[Ham-Computers] Still Need Help

Tue, 18 Nov 2003 03:02:05 -0500

excellent details, exactly why autocad deals well with a GIF

jim VE3 JIW

Jay Eimer wrote:

>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
>>    
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>use -
>  
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>>when not to use it at all, the right resolution setting - 300 pi, 150 pi,
>>    
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>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
>  
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>>>typewriter ink text on a white background.
>>>      
>>>
>>    
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>>>This smudge, or whatever it is, occurs only in text areas. The colors of
>>>      
>>>
>the
>  
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>>>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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>>
>>    
>>
>
>
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>  
>