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The original was posted on /r/stablediffusion by /u/AgeNo5351 on 2025-08-31 21:19:50+00:00.

TLDR:

• You need to find out in how many steps you reach sigma of 0.875 based on your scheduler/shift value.
• You need to ensure enough steps reamain for low model to finish proper denoise.

In the official Wan code https://github.com/Wan-Video/Wan2.2/blob/main/wan/configs/wan_t2v_A14B.py for txt2vid

# inference
t2v_A14B.sample_shift = 12.0
t2v_A14B.sample_steps = 40
t2v_A14B.boundary = 0.875
t2v_A14B.sample_guide_scale = (3.0, 4.0)  # low noise, high noise

The most important parameter here relevant for High/Low partition is the boundary point = 0.875 , This means this is the sigma value after which its recommended to switch to low. This is because then there is enough noise space ( from 0.875 → 0) for the low model to refine details.

Lets take an example of simple/shift = 3 ( Total Steps = 20)

Sigma values for simple/shift=3

In this case , we reach there in 6 steps , so it should be High 6 steps / Low 14 steps.

What happens if we change just the shift = 12

beta/shift = 12

Now we reach it in 12 steps. But if we do partition here, the low model will not enough steps to denoise clearly (last single step has to denoise 38% of noise )So this is not an optimal set of parameters.

Lets compare the beta schedule Beta/ Total Steps = 20 , Shift = 3 or 8

Beta schedule

Here the sigma boundary reached at 8 steps vs 11 steps. So For shift=8 , you will need to allocate 9 steps for low model which might not be enough.

beta57 schedule

Here , for beta57 schedule the boundary is being reached in 5 and 8 steps. So the low-model will have 15 or 12 steps to denoise, both of which should be OK. But now , does the High model have enough steps ( only 5 for shift = 3 ) to do its magic ?

Another interesting scheduler is bong-tangent , this is completely resistant to shift values , with the boundary occurring always at 7 steps.

bong_tangent