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JPEG cannot store transparent pixels. When a browser-based converter turns a transparent PNG into a JPG, those pixels must be composited onto a solid color. White is not the universal default for browser canvas export: the HTML Standard specifies opaque black for canvas serialization to formats that do not support alpha. A converter that produces white is applying white somewhere in its own workflow, but the exact cause depends on the tool.
Why transparency disappears in a JPG
A PNG can represent transparency with an alpha channel, which records how opaque each pixel is. JPEG has no alpha channel. To create a JPEG, the conversion process has to combine transparent and partly transparent pixels with a background color, producing an opaque image.
For browser canvas serialization to a format without alpha, the WHATWG HTML Standard specifies compositing the image onto an opaque black background. That rule describes canvas serialization; it does not establish how every conversion website or app works. A particular tool may first paint its working canvas white or otherwise choose white as its compositing color.
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How to make the JPG background white—or another color
If you control the conversion code, explicitly paint the desired background before drawing the PNG. Then export the canvas as JPEG. This makes the intended color part of the workflow instead of relying on a converter’s default.
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const canvas = document.createElement("canvas");
canvas.width = image.naturalWidth;
canvas.height = image.naturalHeight;
const ctx = canvas.getContext("2d");
ctx.fillStyle = "#fff";
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.drawImage(image, 0, 0);
canvas.toBlob((blob) => {
if (!blob) throw new Error("Could not create image");
// Use or download the JPEG Blob.
}, "image/jpeg", 0.92);Change "#fff" to the color you want. The fill must happen before drawImage(); otherwise the background will not be behind the PNG when it is composited.
Choose an export method and verify its result
| Method | What it returns | When to use it |
|---|---|---|
canvas.toBlob(callback, "image/jpeg", quality) |
An encoded Blob delivered asynchronously to the callback. | Generally preferable for large images because it avoids turning the entire encoded image into an in-memory string. |
canvas.toDataURL("image/jpeg", quality) |
A data URL string containing the encoded image. | Useful when a string representation is needed, but large images can create memory and URL-length drawbacks. |
Both methods accept a requested image type and a quality value for supported lossy formats. The requested type is not a guarantee: PNG is required, and a browser may fall back to PNG if it does not support the requested format. When the actual format matters, check the returned representation. MDN documents the behavior of toBlob() and toDataURL().
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Why a browser converter may still fail to export
If the source PNG is loaded from another origin, canvas export can fail when the canvas is not origin-clean. MDN documents that toBlob() can throw a SecurityError in that situation. This is separate from the background-color issue: an explicit fill controls the output color, but it does not bypass cross-origin restrictions.
For a converter you do not control, check for a background-color setting or inspect its implementation to learn why it chose white. The canvas standard explains the serialization rule, but without knowing the tool, it cannot identify which step produced a white background.
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