible to define in MSS this variable @way_pixels: "([way_area]*pow(4,@zoom)/24505740000)"; and then use @way_pixels (instead of the SQL based way_pixels) later on. But it does not work well: While it works in Kosmtik for normal rendering, but yet for export is fails (completly, even when exporting at 100% scale). It seems that @zoom is not reliably available. scale_denominator isn't available in MSS, is it? I've also asked here.

[pnorman]

Way pixels has two uses - MSS filtering and SQL filtering. The SQL filtering is important for performance.