sdf metadata

[ansis]

It would be nice to include metadata about how the sdfs are rendered.

• font size used in the sdf (currently 24)
• distance units per pixel (currently 256 / 8 = 32)
• shift (currently -2 * 32 = -64)
• sdf granularity

Not urgent, but it would be nice to not hardcode these in the renderers.

Explanation of how this metadata is used at mapbox/mapbox-gl-style-spec#66 (comment)

[mikemorris]

👍

Should buffer size (currently hardcoded to 3) be configurable too?

[yhahn]

Hrm, let's not worry about this until we have a reason to change these things. Picking good defaults and not changing them beats making anything possible and giving the impression that they will be handled by implementers.

[mikemorris]

In testing SDF granularity, it appears that our current defaults, while sensible for most serif and sans-serif fonts, are quite inadequate for curvy, swooping fonts.

[mikemorris]

@ansis Can you link to the lines in the code where distance units per pixel and shift are implemented?

[ansis]

They aren't separate variables yet, but the values are part of the equations for u_buffer and u_gamma here https://github.com/mapbox/mapbox-gl-js/blob/1e8bb1120da09db7eaa804581204acfa36fe4d97/js/render/drawtext.js#L93-L106

@lbud is currently working on updating this part for v4 styles mapbox/mapbox-gl-js#514

[mikemorris]

https://github.com/mapbox/mapbox-gl-js/blob/891cb690d8e8f3d770cbfd2fe14ba4aef3f1a1fa/js/render/drawsymbol.js#L109-L132

[mattdesl]

👍

Even if things like buffer aren't (yet) adjustable it would be great to export meta data so there is less magic going on in tools that use fontnik. For example:
https://github.com/Jam3/sdf-bitmap-glyphs/blob/master/index.js#L6

A few other metrics like lineHeight, baseline, etc would also be great to add to the load() data. Not sure how MapboxGL is currently handling those?

[mikemorris]

offset and radius, more hardcoded defaults that should be exposed/adjustable https://github.com/mapbox/node-fontnik/blob/master/src/glyphs.cpp#L614-L615