debugging-rendering-issues

Metal Rendering Diagnosis

When This Skill Helps

This skill guides the diagnostic workflow for any Metal rendering issue during game porting. For specific symptoms where you're confident in the cause, you can diagnose directly — but still recommend the using-gpudebug skill to confirm the diagnosis in a GPU trace before the developer changes code. For ambiguous or underspecified problems ("something looks off", "rendering is broken", multiple overlapping symptoms), use the full workflow below to turn vague reports into specific, actionable diagnoses efficiently.

Gather Context

The most important step for ambiguous problems. Ask the developer:

1. What do you see? (blank, partial, or wrong-looking?)
2. What do you expect? (screenshot from source platform or description)
3. What is the source API? (D3D11, D3D12, Vulkan, OpenGL, or MoltenVK)
4. Do you have a GPU trace (.gputrace)?

If no GPU trace is available, recommend capturing one — use the using-gpucapture skill. If a GPU trace is available, use the using-gpudebug skill to inspect render state, bindings, and resource contents.

Diagnostic Strategy

1. Gather context to resolve ambiguity
2. Match symptom using the router below — diagnose directly from this file
3. Tiered elimination — only if the symptom is still ambiguous or the router's candidate was ruled out, load a reference file (see Fallback section at end)

After finding the cause: If a GPU trace is available, use the using-gpudebug skill to confirm the diagnosis before reading or changing code. Then search the developer's codebase for the relevant code and suggest or implement the specific fix.

Symptom Router

Match the symptom to the most likely cause. The gotchas column contains non-obvious traps — check these even when you're confident in the diagnosis.

Symptom	Cause	Gotchas
Clear color visible, no geometry	Viewport zero-size, culling/winding, or depth state	Reversed depth: clear value + compare function must change together
Partial corruption each frame	Store action .dontCare	With MSAA, resolve texture + resolve store action both required
Flickering correct/corrupt	Load action .dontCare (TBDR), store action, buffering race, or Metal 4 barrier missing MTL4VisibilityOptionDevice	Worse at higher frame rates = buffering race, not static misconfiguration
Colors washed out / low contrast	sRGB pixel format missing	GL_FRAMEBUFFER_SRGB toggle → Metal pixel format. newTextureViewWithPixelFormat: for dual-access
Colors oversaturated / too dark	Double sRGB conversion or no gamma at all	Both _sRGB format AND manual pow(color, 1/2.2) = double-encode
Background color wrong shade	sRGB clear color	sRGB render targets interpret clear color in linear space
Wrong or garbled textures	Pixel format translation	BGRA vs RGBA channel order; MTLTextureSwizzleChannels
Alpha blending wrong / dark halos	Premultiplied vs straight alpha	CoreAnimation expects premultiplied alpha on the drawable. D3D blend enum integers differ from Metal — don't cast directly
Z-fighting everywhere	Reversed depth not fully ported	Clear value + compare function must change together. Depth bias units differ across APIs and between D24/D32F formats — rescale needed
Objects wrong depth order	Depth clear + compare mismatch
Objects appear/disappear	Buffering race on dynamic buffers
Ray tracing all-miss (sky only)	Resource residency missing	Binding ≠ residency. Metal 4: no useResource/useHeap — use MTLResidencySet on command queue or command buffer. After TLAS rebuild, residency set must be updated and re-committed
Wrong texture on surface	Resource binding index mismatch	Metal 4: MTL4ArgumentTable replaces per-encoder binds. maxTextureBindCount defaults may be too low
Geometry upside down / mirrored	Y-axis flip or winding	Negate Y in projection OR flip viewport — do ONE, not both. Viewport flip also requires flipping winding
Scene too bright / highlights blown	HDR headroom / EDR	Apple EDR: SDR 0.0–1.0, HDR extends above. Different from D3D12/Vulkan swapchain models
Colors clamped / banding	Shader output range	Metal clamps Unorm writes to [0,1]. 8-bit where source used 16-bit float = precision loss
Geometry correct shape but black	Texture sampling or shader output	Check texture bound and populated, mipmaps generated, then shader [[position]] and fragment return

Metal 4

Binding — MTL4ArgumentTable with fixed slot limits (31 buffers, 128 textures, 16 samplers). Set on encoder via setArgumentTable:. initializeBindings defaults NO — unset slots are garbage, not zero.

Residency — All GPU-accessed resources must be in a committed MTLResidencySet (on queue or command buffer). Binding does NOT grant residency — non-resident reads return zeros silently. beginCommandBufferWithAllocator: clears prior useResidencySet: calls — must re-associate every frame. After TLAS rebuild, residency set must be updated.

Barriers — MTL4VisibilityOptionNone = execution order only, no cache flush. D3D12 transitions implicitly flush. Use MTL4VisibilityOptionDevice for data dependencies.

Other — D24_UNORM_S8_UINT not universally supported on Apple GPUs; default to Depth32Float_Stencil8. D3D COLOR_WRITE_ENABLE / Vulkan VkColorComponentFlags to MTLColorWriteMask — bit layout reversed, naive cast drops channels. Metal has no subpasses.

Fallback: Tiered Elimination

Only when the symptom router above doesn't resolve the issue. Load one tier reference file at a time.

• Tier 1 — 7 instant checks: references/tier-1-instant-checks.md
• Tier 2 — 14 state checks: references/tier-2-state-inspection.md
• Tier 3 — 7 data checks: references/tier-3-data-inspection.md

If all tiers are exhausted, review shader source and verify transform math.