measure-on-chain-svg-gas-cost

Measure on-chain SVG gas cost

Rendering an SVG inside the contract pushes bytes into tokenURI; naive concatenation can cost hundreds of thousands of gas per call and blow the eth_call gas limit some wallets use.

Procedure

1. Baseline the byte size of a metadata read:

   cast call $NFT "tokenURI(uint256)(string)" 1 --rpc-url $RPC | wc -c
   cast call $NFT "tokenURI(uint256)(string)" 1 --rpc-url $RPC --gas-limit 50000000 >/dev/null && echo "renders"
   

2. Measure the actual gas with forge:

   forge test --match-test testTokenURIGas --gas-report
   

   or in a script: forge script script/UriGas.s.sol --gas-limit 50000000.

3. Decompose the cost. Static bytes read from storage are cheap; repeated string.concat in a loop grows O(n²) because each concat copies the whole running string. Prefer one abi.encodePacked pass.

4. Compare storage strategies per byte: a bytes slot costs 20000 gas SSTORE per 32-byte word, while SSTORE2 bytecode costs ~200 gas to deploy per byte and ~200 to read — cheaper for large static blobs read rarely.

5. Benchmark the base64 encoder separately. Pure-Solidity base64 of a fixed string is fine; encoding a long dynamic string in the same call nearly doubles the work.

6. Verify the rendered output parses before optimizing further:

   cast call $NFT "tokenURI(uint256)(string)" 1 --rpc-url $RPC \
     | jq -r '.image' | sed 's#data:image/svg+xml;base64,##' | base64 -d | xmllint --noout -
   

7. Set a budget: if tokenURI exceeds ~30M gas, some RPCs and wallets drop the call and the collection renders blank. Cut bytes, not promises.

Pitfalls

• Optimizing storage misses the true cost: string concatenation and base64 dominate.
• SVG path data stored as string[] becomes one SLOAD per element per trait; pack the whole drawing into a single blob.
• tokenURI that reverts on a gas limit makes every token render blank across marketplaces at once.
• Off-by-one in base64 padding silently corrupts the image while still returning text.
• Gas measured by eth_estimateGas excludes part of the read path; measure with eth_call at an explicit high gas.

Verification

forge test --match-test testTokenURIGas --gas-report | grep -i tokenURI

Pass: the reported tokenURI gas is under your budget and xmllint --noout - exits 0 on the decoded SVG. Report the measured gas, the biggest cost contributor, and the storage strategy chosen.