Resolution for scanning images for the web

[Chuck Bearden]

On Tue, 17 Aug 1999, Bob Rasmussen wrote:

> On Tue, 17 Aug 1999, Yvonne Reynolds wrote:
> 
> > 
> > Hi!
> > May I ask a very basic question?  Does everyone scan images to be 
> > used on the web (slides and photographs) at 72dpi/ppi?  Or do you 
> > scan at a higher resolution and then resample?  Pros and cons of 
> > either?   Which is the preferred format for storing the images - tiff 
> > or bmp before converting to jpeg?
> 
> I know some general cross references have been given, but let me point out
> just a couple things:
> 
> 1. 72 dpi is a theoretical pixel density for a screen. It's related to your
> scanning decision, but indirectly. Decide how many pixels wide (or high) you
> want the picture on the screen. Then measure your picture and do the math.
> 
> 2. I prefer to scan at the intended display density. Resampling (resizing)
> never gives you MORE information than you started with, and often gives you
> LESS.
> 
> 3. If your scanning software can't output jpeg, then tiff vs. bmp shouldn't
> matter much. Note, though, that there are many "flavors" of TIFF, and you
> could have problems with an unsupported flavor. I've never had that problem
> with BMP.

I would concur with Bob Rasmussen's advice, except that on #3, I 
wouldn't output my scans directly to JPEG which, even at its best, 
loses color information.  If you are loading your scans into 
Photoshop, I suspect you won't have problems with TIFF format issues.  
I've moved TIFFs back and forth between Photoshop 4.0 for PC and 
for Mac, and the GIMP under Linux with no problems.  My guess is that 
PaintShop Pro will also understand many TIFF variations.  BMP is 
probably also fine.  I'm not sure, but I don't think it loses color 
information.

A good general guideline is to save a high-quality TIFF of your scan 
as a reference copy, and then convert copies of it either to JPEG or 
to GIF.  

As for scanning at a higher resolution and rescaling down, I find 
little difference if any in photographic images between ones scanned 
at the target resolution and ones scanned at a higher resolution and 
sampled down.  My suspicion is that the same limitations of human 
perception that permit JPEGs to lose a fair amount of color 
information with little detectible difference in quality also make it 
difficult to detect the above-mentioned differences between resized 
images and those scanned at the target resolution, when the images 
posess large color palettes and complex gradations of shade and hue.
BTW, try to make saving the image as a JPEG the last thing you do, 
since committing it to JPEG means losing color information.

Line art, maps, and logos are a different mantter.  Because of their 
typically high degree of contrast and limited color palettes, 
rescaling down produces more noticeable effects.  See a small website 
of examples I created at:

  http://edwards.rice.edu/scanning/examples/example_03.html

BTW, while indexed GIFs or PNGs are typically the target formats of 
choice for such images, never convert your image to indexed color 
until you have completed all manipulations to the image.  The only 
manipulations you should perform after indexing the palette of an 
image are adding transparency and interlacing.

I hope this helps.

Chuck
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 Chuck Bearden                                      cbearden at rice.edu
 Electronic Resources Librarian                                   
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 P.O. Box 1892                                                    
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