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fix: align Qwen Image 2.1 flow schedule with official defaults #2048

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Merged
leejet merged 1 commit into from
Sep 24, 2026
+27 −11
Merged

fix: align Qwen Image 2.1 flow schedule with official defaults #2048

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38 changes: 27 additions & 11 deletions src/runtime/denoiser.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -728,12 +728,20 @@ inline float flux_time_shift(float mu, float sigma, float t) {

// https://github.com/black-forest-labs/flux/blob/main/src/flux/sampling.py#L289
struct FluxScheduler : SigmaScheduler {
int image_seq_len = 0;
float base_shift = 0.5f;
float max_shift = 1.15f;

explicit FluxScheduler(int image_seq_len, const char* extra_sample_args = nullptr)
int image_seq_len = 0;
int base_image_seq_len = 256;
int max_image_seq_len = 4096;
float base_shift = 0.5f;
float max_shift = 1.15f;
float shift_terminal = 0.0f;

FluxScheduler(int image_seq_len, SDVersion version, const char* extra_sample_args = nullptr)
: image_seq_len(image_seq_len) {
if (version == VERSION_QWEN_IMAGE_2_1) {
max_image_seq_len = 8192;
max_shift = 0.9f;
shift_terminal = 0.02f;
}
parse_extra_sample_args(extra_sample_args);
}

Expand All @@ -752,10 +760,8 @@ struct FluxScheduler : SigmaScheduler {
}

float compute_mu() const {
constexpr float base_shift_anchor = 256.0f;
constexpr float max_shift_anchor = 4096.0f;
float m = (max_shift - base_shift) / (max_shift_anchor - base_shift_anchor);
float b = base_shift - m * base_shift_anchor;
float m = (max_shift - base_shift) / static_cast<float>(max_image_seq_len - base_image_seq_len);
float b = base_shift - m * static_cast<float>(base_image_seq_len);
return static_cast<float>(image_seq_len) * m + b;
}

Expand All @@ -764,7 +770,7 @@ struct FluxScheduler : SigmaScheduler {
sigmas.reserve(n + 1);

float mu = compute_mu();
LOG_VERBOSE("Flux scheduler: image_seq_len=%d, steps=%u, mu=%.3f", image_seq_len, n, mu);
LOG_VERBOSE("Flux scheduler: image_seq_len=%d, steps=%u, mu=%.3f, shift_terminal=%.3f", image_seq_len, n, mu, shift_terminal);

if (n == 0) {
sigmas.push_back(1.0f);
Expand All @@ -780,6 +786,16 @@ struct FluxScheduler : SigmaScheduler {
}
}

if (shift_terminal > 0.0f && n > 1) {
// The terminal shift applies to the last model evaluation, not the final zero sigma.
float scale_factor = (1.0f - sigmas[n - 1]) / (1.0f - shift_terminal);
if (std::isfinite(scale_factor) && scale_factor > 0.0f) {
for (uint32_t i = 0; i < n; ++i) {
sigmas[i] = 1.0f - (1.0f - sigmas[i]) / scale_factor;
}
}
}

sigmas[n] = 0.0f;
return sigmas;
}
Expand Down Expand Up @@ -1178,7 +1194,7 @@ struct Denoiser {
}
case FLUX_SCHEDULER: {
LOG_INFO("get_sigmas with Flux scheduler");
scheduler = std::make_shared<FluxScheduler>(image_seq_len, extra_sample_args);
scheduler = std::make_shared<FluxScheduler>(image_seq_len, version, extra_sample_args);
break;
}
default:
Expand Down
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