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I've heard bitmap (.bmp) is an uncompressed image format to store images. Does it mean that it can store images in high quality i.e. with no loss of quality in other words, lossless format?

I'm confused between the terms 'uncompressed' and 'lossless' here. are they both same?

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3 Answers 3

up vote 11 down vote accepted

The terms "uncompressed" and "lossless" mean two different things. Compression exists in two forms, lossy (like JPG) and lossless (like ZIP).

Lossless compression is typically used for programs and text files. It removes redundant information from the file to compress it. It generally doesn't do a very good job of compressing media files.

Lossy compression achieves a high compression rate, even for data types that usually don't compress well using lossless compression. It does this be removing information that is non-critical. JPG compression, for example, removes image details that are virtually invisible to the human eye. MP3 compression removes sounds that the human ear can barely, if at all, hear.

The BMP format is both lossless and uncompressed.

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2  
The BMP format is both lossless and uncompressed, and can still be compressed in say an archive, making it lossless only. They compress up well in a compression archive. –  Psycogeek Feb 4 '12 at 21:12
    
Is BMP the only uncompressed image format around? –  user1032421 Feb 4 '12 at 21:29
    
Not necessarily -- both GIF and TIFF have uncompressed options. –  Mike Scott Feb 4 '12 at 21:32
    
Sorry, I don't understand how BMP is both lossless and uncompressed file format. How I understand is that a file can be either uncompressed or compressed (lossless), so how can it be both? –  user1032421 Feb 5 '12 at 0:41
3  
@user1032421 Pretty much every uncompressed format is lossless. –  David Schwartz Feb 5 '12 at 2:16

Uncompressed and lossless are not the same. Some compression formats are lossy (for images, the most common one is JPEG) and others are not (the compression methods used by GIF, PNG and TIFF images, for example). You can use BMP images to prevent loss of quality, but you can also use PNGs and save your disk space.

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For storing your images with no loss in quality (if this is what you wanted to do) I recommend .png. You can make the size smaller than .bmp and the image will still be 100% perfect.

A longer explanation:

Uncompressed

Not compressed. Not made smaller. If we are dealing with Images then I believe .bmp is the only (or at least the most 'common') uncompressed format available. Not recommended for nearly any use these days. To make a metaphore we could say that if we wanted to leave a blanket uncompressed, we would do nothing to the blanket and just spread it out like normal.

Compressed

File has been made smaller. Can be one of two general styles of compression: Lossless or lossy. Both make the file smaller but in different ways. Lossy generally makes the file much smaller than lossless but loses information about the original. Lossy compression is used with media files much more often than lossless.

Lossless

Does not lose any of the original information. The original uncompressed file can be reassembled from this. If we are talking about images the most common format is .png. If it's just generic files the most common is .zip. To continue the blanket metaphore: if we wanted to compress the blanket in a lossless way we would fold it a few times so it is smaller.

Lossy

Loses information about the original to save space. Images and music and video are commonly compressed in a lossy way. Information about the original file is irreversibly lost. In music for instance when compressing in a lossy manner we might lose the very high or the very low notes on the assumption that most of us wouldn't hear them anyway. For music .mp3 is the most common, and for images .jpg/.jpeg (same thing) is most common. For video, the file extension doesn't represent the way it was compressed -- the file type is just a 'container' for video and audio media. A few common methods of video compression are divx (not used these days) xvid (falling out of favor but still used) and h264 (new and gaining favor) with .avi being the most common container. You cannot use lossy compression on non-media files; it will break them because all of the information is needed. An example of lossy 'compression' of the blanket could be cutting the edges of the blanket until it fits on your bed perfectly, then folding it up.

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