Drape vector polygons onto terrain via GPU lookup

[danielzhong]

Description

Follow-up to #13577, which added GPU-lookup draping for clamped polylines on
terrain. This PR extends the same architecture to polygon fills and
points

Issue number and link

Testing plan

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

AI acknowledgment

• I used AI to generate content in this PR
• If yes, I have reviewed the AI-generated content before submitting

If yes, I used the following Tools(s) and/or Service(s):

GithubCopilot
If yes, I used the following Model(s):
Claude Fable 5

PR Dependency Tree

• PR Drape vector polygons onto terrain via GPU lookup #13627 👈
  • PR Clipping polygon buffer migration #13623
    • PR Vector Textures for Polygon Clipping #13635
      • PR Clipping texture plumbing #13636
        • PR Shader rework for terrain polygon clipping #13637
          • PR Remove SDF polygon clipping from Globe #13638

This tree was auto-generated by Charcoal

[github-actions]

Thank you for the pull request, @danielzhong!

✅ We can confirm we have a CLA on file for you.

[mzschwartz5]

Something I didn't think about on the first PR - have we determined that we'll never exceed the max texture size for the grid cell indices? It looks like, right now, it's expected to be a single row.

Seems reasonable to me that a given tile wouldn't need >4k grid cells, but just checking

[danielzhong]

Yep, fixed! The header texture now grows vertically too instead of being a single row, I did the same for GS few months ago

[mzschwartz5]

I know it's alluded to above, but I think it would be helpful to leave a short comment that the texture coords are offset by 1, to account for the grid width + height headers. (And, importantly: offset by 1 and not by 2, because the indexStart for cell 0 is implicit, not stored explicitly).

[mzschwartz5]

I find it a little difficult to digest this loop; the nested conditionals and the last-index-special-case are making already complex code more complicated. I think by moving the special-last-index case out of the loop and using some helper functions, this could be more readable (this is just a suggestion, and it could probably be improved even more):

// Composite a polygon's fill over baseColor when this pixel is inside it. A
// negative index (empty cell or first iteration) or an outside pixel is a no-op.
vec4 vectorCompositePolygonFill(vec4 baseColor, int primitiveIndex, bool inside, ivec2 primitiveTextureSize)
{
    if (!inside || primitiveIndex < 0)
    {
        return baseColor;
    }

    ivec2 primitiveUv = vectorIndexToUv(primitiveIndex, primitiveTextureSize);
    vec4 fillColor = texelFetch(u_vectorColorTexture, primitiveUv, 0);
    return fillColor * vec4(fillColor.aaa, 1.0) + baseColor * (1.0 - fillColor.a);
}

// True if a horizontal +x ray from p crosses the edge. The half-open interval
// (> vs <=) counts a ray through a shared vertex exactly once.
bool vectorEdgeCrossesRay(vec4 edge, vec2 p)
{
    if ((edge.y > p.y) == (edge.w > p.y))
    {
        return false;
    }

    float t = (p.y - edge.y) / (edge.w - edge.y);
    float xIntersect = edge.x + t * (edge.z - edge.x);
    return p.x < xIntersect;
}

vec4 vectorPolygonRender(vec2 vectorUv, vec4 baseColor)
{

   ...

    for (int i = indexStart; i < indexEnd; i++)
    {
        ivec2 edgeUv = vectorIndexToUv(i, edgeTextureSize);
        vec4 edge = texelFetch(u_vectorPolygonEdgeTexture, edgeUv, 0);
        int primitiveIndex = int(texelFetch(u_vectorPolygonEdgePrimitiveIndicesTexture, edgeUv, 0).r);

        // A new primitive means the previous group is complete: composite it,
        // then start counting the new one fresh.
        if (primitiveIndex != currentPrimitive)
        {
            baseColor = vectorCompositePolygonFill(baseColor, currentPrimitive, inside, primitiveTextureSize);
            currentPrimitive = primitiveIndex;
            inside = false;
        }

        if (vectorEdgeCrossesRay(edge, vectorUv))
        {
            inside = !inside;
        }
    }

    // The last primitive group has no trailing edge to trigger its composite.
    baseColor = vectorCompositePolygonFill(baseColor, currentPrimitive, inside, primitiveTextureSize);

[mzschwartz5]

I don't think we need to pass width, since we already have rectangle. Could just call rectangle.width getter directly.