Thinking the erc721 JSON metadata standard image field is a bit inadequate for on-chain (non solidity) generative art.

still forces you to use a static file (ipfs or a centralized url) for each token ID, which works for pngs but isn’t so suited for generative art.

is this not what the onchain html people are on about? injecting html/js inside and svg etc

Yep! There’s two popular flavours: onchain via solidity (code that draws the token on the fly in tokenUri, usually SVG) or onchain by storing some code in state and having a client render it (Artblocks, Deafbeef)

The latter allows you to be more expressive w/o worrying about EVM gas. But for the tokens to appear properly on OpenSea and listing sites, you need metadata that points to a PNG file or HTML file, usually that means a rendering system that generates thumbs and and saves them on a url or ipfs link

terraforms contract delivers an svg for the "image": that opensea shows in the collection view i believe