One-color stencil from an AI image: Pillow threshold for screen print

Convert to grayscale, blur lightly, and apply point(lambda v: 255 if v > T else 0) with a cutoff T, then convert to mode 1. The output has two states only, which is what a one-color stencil, vinyl cut, or screen print needs. Choose T so the black share lands between about 25 and 45 percent, instead of guessing a number like 128.

What a stencil needs

A screen printer wants solid areas of one ink and clean gaps. Anti-aliased gray edges are a problem, since a screen cannot hold them. A threshold forces every pixel to ink or no ink.

Generate with a prompt that already helps: bold shapes, high contrast, a plain light background, no fine shading. A model that is asked for a woodcut or linocut look gives you a head start. The picture still needs to be thresholded, because even a woodcut look contains grays.

Choose the cutoff from the histogram

A fixed cutoff of 128 is a poor default. A dark image turns solid black and a light image turns blank. The code instead walks cutoffs and picks the first one where the dark share reaches a target, here 35 percent.

import os, io, requests
from PIL import Image
H = {"Authorization": "Bearer " + os.environ["SUME_API_KEY"]}
def gen(**body):
    r = requests.post("https://api.sume.com/v1/images", json=body, headers=H, timeout=60)
    r.raise_for_status()
    if r.status_code == 202:
        raise SystemExit("queued, read /v1/jobs/{id}/result: " + r.text)
    return r.json()
def fetch(u):
    return Image.open(io.BytesIO(requests.get(u, timeout=60).content))
from PIL import ImageFilter, ImageOps
out = gen(model="bytedance-seed/seedream-4.5", aspect_ratio="1:1",
          prompt="Bold linocut print of a fox head, thick shapes, plain white background")
g = ImageOps.autocontrast(fetch(out["data"][0]["url"]).convert("L"))
g = g.filter(ImageFilter.GaussianBlur(1.2))
hist, total, run, T = g.histogram(), g.width * g.height, 0, 128
for v in range(256):
    run += hist[v]
    if run / total >= 0.35:
        T = v
        break
stencil = g.point(lambda v: 255 if v > T else 0).convert("1")
stencil.save("stencil.png")
print("cutoff", T, "cost", out["usage"]["cost"])

Tuning the look

Two knobs change the result: blur radius before the threshold and the dark-share target after.

Cleanup before you send it

• Small specks (islands) are hard to cut and to burn onto a screen. Increase blur or remove them with a median filter such as ImageFilter.MedianFilter(5) before thresholding.
• Save as PNG in mode 1. JPEG would add gray ringing.
• Check the physical size. A 1024 pixel stencil printed at 8 inches is 128 dpi, which is soft for fine lines. Generate larger or vectorize the result in a dedicated tool.
• Invert with ImageOps.invert if your process needs ink where the image is light.

Related

The no-JPEG rule applies here too, see save PNG between passes. Once the stencil is fixed, derive other sizes from one master with the srcset recipe, and check any brand color of the ink with the delta E script.