Question 1

Why did my image save as WebP when the page showed a JPG?

Accepted Answer

Because the server chose the format after seeing your browser's request. A browser that can display WebP says so in the Accept header it sends with every image request, and image CDNs and many hosting platforms use that to send WebP instead of the JPG or PNG the page refers to, since it is smaller. The URL can still end in .jpg while the bytes are WebP. Saving the image keeps what was actually delivered, which is why the file turns up as .webp, or as a .jpg that some programs then refuse to open.

Question 2

Does converting WebP to PNG lose quality?

Accepted Answer

No further quality is lost. PNG is lossless, so the pixels decoded from the WebP are the pixels written into the PNG. What the conversion cannot do is undo loss that already happened: if the WebP was saved lossy, its compression artefacts are now part of the picture and the PNG keeps them faithfully. The one small exception is half-transparent pixels, covered further down, where the browser's canvas can shift a color value by a step or two.

Question 3

Why is the PNG so much bigger than the WebP?

Accepted Answer

Lossy WebP gets its size by discarding detail the eye is unlikely to notice. PNG discards nothing, so it has to describe every pixel, including the faint noise every photograph carries, and DEFLATE compression has little purchase on noise. Three to ten times the size is normal for a photo. Screenshots, logos and flat graphics grow far less, since PNG compresses large areas of identical color very well. If the size matters more than exactness, convert to JPG instead.

Question 4

Is the transparent background kept?

Accepted Answer

Yes. WebP and PNG both store a full alpha channel, so fully transparent areas stay transparent and soft edges such as shadows or anti-aliased outlines stay soft. The checkerboard on each card is there so you can see the transparency survived. If you would rather have a solid background, switch to JPG and pick the color, since JPG cannot hold transparency at all.

Question 5

What happens to an animated WebP?

Accepted Answer

Only the first frame is converted, and its card says so. The converter reads the file's VP8X header to find the animation flag rather than guessing from the name, because a browser canvas only ever exposes one frame and the result would otherwise look like a quiet failure. PNG does have an animated variant, APNG, but it is not something a canvas can write. For an animation, keep the WebP or convert it with a video or GIF tool.

Question 6

Why are a few semi-transparent pixels slightly different?

Accepted Answer

Browsers store canvas pixels with the color already multiplied by the opacity, which is faster to draw with. Writing the PNG means dividing that back out, and for a pixel that is, say, 3 percent opaque there are only a handful of possible values left to divide, so the color can come back a step or two off. The HTML specification itself warns that converting to and from premultiplied values is lossy. It only touches pixels that are nearly invisible to begin with, and opaque pixels come out unchanged.

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