[Batch 1] Billboard/cross-model rendering for vegetation

[MichaelFisher1997]

Summary

Flowers, tall grass, saplings, bamboo, and dead bushes are currently rendered as solid cubes in the cutout render pass. They should use a cross/billboard model (2 diagonal quads) like Minecraft. This reduces vertex count by ~70% for vegetation blocks and dramatically improves visual quality.

Current Behavior

All 46 block types use the same cube meshing path in greedy_mesher.zig. The block registry has a render_pass field (.solid, .cutout, .fluid, .translucent) but no shape distinction. Flowers and grass get 6 cube faces with the texture on all sides — they look like cubes with a flower texture.

Target Behavior

Add a RenderShape enum to block definitions:

• .cube — standard 6-face cube (current behavior, default)
• .cross — 2 diagonal quads crossing at center (flowers, saplings, dead bush)
• .tall_cross — 2 tall diagonal quads, height = 2 blocks (tall grass, bamboo may use this or cross)

Affected blocks:

Block	Shape	Notes
flower_red	cross
flower_yellow	cross
tall_grass	cross	Slightly shorter quad
dead_bush	cross
acacia_sapling	cross
bamboo	cross	Thin variant
torch	cross	Single thin quad pair, emissive

Implementation Plan

Step 1: Add RenderShape to block registry

In src/world/block_registry.zig:

• Add render_shape field to BlockDefinition struct
• Populate for all blocks (.cube default, .cross for vegetation)
• Expose helper: getBlockDefinition(block).render_shape

Step 2: Cross-pattern vertex generation

In src/world/chunk_mesh.zig or meshing/greedy_mesher.zig:

• When encountering a .cross block, skip the normal cube face generation
• Instead emit 2 quads forming an X shape:

  Quad 1: (-0.15, 0, -0.15) → (0.15, 1, 0.15) diagonal
  Quad 2: (-0.15, 0,  0.15) → (0.15, 1, -0.15) diagonal
  

• Both quads have double-sided faces (4 triangles total, 12 vertices with front+back)
• UV mapping: full texture on both quads
• AO: can use simple vertex AO or disable for cross shapes
• Biome tint applied via vertex color

Step 3: Greedy mesher skip

• greedy_mesher.zig should NOT merge cross blocks with adjacent cross blocks
• Each cross block is meshed independently (4 triangles is already minimal)
• Cross blocks still go through the cutout pass for alpha testing

Step 4: Torch special case

• Torch could use a single thin quad or a 3-cross pattern
• Must preserve emissive block light (already handled by block registry)

Files to Modify

• src/world/block_registry.zig — add RenderShape enum, populate per block
• src/world/chunk_mesh.zig — branch on render_shape during mesh building
• src/world/meshing/greedy_mesher.zig — skip greedy merge for cross blocks

Testing

• Cross blocks render as X-shaped billboards
• Cube blocks unaffected (no regression)
• Biome tinting still applied (grass, leaves)
• Torch renders and emits light
• Cutout alpha testing works on cross quads
• No z-fighting between cross quads
• Block outline still highlights cross blocks correctly

Impact

~70% fewer vertices for vegetation blocks. Visual quality improvement — the #1 visual sore thumb in the current engine.

Roadmap: docs/PERFORMANCE_ROADMAP.md — Batch 1, Issue 1C

[opencode-agent]

Based on my analysis of the codebase, I can provide a comprehensive assessment of this feature request.

Classification: Feature Request

Analysis:

This is a well-structured feature request with a clear implementation path. After reviewing the relevant code, I can confirm:

1. Current architecture supports this well: The BlockDefinition struct in block_registry.zig:31-80 already has the render_pass field, and adding a render_shape field would be straightforward.

2. Affected blocks (currently using .cutout pass):

   • .flower_red, .flower_yellow, .tall_grass, .dead_bush, .acacia_sapling, .bamboo, .torch (lines 303 in block_registry.zig)

3. Implementation considerations:

   In block_registry.zig: Add the RenderShape enum and field to BlockDefinition. The comptime registry population pattern (lines 83-329) makes it easy to set .cross for specific blocks.

   In chunk_mesh.zig: The current architecture builds meshes through buildSubchunk() which calls greedy_mesher.meshSlice(). You'll need to add a secondary path that iterates individual cross blocks and emits the X-shaped quads.

   In greedy_mesher.zig: The greedy algorithm (lines 46-164) builds 16x16 face masks. Cross blocks should be excluded from this process entirely. The simplest approach is to filter them out in the mask-building phase (around line 72-94).

4. Vertex format compatibility: The Vertex struct in rhi_types.zig:93-102 already supports everything needed—position, color, normal, UV, lighting, and AO.

5. No missing information - The issue is complete with implementation steps, affected blocks, and testing criteria.

Recommendation: This is a focused, achievable feature. The scope is appropriate for a single PR. The ~70% vertex reduction claim is accurate (cross = 4 triangles vs cube = 12 triangles visible on average).

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