Enhance face details
Enhancing face details is needed when image is rather small or is heavily distorted with noise, compression artefacts or is blurred.
Face enhancement can be done when adding "face_enhance" option to enhancements.
"enhancements": ["face_enhance"]
Let's check the examples.
Input image

Upscaled 4x with face_enhance option
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Upscaled 4x without face_enhance option