Basic Image
Information: (gifs, jpegs, and pngs)What
is the difference between JPEG, GIF and PNG?
jpeg
JPEG (Joint Photographic Experts Group)
JPEG is a standard format for compressing either
full-color (24 bit) or grey-scale digital images of "natural" (real-world)
scenes. In other words, it works best with things like photographs, high-grade
artwork, real-life scans, etc.; not so well on lettering, simple cartoons, or
black-and-white line drawings. JPEG handles only still images, but there is a
related standard called MPEG for motion pictures.
JPEG is "lossy", meaning that the image
you get out of decompression isn't identical to what you originally put in. The
compression algorithm uses the known limitation of the human eye, especially the fact that
the eye sees small details of light-and-dark better than it sees small color details.
So, JPEG compresses images that will be looked at by humans. It just happens
to do it well enough that we usually can't tell that information in the image has been
lost .....usually.
Quality vs. Compression qvc
A useful property of JPEG is that you can adjust
the degree of lossinesswhen you save images. This means you can trade off filesize
against image quality. The larger the filesize, the better the quality; the smaller
the filesize, the worse the quality. For good, full-color source images, a quality
setting of 6 to 7 (out of 10)in Photoshop is usually the best choice if quality is a
concern. Except for experimental purposes, never go above a setting of about 9;
using a setting of 10 will produce a file two or three times as large as a 9, but of
hardly any better quality. If you want a very small file
(say for preview or indexing purposes) and are prepared to tolerate large defects, a
quality setting in the range of 1 to 2 is about right.
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gif
GIF (Graphics Interchange Format)
GIF stands for Graphics Interchange Format.
It is either pronounced like the peanut butter (Jif) or like the sportscaster (Giff
Nielsen) ....no one seems to know for sure, but this page claims to have definitive
proof that it's like the peanut butter. At any rate, it was first developed in 1987
for Compuserve. They needed an image format that could be sent over slow network
connections. It is a lossless format, which means it retains the quality of its
image information when it compresses. The maximum number of colors a .gif can
contain is 256.
There are two GIF standards, 87a and 89a
(developed in 1987 and 1989 respectively). The 89a standard added better interlacing, the
ability to make part of an image transparent, and the ability to stack several images
together to create animations.
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png
PNG (Portable Network Graphics format)
In January 1995 Unisys, the company Compuserve
contracted to create the GIF format, announced that they would be enforcing the patent on
the LZW compression technique the GIF format uses. This means that commercial developers
that include the GIF encoding or decoding algorithms have to pay a license fee to
Compuserve. This does not concern users of GIFs or non-commercial developers.
However, a number of people banded together and
created a completely patent-free graphics format called PNG (pronounced "ping"),
the Portable Network Graphics format. PNG is superior to GIF in that it has better
compression and supports millions of colors. PNG files end in a .png suffix.
It stands a good chance of becoming widely supported as a standard Web
graphic, along with its animated sibling, MNG (pronounced "ming").
PNG is supported in Netscape 4.03 and above.
For more information, try the PNG home
page.
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when
When is JPEG best, and when is GIF the best choice?
JPEG is not going to displace GIF entirely.
For some types of images, GIF is superior in image quality, file size, or both.
One of the first things to learn about JPEG is which kinds of images to apply it
to.
Generally speaking, JPEG is superior to GIF for
storing full-color or grey-scale images of "realistic" scenes; that means
scanned photographs and similar material. Any continuous variation in color, such as
occurs in highlighted or shaded areas, will be represented more faithfully and in less
space by JPEG than by GIF.
GIF does significantly better on images with only
a few distinct colors, such as line drawings and simple cartoons. Not only is GIF
lossless for such images, but it often compresses them more than JPEG can. For
example, large areas of pixels that are all exactly the same color are compressed
very efficiently by GIF. JPEG can't squeeze such data as much as GIF does without
introducing visible defects. (One implication of this is that large single-color
borders are quite small in GIF files, while they are best avoided in JPEG files.)
Computer-drawn images (ray-traced scenes, for
instance) usually fall between photographs and cartoons in terms of complexity. The
more complex and subtly rendered the image, the more likely that JPEG will do well on it.
The same goes for semi-realistic artwork (fantasy drawings and such).
JPEG has a hard time with very sharp edges: a row
of pure-black pixels adjacent to a row of pure-white pixels, for example. Sharp
edges tend to come out blurred unless you use a very high quality setting. Edges
this sharp are rare in scanned photographs, but are fairly common in GIF files: borders,
overlaid text, etc. The blurriness is particularly objectionable with text that's
only a few pixels high. If you have a GIF with a lot of small-size overlaid text,
don't JPEG it.
Plain black-and-white (two level) images should
never be converted to JPEG; they violate all of the conditions given above. You need
at least about 16 grey levels before JPEG is useful for grey-scale images. It should
also be noted that GIF is lossless for grey-scale images of up to 256 levels, while JPEG
is not.
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