What's the difference between the two modes?

Grayscale converts colors to tones (with a strength slider for partial desaturation); pure black & white applies a hard threshold — every pixel becomes either black or white.

Which luma formula is used?

Rec. 601 weights (0.299 R, 0.587 G, 0.114 B) — the standard photographic grayscale.

Is the photo uploaded?

No — the conversion is per-pixel math on your device.

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