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.
Create the sprite-export canvas with alpha support before drawing into it. A 2D context initialized with alpha: false has an opaque backing surface. Clearing it does not restore transparent pixels, and exporting that surface as PNG does not reconstruct alpha that was already lost.
Key takeaway: Preserve transparency at the drawing surface, not only in the chosen output file format.
Inspect the context before editing the artwork
The getContext documentation describes the alpha initialization option. The authored test compares two fresh 1×1 canvases, one with alpha enabled and one without it. Both receive clearRect.
| Context option | Recorded pixel after clear |
|---|---|
| alpha: true | RGBA 0, 0, 0, 0 |
| alpha: false | RGBA 0, 0, 0, 255 |
Chromium 156.0.8078.4 produced those values. The different alpha values isolate the surface configuration without relying on a checkerboard background or a screenshot of a transparent image viewer.
Create a new transparent surface
Choose the intended option before acquiring the context:
const exportCanvas = document.createElement('canvas');
exportCanvas.width = width;
exportCanvas.height = height;
const exportContext = exportCanvas.getContext('2d', {alpha: true});
if (!exportContext) throw new Error('2D canvas unavailable');
exportContext.clearRect(0, 0, width, height);
exportContext.drawImage(originalDecodedSource, 0, 0);
Do not assume that calling getContext again with a different options object reinitializes an existing context. Make a new surface for the export contract. Redraw from the original alpha-bearing source rather than copying pixels that were already composited into an opaque surface.
The Canvas specification explains the context and bitmap behavior. The toBlob reference explains serialization into an image file; serialization does not infer missing foreground transparency.
Verify actual alpha rather than the page background
Display the exported asset over both dark and light backgrounds, then inspect its alpha channel with a decoder. A white page can make an opaque white background look correct. A checkerboard UI can also be painted into the file instead of representing transparency.
Do not replace all black or white pixels with transparent pixels as a general repair. Those colors may belong to the character, and antialiased edges may already have been composited. Return to the intact source and preserve its alpha through each processing step.
Evidence and its boundary
Two assertions ran in the local browser checker. The recorded result confirms the two pixel values, and the input records both context options. The test sampled Canvas pixels; it did not encode and decode a PNG or exercise a production export path. The HTML is part of the controlled local harness used for that run.
For the real exporter, add the separate encode/decode check after the surface test. Preserve dimensions and alpha values in its report instead of relying on the file extension.
Distinguish similar edge and background problems
A baked checkerboard needs the checkerboard diagnosis. An edge halo can involve straight versus premultiplied alpha. Hidden color values beneath zero alpha need the transparent RGB QA check.
Keep opaque rendering surfaces for workflows that intentionally need them, and route transparent sprite export through its own explicitly initialized surface.
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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