CODEWITHSUNDEEP
pythonnumpyopencvpngimage-compression
smcs
smcs

Reputation: 2004

Am I creating lossless PNG images?

I am doing image processing in a scientific context. Whenever I need to save an image to the hard drive, I want to be able to reopen it at a later time and get exactly the data that I had before saving it. I exclusively use the PNG format, having always been under the impression that it is a lossless format. Is this always correct, provided I am not using the wrong bit-depth? Should encoder and decoder play no role at all? Specifically, the images I save

Upvotes: 21

Views: 17183

Answers (2)

eric
eric

Reputation: 8108

While png is lossless, this does not mean it is uncompressed by default.

I specify compression using the IMWRITE_PNG_COMPRESSION flag. It varies between 0 (no compression) and 9 (maximum compression). So if you want uncompressed png:

cv2.imwrite(filename, data, [cv2.IMWRITE_PNG_COMPRESSION, 0])

The more you compress, the longer it takes to save.

Link to docs

Upvotes: 17

Dan Mašek
Dan Mašek

Reputation: 19071

PNG is a lossless format by design:

Since PNG's compression is fully lossless--and since it supports up to 48-bit truecolor or 16-bit grayscale--saving, restoring and re-saving an image will not degrade its quality, unlike standard JPEG (even at its highest quality settings).

The encoder and decoder should not matter, in regards of reading the images correctly. (Assuming, of course, they're not buggy).

And unlike TIFF, the PNG specification leaves no room for implementors to pick and choose what features they'll support; the result is that a PNG image saved in one app is readable in any other PNG-supporting application.

Upvotes: 18

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