Check a PNG's Transparency

declared colour type + measured pixels · soft vs hard edges · trimmable border

"Is the background actually transparent, or just white?" You can't tell by looking, because most viewers show transparency as white anyway. This reads what the file declares and counts the pixels, so you get a definite answer before you paste a logo onto a dark background and discover a white box.

Declared versus measured

These are two different questions and the report answers both. Declared comes from the PNG header's colour type: 6 is RGBA and 4 is greyscale-with-alpha, so those files can hold transparency. Types 2 and 0 have no alpha channel at all, though type 3 can fake it with a tRNS chunk. Measured counts actual pixel alpha values.

A file can declare an alpha channel and use none of it — that's an RGBA PNG where every pixel is opaque, which happens constantly when software exports "with transparency support" from a flattened document. It costs you bytes and buys you nothing, and it's the case that looks transparent in a file-info panel while behaving as opaque everywhere.

Soft edges are the detail that matters

The report distinguishes fully transparent, partially transparent, and opaque pixels. That middle number is the one to look at:

Zero partial pixels means hard-edged transparency — every pixel is all or nothing. Fine for pixel art and simple shapes; on a logo or a cut-out photo it means jagged, aliased edges that will look rough at any size. Typically the file came from a GIF, or from a "remove background" tool that thresholded the alpha.

A few percent partial is the healthy signature of properly anti-aliased artwork. The edge pixels blend, so the shape sits cleanly on any background colour.

The trimmable border figure tells you how much of the canvas is empty transparent space around the visible content. If it says 60%, most of your file is nothing — Auto-trim will remove it, or Add padding if the margin was deliberate and you want to control it precisely.

FAQ