Overlay a logo on a QR code, then decode the very same image to check whether it still scans
L, M, Q and H are described as about 7%, 15%, 25% and 30%. According to Denso Wave's official explanation, that is a recovery rate over all codewords. A codeword is the unit that makes up the data, and in QR codes one codeword is 8 bits. It is not a share of the picture area, so "level H lets me hide 30% of what I see" does not follow. How many codewords break depends on where the logo sits.
The three large squares in the corners (finder patterns) and the alternating black and white lines between them (timing patterns) are what a reader uses to locate, orient and size the code. They are not covered by error correction. Cover them and there is nothing to recover from, because the position is never established. This tool checks the overlap in module coordinates and warns you.
Denso Wave's FAQ states plainly that overlaying illustrations or design on a QR code is not recommended, because small stains or damage can then stop it from being read or make scanning slow, and FrameQR is offered for design use. If you still have to add a logo, the least you can do is confirm it reads before shipping it.
QR codes carry error correction, in four levels: L, M, Q and H. According to Denso Wave's official explanation, the percentage is a recovery rate over all codewords (one codeword is 8 bits), not over area. So "level H lets you hide 30% of the picture" does not follow. The reliable way is to decode the image you actually produced.
No. Denso Wave's FAQ states that overlaying illustrations or design on a QR code is not recommended, because small stains or damage can then make it unreadable or slow to scan, and points to FrameQR for design use. This tool exists for the case where you still have to do it, so you can check the result yourself.
The three large squares in the corners (finder patterns) and the alternating black and white lines that join them (timing patterns). A reader uses these to locate and size the code, and they are not covered by error correction. This tool warns you when the logo overlaps them.
No. Generating the QR code, compositing the logo and decoding it for verification all happen inside your browser. Neither the image nor the text leaves your machine.