Bloom

Treat bloom as the broad tail of an imaging point-spread function. Its closest built-in optical input is all scene-linear HDR radiance. Selective contribution is an art-direction branch, not an automatic quality upgrade.

1. Validate the source scene

Capture scene-linear HDR, false-color pre-tone luminance, and the final image with bloom disabled. Repair the source when highlights were clamped or when silhouette, material, or lighting identity disappears without bloom.

This step is complete when the bloom-off scene remains readable and the input is unclamped scene-linear HDR in a declared working basis.

2. Choose the signal

Stabilize exposure before comparing branches. Inspect both false-positive and false-negative bloom energy, including bright mirrors and transmissive surfaces.

This step is complete when one source branch is named, its omissions are accepted, and its contribution error and target-device cost meet declared limits.

3. Build one scene pass

Use one HDR scene traversal for both full-scene and selective bloom. Full-scene bloom reads the scene output. Selective bloom adds an emissive MRT output to the same pass and gives it BlendMode(MaterialBlending) so transparent contributions follow the material blend state.

In r185, material-level mrtNode merging can lose the operative blend-mode map. Keep the canonical selective path on the regular emissiveNode; use a separate measured contribution pass or source-verified custom merge only when visible emission and bloom contribution must diverge.

Choose the final-output alpha branch before adding bloom RGB:

• opaque or already composited output: use

vec4(scene.rgb + bloom.rgb, 1);

• transparent coverage-clipped output: use

vec4(scene.rgb + bloom.rgb, scene.a) and explicitly accept that glare outside source coverage is discarded;

• transparent output that must preserve halos outside source coverage:

composite over the known background before renderOutput(), or publish separate glare RGB with an explicit coverage/compositing contract.

Unchanged source alpha cannot claim preserved transparent halos. Verify transparent contribution overlap in both insertion orders and under the chosen premultiplied/straight-alpha policy.

This step is complete when scene traversal count is known, every contribution uses the intended depth/sort/blend/alpha state, overlapping contributors accumulate correctly, and the final-output alpha branch states whether halo outside source coverage is discarded, composited, or represented separately.

When implementing the full-scene, selective, or hybrid graph, or transparent selective contribution, read [r185 graphs and transparent blending](references/hdr-bloom-system.md#r185-graphs-and-transparent-blending).

4. Set threshold and footprint

Name the threshold domain:

• scene-referred: fixed in the bloom input's radiance basis;
• exposed-linear: convert from the current adapted exposure;
• display-referred: use only with a stable inverse of the declared tone/output

path.

Convert threshold and soft-knee width together when exposure or calibration changes. Treat radius as cross-mip spread, not a physical radius. Validate minimum and maximum viewport/DPR. Stock r185's five-level chain requires:

floor(bloomScale * min(drawingBufferWidth, drawingBufferHeight)) >= 16

This step is complete when threshold units and update owner are explicit, the halo passes endpoint views, and the deepest mip remains valid at every shipping extent.

When tuning spread, scale, or a nonstandard optical footprint, read [BloomNode PSF and work](references/hdr-bloom-system.md#bloomnode-psf-and-work).

5. Join the final-image graph

Use this default order:

stable scene-linear HDR
  -> temporal resolve, when present
  -> excluded transparent/refractive layers
  -> pre-bloom meter tap
  -> bloom extraction, blur, and RGB add
  -> adapted exposure
  -> tone map, grade, and one output conversion

Selective input that bypasses temporal resolve still needs a stability decision for subpixel or discontinuous emission. Keep UI and diagnostics outside HDR bloom unless the visual contract includes them. After changing the active output graph, set RenderPipeline.needsUpdate = true.

This step is complete when bloom has one owner, every admitted meter, exposure, tone map, alpha operation, and output conversion has one owner, the selected output-alpha branch is preserved through presentation, and the bloom-off graph is genuinely reachable.

When coupling threshold to exposure or selecting final output ownership, read [Exposure, output, and lifecycle](references/hdr-bloom-system.md#exposure-output-and-lifecycle).

Use $threejs-exposure-color-grading for exposure-coupled thresholds, tone mapping, grading, and output conversion. Use $threejs-image-pipeline when the scene pass, MRT signals, transparent ordering, or final graph is shared with other effects.

6. Measure, degrade, and dispose

Measure warmed paired graphs: scene alone, full-scene bloom, scene with contribution MRT, and selective/hybrid bloom where applicable. Report physical pixels, scale, attachment formats, target inventory, transparent screen coverage, sustained timing, and marginal time. Reduce bloom scale first for a pixel-bound miss, then remeasure; full-scene input can remove a costly full-resolution contribution attachment. Disable bloom when the minimum-mip, memory, thermal, or marginal-time gate still fails.

Replace the output node for the disabled path, mark the pipeline dirty, and dispose BloomNode and any exclusive contribution resources after final GPU use. Recreate evidence after renderer/device loss or format generation change.

This step is complete when every shipping tier passes fixed captures and sustained target-device budgets, bloom-off timing loses the bloom work, and repeated enable/disable, resize, and disposal cycles stabilize resources.

When calculating exact internal storage/work or diagnosing a failed gate, read [Budget and acceptance](references/hdr-bloom-system.md#budget-and-acceptance).