Les wrote:
Jpeg is a horrible compression algorithm. I have no idea why it was chosen. It is lossy, has artifacts in the result, often smearing the image or showing a fingerprint like Moire pattern (most image processing software has an anti-moire filter specifically tailored for JPEG ) and other artifacts. There are better compression algorithms, but many if not all are proprietary. Somewhere in Europe about 2005, there was a new lossless algorithm similar to LZ in compression size or slightly better, and HP licensed it, but I don't know the name. LZ and GIF are also lossless, but GIF somehow lost the ability to do color from what I understand. The problems endemic with images deal with the representation, and the problem with something called tilt, which is an artifact of the digital image capture method from CCD's. CMOS devices don't exhibit this or at least don't have to because the underlying technology is different.
Any image format is a compromise between conflicting needs. You want an image format that expands without practical limit? You need an algorithmic format. It's what postscript (without bitmaps) does.
You want an exact image to some defined precision. Choose dots per inch in two dimensions plus some number of bits per pixel to represent colour: 8 bits (256 colours) is generally regarded as pretty horrible.
That's what a bitmap does, and it uses a lot of storage space. Images are compressible to varying degrees depending on the amount of detail they contain.
JPEG is a compromise, chosen because it's more compressible, at some loss of detail. The amount of compression is selectable, and the more compression the more loss of detail.
It's used because it can give good compression, and in many cases the lost detail doesn't matter too much. JPEG images out of my cameras look fine on my computer. My Canon claims to be able to store around 1000 photos at its highest resolution on my 4 Gb CF card. However, if I want to get the best quality images my camera can do, I use its native format, called "raw," and store many fewer images on the CF card.
According to my camera's handbook, raw images occupy about 8.7 Mbytes at its highest resolution. I have several choices with JPEGs, the best quality & largest takes 3.8 Mbytes
Some image compression algorithms are described as "lossless," meaning that they preserve the original detail.
Others, such as that used in JPEG, are lossy - they sacrifice detail to save on storage.
Poor image quality is more likely to result from use of digital zoom. Here is one pixel at the camera's native size.
. Here is the same picture, zoomed to twice the linear size: .. .. Properly scaled, it's a square box. -- Cheers John -- spambait 1aaaaaaa@xxxxxxxxxxxxxxxx Z1aaaaaaa@xxxxxxxxxxxxxxxx -- Advice http://webfoot.com/advice/email.top.php http://www.catb.org/~esr/faqs/smart-questions.html http://support.microsoft.com/kb/555375 Please do not reply off-list
