Disclosure: This article was generated with AI for the FrameSprite team, which has a commercial interest in FrameSprite. It received a separate AI editorial review against the cited sources. Executed synthetic fixtures are identified with their runtime and limits; no other editor, engine, production or performance test is implied.
Wait for every required Canvas toBlob callback before finalizing the sprite archive. The method returns undefined, not a Blob. A preview can be correct while an archive built too early contains no encoded frames. This guide isolates the encoding-completion boundary; it does not test a particular ZIP library.
Key takeaway: Visible pixels are not yet an encoded file. Archive assembly starts after successful encoding results are collected.
Treat null as a failed frame
The toBlob documentation describes the callback result and its possible null value. Wrap that result in a Promise that either resolves to a usable Blob or rejects clearly:
function encodeFrame(canvas) {
return new Promise((resolve, reject) => {
canvas.toBlob(blob => {
if (!blob) return reject(new Error('frame encoding failed'));
resolve(blob);
}, 'image/png');
});
}
const blobs = await Promise.all(canvases.map(encodeFrame));
// Only now hand the complete set of blobs to the archive writer.
Do not count a null result as a successful frame. Preserve its source frame ID in the error report so the user knows what needs retrying. If your application supports partial delivery, make that an explicit policy and manifest state instead of silently publishing a smaller archive.
Keep completion and ordering separate
Promise.all returns results in the input promise order, but the list you supplied still needs a stable frame order. Bind each requested canvas to its intended frame ID before encoding. A list assembled from completion callbacks can have a different order even if every Blob is valid.
The Canvas serialization specification describes bitmap serialization. The Response.blob reference concerns a separate operation on downloaded responses; do not confuse those two completion boundaries.
Read the actual local result
Chromium 156.0.8078.4 ran four assertions in the local checker. Immediately after calling toBlob, its return was undefined and the callback had not run. Awaiting the requested encodes produced two PNG Blobs, each 93 bytes in this browser run. An empty canvas exercised the null-result rejection path.
The recorded output retains those values, and the input records the 2×2 and empty canvas cases. The 93-byte size is a fixture observation, not a stable encoder-size promise or an acceptance threshold.
No ZIP library, game engine or production export path was executed. The browser test proves Blob collection and failure handling only. The downloadable HTML is part of the controlled local harness used for the recorded run.
Finish the archive checks after encoding
Before publishing the archive, compare the collected frame IDs and count with the intended manifest. Let the archive writer complete, then reopen the produced archive and verify its members. A successful Promise.all does not prove that the ZIP writer committed those files.
Use the frame-order guide for the sequence, the archive inventory check for expected members, and the ZIP CRC guide for member integrity.
Retain an intentionally failed encoding case in regression checks. It should block a complete-delivery claim and name the affected frame, rather than producing a success message followed by an empty or incomplete download.
Editorial ownership
Prepared by the workflow team, which has a commercial relationship with this site. This guide was drafted with AI assistance; independent editorial review is recorded separately. Report a technical correction with the relevant input, tool version and observed result.
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