Is QImage BMP lossless?

[jajcek]

Hi,

I hope this question is not inappropriate. I wonder if the BMP created by the QImage::save() method creates a lossless bitmap? I know that MS Paint somehow losses the quality when saved (it is even possible to see the lower quality at save time). How about the QImage class?

Thanks.

[wysota]

BMP doesn't have any internal compression (as far as I know) thus by design it is loseless. If Paint indeed causes some loss in quality, it is because of Paint, not BMP.

[ChrisW67]

There are various forms of BMP, some with lossless run-length encoding, none with lossy image compression like JPEG. Taking a 24-bit image and putting it in a 24-bit BMP will not be lossy. Any alpha channel from a 32-bit image may be lost because only recent variants of BMP support it. Also if Qt (or Paint) produces a 16-bit or colour mapped BMP then it will be lossy with respect to the original image. It seems the Qt output code will do the right default thing from a cursory inspection of the source (src/gui/image/qbmphandler.cpp).