Web Picture Quality

by : Paul Hailey

bitmap, gif and jpeg. picture formats are commonly used.

I use jpeg format for converting most big pictures, and reduce the dots per inch (dpi) to 70 or below, for a web page.

To keep jpeg file sizes under 50k, I use 30 to 60 on the 'compression' scale for jpegs ( a scale runs from 2 as high quality, to 255 for low quality. )

Because many users employ a 256 colour palette, gif's are still much in use, but they only handle up to 256 colours. I use gifs for icons and small pics.

I use 24 bit colour bmp's for archiving (I use 300 dpi in case I might print them later), they can be used as a source for both gif and jpeg.
1.bmp (bitmap)
This is a pixel-mapped format. I scan pics at 300 dpi -this is about the maximum for a printer, but too good for the web.

300 dpi 10 by 8 inch pictures, take around 20 megabytes file size. 300 dpi bitmap scans are ideal for archiving, as the place to start processing.

NOTE - Netscape Browser does not appear to handle bmp's!


Gif's are useful for small pics like icons, and also come in 'animated' form useful for websites.

Any scanned colour pic should be processed to 256 colour FIRST, then the dpi reduced to a maximum of 72dpi for screen use, then saved as the gif.

The basic colour palette options are (in PhotoPaint) :
'uniform palette'
- keeps to colours that a 256 colour screen uses. This is good where multiple pics are on one webpage, because the browser might have only those colours to use, (someone correct me if that's wrong?)

'adaptive palette' and 'optimised palette'
These both select the 'best' 256 colours from the picture itself -. Pics done with these options may not look well on pages with many pictures, as the browser page may be limited to 256 colours in total. A picture on its own on a page will look ok.

3. jpegs (jpg)

The jpeg compression works directly from a 24 bit colour palette if necessary, whereas gifs are 256 colours maximum.

The standard compression method is type 4:4:4 on Photopaint. There is a scale 'compression' from 2 to 255. 2 is highest quality, biggest file, 255 is lowest quality, smallest file.

Here are the pic filesizes I got from different compression of a 70 dpi, 10 by 8 inch scan of a full colour (24bit) photograph:

Original bmp file .. appx 20mb
jpg compression 2: 217kb
jpg compression 15: 79kb
jpg compression 30: 51kb ..reccommended
jpg comression 50: 36kb ..reccommended
jpg compression 150: 20kb.

Even the 20kb version was adequate for pictures say up to 5 by 3 inches on an 800 by 600 pixel screen resolution. 30 to 60kb seems a good trade off between file size and quality, but the file size you get relates to the picture composition. This would seem a good range for 'average' sized jpg web pictures without noticably losing quality.

4. jpg versus gif
gifs are good for icons and small pics - these can be in 16 colours which makes very small files.

jpeg is a compression facility and does not do so well with 16 colour or small pics, unless the compression number is increased. jpeg is really for compressing big pictures, lots of data, 24 bit colour.

With gif you do not get the options for palette selection and compression levels that jpeg gives.

I use gifs for icons and small pics, jpeg for the rest.

5.Black and white.
For texts, scan textual material in B&W even if there are colour pics on the page. Separately scan the colour pics, in colour. Black and white is 1 bit not 8 bit - file sizes are much smaller than the equivalent in colour.

I have received text scans in colour - usually the black is grey rather than black. If 'ocr' software won't scan them because the resolution is too low (and it usually is!), I do the next best thing and convert the pic to B&W. The process usually involves darkening the colour image as much as possible, to increase the blackness of the characters.

Then convert to black and white using the 'threshhold' control. This threshhold is just the point at which the analyser decides it is a 'black' pixel square, not 'white'.

The threshhold range is 0 t0 255, in the range from 127 to 220 does the trick, dependent on the colour original, and the allowable extra black 'dots' you get if the value is set too high.

File sizes can reduce a text page from 100kb jpg to 20kb gifFeature Articles, just by going to b&w. Somehow I prefer gifs for black and white - I feel gifs are better suited to simpler jobs than jpeg.