Fix: Unity URP Render Feature Blit at Wrong Pass Event

Custom Renderer Feature blits a fullscreen shader. Output is black or one frame stale. Wrong RenderPassEvent: the source buffer hasn’t been written yet at that point in the frame.

The Symptom

Renderer Feature compiles, runs, but result is black, garbage, or last-frame content. Same shader applied via Volume effect produces correct output.

The Fix

public class MyEffectFeature : ScriptableRendererFeature
{
    [Serializable]
    public class Settings
    {
        public RenderPassEvent renderPassEvent = RenderPassEvent.AfterRenderingTransparents;
        public Material material;
    }
    public Settings settings;

    private MyEffectPass _pass;

    public override void Create()
    {
        _pass = new MyEffectPass(settings.material) { renderPassEvent = settings.renderPassEvent };
    }

    public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData rd)
    {
        renderer.EnqueuePass(_pass);
    }
}

Inside the pass, get the camera color and blit:

public override void Execute(ScriptableRenderContext ctx, ref RenderingData rd)
{
    var cmd = CommandBufferPool.Get("MyEffect");
    var source = rd.cameraData.renderer.cameraColorTargetHandle;
    cmd.SetRenderTarget(_tempRT);
    cmd.Blit(source, _tempRT, _material);
    cmd.Blit(_tempRT, source);
    ctx.ExecuteCommandBuffer(cmd);
    CommandBufferPool.Release(cmd);
}

Pick the Right Event

• BeforeRenderingOpaques: too early; color empty.
• AfterRenderingOpaques: opaque content only.
• AfterRenderingTransparents: full visible color, pre-postFX.
• BeforeRenderingPostProcessing: pre-tonemap, useful for HDR distortions.
• AfterRenderingPostProcessing: post-tonemap, final overlay.
• AfterRendering: completely final.

Verifying

Frame Debugger → step through. Your custom pass should appear at the expected event. Output should match the camera color at that moment, then be transformed by your shader.

Understanding the issue

Render pipelines have ordering: which pass runs when, what state is bound, which targets are written. Bugs at this layer are often invisible in code review and only manifest at runtime.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Right event for the source. RTHandle for the buffer. Effect renders.”

Related Issues

For Volume not applying, see Volume. For SRP Batcher, see SRP Batcher.

Pick the event. Use the handle. Effect appears.