# Phase 0 — substrate for iter6

Opened **2026-05-19** after [iter5 close GREEN](phase8_iteration5_close.md).

## Locked research question — iter6

> **Render a depth-tested scene into a 128×128 RGBA8 color attachment + 128×128 D32_SFLOAT depth attachment via dynamic rendering:**
>
> - **Triangle A (red):** large, NDC (-0.8,-0.8), (0.8,-0.8), (0.0,0.8), all with z=0.7
> - **Triangle B (green):** small, NDC (-0.4,-0.4), (0.4,-0.4), (0.0,0.4), all with z=0.3 — fully geometrically inside Triangle A
> - Draw A first, then B. Depth test enabled (`VK_COMPARE_OP_LESS`).
> - Triangle B's lower z should make it appear in front of A wherever they overlap.
>
> **Verify specific pixels:**
> 1. `(0, 0)` — clear (outside both, top-left corner)
> 2. `(127, 127)` — clear (outside both, bottom-right corner)
> 3. `(64, 64)` — inside both, GREEN (B in front)
> 4. `(64, 30)` — inside A only (above B's reach in pixel-y), RED
> 5. `(64, 100)` — inside A only (below B's reach), RED

## Why this shape

iter5 closed all single-draw, no-depth, descriptor-binding paths. iter6 adds:

- **Depth/stencil attachment** (D32_SFLOAT) — new image format, new aspect, new usage flag (DEPTH_STENCIL_ATTACHMENT)
- **Depth test + depth write** in pipeline state (`VkPipelineDepthStencilStateCreateInfo`)
- **Multi-draw within one render pass** — two `vkCmdDraw` calls between begin/end
- **z-coordinate handling** in the vertex shader (gl_Position.z affects depth)
- **128×128 image** instead of 64×64 (more tiles — 8×8 grid of 16×16 = 64 tiles)
- **Depth attachment format** in `VkPipelineRenderingCreateInfoKHR.depthAttachmentFormat`
- **Depth attachment** in `VkRenderingInfoKHR.pDepthAttachment`

## Hypothesis space

1. **D32_SFLOAT depth format on Bifrost.** Bifrost packs depth into tiles; the layout differs from color. First time we use a non-color attachment.
2. **Depth-stencil image creation + layout (`DEPTH_STENCIL_ATTACHMENT_OPTIMAL`).** New layout never used.
3. **Depth test plumbing.** PanVk-Bifrost's path from `VkPipelineDepthStencilStateCreateInfo` → tile descriptor depth-state fields.
4. **Depth write back to depth attachment.** Mali stores depth in tile memory then flushes; per-tile flush + cache invalidation.
5. **Multi-draw within one render pass.** Bifrost's binner may have bugs handling N>1 jobs per render pass — particularly around per-draw state changes.
6. **z-coordinate from vertex shader.** Vertex output position.z passes through to rasterizer.
7. **Tile binning at 128×128** (64 tiles vs. iter3's 16). More potential for binner state bugs.

## In-scope (LOCKED 2026-05-19 for iter6)

- 128×128 RGBA8 color + 128×128 D32_SFLOAT depth attachment.
- 6 vertices (2 triangles) in one vertex buffer, vec3 pos + vec3 color.
- Depth test enabled, depth write enabled, compare LESS.
- Two `vkCmdDraw(3, 1, *, 0)` calls within one render pass.
- 5-point pixel-level verification.

## Out-of-scope (LOCKED 2026-05-19 for iter6)

- Stencil (D32_SFLOAT has no stencil aspect anyway).
- D24_UNORM_S8_UINT or other depth formats (iter would explore if iter6 fails).
- Depth clear via load-op only (no separate clear-image).
- Front-face culling, polygon-mode lines.
- Indexed draws.
- More than 2 triangles.
- WSI / surface — still off-screen attachment + buffer readback.

## Reference

- Prior closes: [iter1](phase8_iteration1_close.md) — [iter5](phase8_iteration5_close.md).