jpeg vs bmp?

[JT]

What is the difference between jpeg and bmp? The obvious difference is file
size but when I compare a picture in jpeg and bmp I see no difference. I
assume that bmp is better quality? If I save a bmp as a jpeg is information
lost or does returning the jpeg to a bmp restore everything?

 

[John Dingley]

You are correct. The file size of these image files usually indicate the
"quality" of the image.

You may not notice any difference at lower resolutions/magnification but
when you try and zoom in on the lower quality image the difference becomes
clear.

It only really makes a difference when you are putting images on web sites
when it needs to look good at the lowest possible file size.

 

[JW]

The BMP (Bitmap) will not loose any quality no matter how
many times you open it or zoom into it, however the
compressed jpeg will gradually loose quality, particularly
if you zoom in to max on a specific area (say a face for
example). Jpegs are substantially smaller so better for e-
mailing etc.

 

[Larry(LJL269)]

On Tue, 07 Oct 2003 09:16:00 GMT, "JT"

|If I save a bmp as a jpeg is information
|lost or does returning the jpeg to a bmp restore everything?

YES & NO. U need different filetyped for differnt
purposes.
X overview http://www.scantips.com/basics09.html
Clk JPG link @ bottom 4 all gory details.

HTH - Larry

Any advice given is my attempt to show appreciation for all
the excellent help I've received here but I'm no MVP so it
may only apply NUGS. Personal attacks, nitpicking & criticism
of anything but content will NOT be responded to. Those
posters should spend their time taking the test @
http://www.nimh.nih.gov/publicat/ocdtrt1.htm

 

[Richard Mason]

What is the difference between jpeg and bmp?

On screen a pixel color is defined by three bytes, one each for
Red-Green-Blue. For a 24 bit image the color data for each pixel in a
bmp file is stored in three byte order but reversed i.e. B-G-R. What
one sees on screen is exactly what's in the file. A bmp is actually a
file format that describes how a bitmap is stored.

Jpeg is not a file format but a complicated compression scheme. The file
format is JFIF, but nobody calls it that. Jpeg compression is also used
with the Exif file format used in digital cameras.

The obvious difference is file size but when I compare a picture in
jpeg and bmp I see no difference. I assume that bmp is better quality?

There is a difference as jpeg is a lossy compression scheme, its just
that the difference may not be immediately apparent to the eye. It
depends on the degree of compression and how abruptly colors change in
the image. Convert a bmp consisting of, say, color bars to jpeg and the
difference will be readily apparent even at minimum compression (best
quality).

If I save a bmp as a jpeg is information lost or does returning the
jpeg to a bmp restore everything?

Information is always lost and you can't restore the lost information by
converting back to a bmp.

 

[Richard Mason]

On Tue, 7 Oct 2003, JW wrote in

The BMP (Bitmap) will not loose any quality no matter how many times
you open it or zoom into it,

Zooming in on a bitmap, i.e. magnifying, doesn't reveal more detail in
the image because there is no more detail to reveal. When magnifying,
the display software adds pixels that were never in the original image
so, while it may not look too bad, zooming in is actually a distortion
of the original image.

however the compressed jpeg will gradually loose quality, particularly
if you zoom in to max on a specific area (say a face for example).
Jpegs are substantially smaller so better for e- mailing etc.

A jpeg is decompressed before it is displayed and once decompressed it
is just a bitmap and no different to display of the bitmap in a bmp. The
lower quality (artefacts) of a jpeg are already there as a result of
the compression process and there is no "extra" loss of quality in
zooming a bitmap from a jpeg then there is from a bmp.

 

[Larry(LJL269)]

I think of it in terms of transforms, each time u
introduce a little error. But 2nd time u open it, u get
the little error + error from 1st time, And so on.
Probably by 1000 time u open it, it looks like hell.

I think if u open it & close without rewriting(which u
can do with PI), image wont be any worse.

Point too is u may need several different files of same
pic- jpeg for web, gif 4 thumbnail, tif for printing &
archival storage.

URL in my other post has best explanation I ever read
but is for advanced(picky) photo nuts like me.

Comments/suggestions/corrections appreciated.
Larry

On Wed, 8 Oct 2003 00:53:33 +1300, Richard Mason

|On Tue, 7 Oct 2003, JT wrote in <<kvvgb.35127$9l5.31201@pd7tw2no>>
|
|>What is the difference between jpeg and bmp?
|
|On screen a pixel color is defined by three bytes, one each for
|Red-Green-Blue. For a 24 bit image the color data for each pixel in a
|bmp file is stored in three byte order but reversed i.e. B-G-R. What
|one sees on screen is exactly what's in the file. A bmp is actually a
|file format that describes how a bitmap is stored.
|
|Jpeg is not a file format but a complicated compression scheme. The file
|format is JFIF, but nobody calls it that. Jpeg compression is also used
|with the Exif file format used in digital cameras.
|
|>The obvious difference is file size but when I compare a picture in
|>jpeg and bmp I see no difference. I assume that bmp is better quality?
|
|There is a difference as jpeg is a lossy compression scheme, its just
|that the difference may not be immediately apparent to the eye. It
|depends on the degree of compression and how abruptly colors change in
|the image. Convert a bmp consisting of, say, color bars to jpeg and the
|difference will be readily apparent even at minimum compression (best
|quality).
|
|>If I save a bmp as a jpeg is information lost or does returning the
|>jpeg to a bmp restore everything?
|
|Information is always lost and you can't restore the lost information by
|converting back to a bmp.


Any advice given is my attempt to show appreciation for all
the excellent help I've received here but I'm no MVP so it
may only apply NUGS. Personal attacks, nitpicking & criticism
of anything but content will NOT be responded to. Those
posters should spend their time taking the test @
http://www.nimh.nih.gov/publicat/ocdtrt1.htm

 

[Richard Mason]

On Wed, 8 Oct 2003, Larry(LJL269) wrote in

I think of it in terms of transforms, each time u introduce a little
error. But 2nd time u open it, u get the little error + error from 1st
time, And so on. Probably by 1000 time u open it, it looks like hell.
I think if u open it & close without rewriting(which u can do with PI),
image wont be any worse.

You get a loss of quality (error) only in saving an image to a jpeg
file, not when opening it.

URL in my other post has best explanation I ever read but is for
advanced(picky) photo nuts like me.

Yes, it's a good explanation.

 

[Alex Nichol]

What is the difference between jpeg and bmp? The obvious difference is file
size but when I compare a picture in jpeg and bmp I see no difference. I
assume that bmp is better quality?

Both start as bitmaps, one group of bytes per pixel, and BMP is a
straightforward saving of those (plus a little information on things
like how many bytes per pixel (color depth) and dimensioning, in a
header)

JPEG files are compressed, according to methods worked out by the Joint
Photographic Experts Group, so as to make a smaller file. That implies
loss of detail; the trick is that this is done in ways that do not show
up to the human eye when doing it on photographs. Saving in JPEG from
any good program allows you to select a compromise between files size
and loss of quality. Only if you go right to the smallest size end are
you likely to see appreciable loss. But if you load the result into a
graphics program and save as JPEG again, the losses are compounded - so
if you edit photos you should retain the original.

There are also TIFF which will compress without loss, but not usually by
much on a photo, and GIF which will compress without loss quite a lot on
text or simple areas of uniform color and is used for logos and the like
in web pages

 

[Larry(LJL269)]

On Wed, 08 Oct 2003 02:44:13 GMT, "Larry(LJL269)"

|Point too is u may need several different files of same
|pic- jpeg for web, gif 4 thumbnail, tif for printing &
|archival storage.
1-TIFF & BMP's are same size but TIFF with LZW
compression (16k,?) is 1/3 size of BMP
2-TIFF image files with LZW lossless compression.
Lossless means there is no quality loss due to
compression. Lossless guarantees that you can always
read back exactly what you thought you saved,
bit-for-bit identical, without data corruption. This is
very critical factor for archiving master copies of
important images.

Any advice given is my attempt to show appreciation for all
the excellent help I've received here but I'm no MVP so it
may only apply NUGS. Personal attacks, nitpicking & criticism
of anything but content will NOT be responded to. Those
posters should spend their time taking the test @
http://www.nimh.nih.gov/publicat/ocdtrt1.htm