[Ham-Computers] Still Need Help

Sat, 15 Nov 2003 10:11:00 -0600

I can't give you all the answers you requested, but here's a basic.

.bmp stores the actual color values for each pixel.  Images are very large
files.  Images with high resolutions are MUCH larger.  Avoid if possible,
especially for web presentation.

.jpg uses aliasing for compression.  This means that for each "level" of
compression, a square of 9 pixels are turned into 1 pixel, by setting it to
the average of all of their color values.  The next higher compression would
be to average 16 pixels into 1 (4x4 square) and then 25 pixels (5x5), etc.
With photo realistic color pictures, this results in loss of detail.  You'll
lose "fine lines".  On the other hand, gradual color changes (gradients,
fades) just become more abrupt, but are often not noticable unless compared
to the original.  On the other hand, very sharp, high contrast transitions
are "blurred" because the light and dark pixels immediately adjacent to each
other are averaged into a medium color.  A black and white line drawing
(like a blueprint) gets turned into something that looks like it was drawn
by a three year old with a crayon.

.gif uses lossless "repeating bits" compression.  For gradual color changes,
this accomplishes nothing, since, say in a photo, you rarely have 2 colors
adjacent to each other that are "exactly" the same.  There are millions of
variations in skin tone, for example, even in one face.  But for a line
drawing, it's not uncommon for the compression to do things like this:

One black pixel, followed by 4000 white pixels, followed by one black pixel
(stored by a "color" value and a "repeat" value in just two bytes).  Repeat
this a few hundred times, and you'll see that it comes out as two black
vertical lines, 4000 pixels apart, but each "line" of pixels is stored in
only 6 bytes.  The picture can be recreated "mathematically" and exactly,
with no JPG style "jaggies" and no loss, and it will be significantly
SMALLER than a jpg of the same image.

So, JPG for photos or items with lots of COLOR.  GIF for line drawing or
things with lots of CONTRAST.

Jay
AD5PE
----- Original Message ----- 
From: "Duane Fischer, W8DBF" <[email protected]>
To: <[email protected]>
Sent: Friday, November 14, 2003 1:47 PM
Subject: Re: [Ham-Computers] Still Need Help

> What might be most helpful to many list subscribers, is for one of you
photo
> and/or scanner wizards to explain what type of image (.bmp, .gif, .jpg
etc)
> should be used for what and when, when to use compression - how much to
use -
> when not to use it at all, the right resolution setting - 300 pi, 150 pi,
75 pi
> etc.
>
> Duane W8DBF
>
>
> ----------
> From: Michael Neverdosky <[email protected]>
> To: [email protected]
> Subject: Re: [Ham-Computers] Still Need Help
> Date: Friday, November 14, 2003 1:56 PM
>
> "Duane Fischer, W8DBF" wrote:
> >
> > I then changed it from .bmp to .jpg, lowered the compression down to 90%
and
> > saved the image.
> >
> > When a friend viewed it there were some black smudges in text areas,
black
> > typewriter ink text on a white background.
>
>
> > This smudge, or whatever it is, occurs only in text areas. The colors of
the
> > card are fine and clear.
> >
>
> The smudge is an artifact of .jpg compression.
> For line art or text you want to use .gif if you need a compressed
> format.
> .jpg is great for photo realistic scans but can't handle very high
> contrast
> transitions like black text on white background.
>
> Use gif.
>
> michael N6CHV
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