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Is this possible convert BMP to PNG while maintaining quality? I am trying to reduce the size of some albums that are in BMP, but still maintain the lossless quality.

Just tried XnConvert and setting it at “1” compression I noticed quality loss. Looking for batch conversion or a way to convert 1 at a time automatically. Conversion time/speed is not a problem but I do feel safer if it took a tad bit longer than XnConvert which converted 407 BMPs to PNG, 0 compression in 39 seconds. I’m not sure if that’s normal.

  • If you lose bits, you lose quality. Question is whether you can really see the difference. I use Paint for those conversion tasks. – whs Apr 28 '15 at 3:07
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    There is something inherently wrong with the conversion and/or result preview process because you cannot lose quality by converting to PNG. You will need to specify how you are seeing this loss of quality. – Nelson Apr 28 '15 at 4:23
  • I just tried using nconvert (command line version of XnConvert) with various -clevel PNG compression settings (default is 6) and there was no quality loss. – Karan Apr 28 '15 at 4:36
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    @HyeVtlg3 Can you try the conversion again with XnConvert? According to Jake's answer below, you should not be experiencing any quality loss at all. Nelson's comment above concurs with Jake's answer. Maybe you can try it again and report back? – RockPaperLizard Apr 28 '15 at 4:57
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    Can you provide an example of the quality loss? As far as I'm aware the quality would be exactly the same as they are both lossless formats. – Mokubai Apr 28 '15 at 5:20
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Conversion time/speed is not a problem but I do feel safer if it took a tad bit longer than XnConvert which converted 407 BMPs to PNG, 0 compression in 39 seconds. I’m not sure if that’s normal.

You shouldn’t be that concerned about speed in a conversion process like this since BMP and PNG are lossless image formats, but PNG is lossless and compressed. Meaning you should not experience any quality loss at all as your conversion program simply shuffles the bits from BMP to PNG format.

As explained on Wikipedia, this is what BMP is all about; bold emphasis is mine:

Typically, BMP files are uncompressed, and therefore large and lossless; their advantage is their simple structure and wide acceptance in Windows programs.

And here is what PNGs are all about from that same article; again bold emphasis is mine:

Compared to JPEG, PNG excels when the image has large, uniformly colored areas. Even for photographs – where JPEG is often the choice for final distribution since its compression technique typically yields smaller file sizes – PNG is still well-suited to storing images during the editing process because of its lossless compression.

So depending on your computer’s speed and power—as well as the size of the images being handle—being “slow” when converting from BMP to PNG is really a non-factor. Both are lossless formats with the main difference being that PNG compresses the data while BMP is basically raw and uncompressed data.

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    Although this has good information, I'm not sure this answers the question. The OP reported that they experienced a loss of quality during the conversion. According to this answer, that can't happen, as both are supposed to be lossless. So something is wrong here or with the OP's experience. This answer does not address that discrepancy. – RockPaperLizard Apr 28 '15 at 4:10
  • @RockPaperLizard Good point. But no clear solution from my perspective and lack of understanding of XnConvert settings. – JakeGould Apr 28 '15 at 4:12

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