fix(core): keep the cartesian z scale continuous at zoom 12

[mfrnd]

For #7064. Follow-up to #10790, from ibgreen's note.

Background

Since #10790, 'cartesian' positions below zoom 12 use the viewport center as geospatial origin, so their second-order z correction is nonzero away from the equator. In WEB_MERCATOR mode, project_offset_ multiplies it by the absolute common y instead of the offset from the center: below zoom 12, 1,000 m at the viewport center is drawn as 1,000.40 m at latitude 80 and 999.95 m at latitude -80, from zoom 12 as 1,000.00 m.

Change List

• calculateViewportUniforms: cartesian z correction in WEB_MERCATOR_AUTO_OFFSET mode only
  • Omitted below zoom 12 rather than made center-relative: unitsPerMeter2 is per meter, but cartesian y offsets are in common units, so even from zoom 12 it changes z by less than a millionth across the view
• Unit tests: projected z on the GPU at zoom 11.99 and 12, latitudes 80 to -80

Ran lint, build, the core shaderlib and viewport, extensions and TerrainLayer headless tests, and the terrain render tests on WebGL.

---

Note

Medium Risk
Touches core projection uniforms used by all cartesian/preprojected layers; behavior change below zoom 12 is intentional but could affect subtle elevation rendering.

Overview
Fixes a discontinuity at zoom 12 for COORDINATE_SYSTEM.CARTESIAN geometry (e.g. terrain meshes) where z is in meters but x/y are already in common space.

In calculateViewportUniforms, the cartesian branch no longer always sets commonUnitsPerWorldUnit2 from unitsPerMeter2. The second-order z correction is applied only when projectionMode is WEB_MERCATOR_AUTO_OFFSET. In plain WEB_MERCATOR (zoom < 12), it is zeroed so the shader does not scale that term by absolute common y—which had caused latitude-dependent elevation error and a step when the viewport switches modes at zoom 12.

Adds a GPU regression test that projects a 1000 m z at the viewport center across zoom 11.99 vs 12 and latitudes 80° to −80°, asserting recovered elevation stays ~1000 m.

^{Reviewed by Cursor Bugbot for commit e2be793. Bugbot is set up for automated code reviews on this repo. Configure here.}

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge.

Summary

The PR removes the incorrect cartesian z correction below zoom 12 while keeping the existing correction above it.

• Cartesian heights stay continuous as zoom crosses 12.

_{Reviews (1) · Last reviewed commit: "fix(core): keep the cartesian z scale co..." · Reviewed by Greptile}

[coveralls]

coverage: 83.704% (+0.001%) from 83.703% — mfrnd:fix/7064-cartesian-z-zoom-12-step into visgl:master