[DrMkdaddy]

get_image_seq_len() returned latent pixels. The Flux shift formula is fitted on 2x2 patch tokens, so 1024x1024 produced image_seq_len=16384 instead of 4096, mu=3.230 instead of 1.150, and the 4 px grid in #2267.

This adds DiffusionModelRunner::get_patch_size() with a default of 1 and divides the latent grid by it in get_image_seq_len(). The Flux runner returns config.patch_size: 2 for Flux and Chroma, 1 for Flux.2, 16 or 32 for Chroma Radiance.

Verified on Chroma1-HD Q2_K:

• 1024x1024: Flux scheduler: image_seq_len=4096, steps=1, mu=1.150.
• 256x256, one step, end to end: image_seq_len=256, 47.4s sampling, 4.0s VAE decode, PNG written.
• Scheduler harness: 16384 -> mu=3.230, 4096 -> mu=1.150.

@cdamiens, can you confirm the grid is gone on your setup?

Fixes #2267

Summary by CodeRabbit

• Bug Fixes
  • Image sequence length now accounts for the diffusion model’s patch size, improving handling of models that process images in patches.

[coderabbitai]

Walkthrough

The diffusion model API now exposes patch size. get_image_seq_len uses that value to round latent dimensions up to whole patches before calculating the sequence length.

Changes: Patch-aware sequence length

Layer / File(s)	Summary
Expose diffusion model patch size src/model/diffusion/model.hpp, src/model/diffusion/flux.hpp	DiffusionModelRunner adds a virtual get_patch_size() method that defaults to 1. FluxRunner returns its configured patch size.
Calculate sequence length from patches src/pipeline/diffusion_engine.cpp	get_image_seq_len rounds each latent dimension up to a whole number of patches before multiplying them. If the diffusion model is unavailable, the patch size defaults to 1.

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: leejet

Merge Risk: Minimal · up to d6ef6

Flux scheduling now uses patch counts. Artifact removal has not yet been confirmed, but no merge-blocking regression was established.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title "fix: use patch token count for the Flux scheduler" matches the main change. The PR divides the latent grid by the diffusion model's patch size in get_image_seq_len(), so the Flux scheduler …
Linked Issues check		[#2267] Objective: the Flux-family scheduler must use 2x2 patch tokens, not latent pixels. At 1024x1024 the expected count is 4096. Implementation at the reviewed head: - src/model/diffusion/model.hp…
Out of Scope Changes check		The whole-PR diff (merge base 7bcd4ce to head d6ef616) changes three files only: - src/model/diffusion/model.hpp: adds the default virtual get_patch_size() hook. - src/model/diffusion/flux.hpp: adds t…
Description check		The description is clear, specific, and on-topic. It explains the bug (get_image_seq_len returned latent pixels instead of patch tokens, giving image_seq_len=16384 instead of 4096 and mu=3.230 instead…

Full details: Title check

Explanation

The title "fix: use patch token count for the Flux scheduler" matches the main change. The PR divides the latent grid by the diffusion model's patch size in get_image_seq_len(), so the Flux scheduler receives the patch token count (4096 at 1024x1024) instead of the latent pixel count (16384). The title is concise, specific, and follows a conventional prefix.

Full details: Linked Issues check

Explanation

[#2267] Objective: the Flux-family scheduler must use 2x2 patch tokens, not latent pixels. At 1024x1024 the expected count is 4096.

Implementation at the reviewed head:

• src/model/diffusion/model.hpp: new virtual DiffusionModelRunner::get_patch_size() const returns 1 by default.
• src/model/diffusion/flux.hpp: FluxRunner::get_patch_size() returns config.patch_size. FluxConfig defaults patch_size to 2, and Chroma keeps that default.
• src/pipeline/diffusion_engine.cpp: StableDiffusionGGML::get_image_seq_len() divides the latent grid by the patch size using ceiling division. It falls back to 1 when diffusion_model is null.

At 1024 px, vae_scale_factor 8 gives a 128x128 latent grid. Dividing by 2 gives 64x64 = 4096, which matches the issue's expected token count. Runners that do not override get_patch_size() keep patch size 1, so their result is unchanged.

The issue's second item (switch Chroma to beta scheduler with no shift) is a separate alternative. The PR does not implement it, and the primary requested outcome does not depend on it.

Unverified: the issue's 4 px grid artifact. The PR reports no FFT check. Its verification is the scheduler log value and a 256x256 end-to-end run. Removal of the grid is therefore not established by this PR.

No automated test is added. The issue does not request one.

Full details: Out of Scope Changes check

Explanation

The whole-PR diff (merge base 7bcd4ce to head d6ef616) changes three files only:

• src/model/diffusion/model.hpp: adds the default virtual get_patch_size() hook.
• src/model/diffusion/flux.hpp: adds the FluxRunner override that returns config.patch_size.
• src/pipeline/diffusion_engine.cpp: patch-aware get_image_seq_len().

Each change serves the sequence-length objective of #2267. The issue names Flux-family models as the target and states that other FLUX_SCHEDULER models may also be affected. The FluxRunner override applies to those models, which is consistent with that statement.

No unrelated change is present. The Chroma default scheduler is not changed, which matches the PR scope. Non-Flux runners inherit patch size 1, so their token count is unchanged.

Not established: the ceiling division differs from the floor division in the diffusers reference for sizes that are not multiples of the patch size. The issue does not cover those sizes, and this is not a scope violation. Chroma Radiance (patch size 16 or 32) was not checked against its VAE scale factor. The PR does not claim verification for that model.

Full details: Description check

Explanation

The description is clear, specific, and on-topic. It explains the bug (get_image_seq_len returned latent pixels instead of patch tokens, giving image_seq_len=16384 instead of 4096 and mu=3.230 instead of 1.150) and the fix (a new DiffusionModelRunner::get_patch_size() with default 1, overridden by the Flux runner to return config.patch_size). It lists per-model patch sizes and gives concrete verification on Chroma1-HD Q2_K, including a 1024x1024 scheduler log, an end-to-end 256x256 run, and a scheduler harness comparison. It links the issue with 'Fixes #2267'. The template's Related Issue section is satisfied by that link. The Checklist item confirming contribution-guideline compliance is not ticked, and the template's required checkbox is missing from the description. The Summary and Additional Information sections are present in substance, though not under the template's exact headings.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 1 functions across 3 files.

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

Finishing Touches

Generate unit tests (beta)

• Create a new PR

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Warning

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Cppcheck (2.22.0)

src/pipeline/diffusion_engine.cpp

Cppcheck timed out; analysis of this file is incomplete

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Failed pre-merge checks:
- Docstring Coverage:
Write docstrings for the functions missing them to satisfy the coverage
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Reviewing files that changed from the base of the PR and between 7bcd4ce and d6ef616.

Files selected for processing: 3

• src/model/diffusion/flux.hpp
• src/model/diffusion/model.hpp
• src/pipeline/diffusion_engine.cpp

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