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fix: avoid issues when sigma_min is close to 0 #1138

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fix: avoid issues when sigma_min is close to 0 #1138

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f287b5a
fix: avoid issues when sigma_min is close to 0
daniandtheweb Dec 23, 2025
f902205
fix: clamp sigma_min in Karras and KL Optimal to avoid NaNs
daniandtheweb Dec 25, 2025
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24 changes: 17 additions & 7 deletions denoiser.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -276,6 +276,10 @@ struct KarrasScheduler : SigmaScheduler {
// but does anybody ever bother to touch them?
float rho = 7.f;

if (sigma_min <= 1e-6f) {
sigma_min = 1e-6f;
}

std::vector<float> result(n + 1);

float min_inv_rho = pow(sigma_min, (1.f / rho));
Expand Down Expand Up @@ -347,35 +351,41 @@ struct SmoothStepScheduler : SigmaScheduler {
}
};

// Implementation adapted from https://github.com/AUTOMATIC1111/stable-diffusion-webui/pull/15608
/*
* KL Optimal:
* Original work from https://github.com/AUTOMATIC1111/stable-diffusion-webui/pull/15608.
* Implemented using https://github.com/comfyanonymous/ComfyUI/pull/6206 as a reference.
*/
struct KLOptimalScheduler : SigmaScheduler {
std::vector<float> get_sigmas(uint32_t n, float sigma_min, float sigma_max, t_to_sigma_t t_to_sigma) override {
std::vector<float> sigmas;

if (n == 0) {
return sigmas;
}

if (n == 1) {
sigmas.push_back(sigma_max);
sigmas.push_back(0.0f);
return sigmas;
}

if (sigma_min <= 1e-6f) {
sigma_min = 1e-6f;
}

sigmas.reserve(n + 1);

float alpha_min = std::atan(sigma_min);
float alpha_max = std::atan(sigma_max);

for (uint32_t i = 0; i < n; ++i) {
// t goes from 0.0 to 1.0
float t = static_cast<float>(i) / static_cast<float>(n - 1);

// Interpolate in the angle domain
float t = static_cast<float>(i) / static_cast<float>(n - 1);
float angle = t * alpha_min + (1.0f - t) * alpha_max;

// Convert back to sigma
sigmas.push_back(std::tan(angle));
}

// Append the final zero to sigma
sigmas.push_back(0.0f);

return sigmas;
Expand Down
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