Question 1

Why does my transparent PNG turn black in other converters?

Accepted Answer

Because JPG has no alpha channel and the transparent pixels have to become something. A transparent pixel in a canvas has a color of zero, which is black, and a converter that encodes without first painting a background passes that black straight through. Here the background color is painted underneath before the image is drawn, so transparency becomes white, or whatever you choose, and semi-transparent edges blend into it properly.

Question 2

Which background color should I pick?

Accepted Answer

The color of wherever the image is going. For a document, an upload form or printing, white. For a web page, the page's background color. Matching matters more than it seems, because anti-aliased edges and soft shadows are partly transparent and get blended with this color. A logo filled onto white and then placed on a grey page shows a faint pale outline, and that halo is the giveaway of a mismatched fill.

Question 3

When should I not convert a PNG to JPG?

Accepted Answer

When the image is mostly text, lines, a logo, an interface screenshot or a chart. JPEG compresses by smoothing out fine detail in 8×8 blocks, which works beautifully on photographs and badly on sharp edges, leaving faint ripples around letters. Those images also tend to compress well as PNG already, so the JPG is not much smaller. Keep PNG for them, or try WebP if you need both sharpness and a smaller file.

Question 4

How much smaller will the JPG be?

Accepted Answer

For a photograph, typically 70 to 90 percent smaller, which is the whole reason to convert. For a screenshot with large flat areas, sometimes barely smaller, and occasionally larger, because PNG was already compressing those areas very efficiently. Each card shows the exact change, so there is no need to guess. If a file comes out larger, keep the PNG.

Question 5

What quality should I use?

Accepted Answer

Leave it at 90 unless you have a size limit to meet. At 90 a converted photograph is very hard to tell from the PNG. 80 gives a noticeably smaller file and still looks clean at normal viewing size. Below about 70, blocks start to show in skies and smooth gradients. Since PNG is lossless, this is the only compression the image ever goes through, so a high setting here costs less than it would on an image that has been saved as JPG before.

Question 6

Will the JPG look exactly like the PNG?

Accepted Answer

Close, but not identically, because JPG is lossy by design. Apart from the compression itself, JPEG encoders store color at reduced resolution, one color sample for each 2×2 block of pixels, which is invisible on photographs and can soften thin, saturated colored lines such as red text on a dark background. Use the comparison slider at full size on anything like that before relying on the result.

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