--- In anaglyphs(-at-)yahoogroups.com, mark sroufe <markgambier(-at-)...> wrote:
>
> Fred, ... be sure that the resolution is at 72, not 300, or 120 (or any other number). 72 is the standard for images displayed on the web. You will notice that when you reduce the resolution that the pixel width and depth are automatically reduced as well.
>
>
Mark, I don't know PS but I doubt that it behaves like that.

The figures mentioned are a measure of the resolution of the output devices (screen, printer,...).
They are given in DPI and/or PPI and define only the output image size.
They have nothing to do with the (input) file size(kilobytes) or resolution given in pixel count (megapixel).

To verify: open an image in Irfanview and hit the 'I' key (ImagePropertees): type in new dpi values and verify that the filesize remains the same and only printsize changes.

In practice you can put any value as DPI/PPI because the output size in almost all cases is fixed: a 10x15cm (4x6inch) photoprint at 200dpi would require a 800x1200pixel input image and at 300dpi a 1200x1800pixel, but most labs don't even mention their dpi, so...
The same goes for your fixed sized screen of course.

One time where it cares is when you put images in PDF files.
Once the output size is fixed, say A4 (21x29,7cm) you need to specify the DPI to make the image fit in. For example if you want a 800x600pixel image to be 10x7,5cm (4x3inch) on a A4 PDF the DPI has to put to 200.

Hint for Fred:
Irfanview allows you to compress to a given file size via
menu File:Save for Web.
Put the Chromasubsampling to 'None' and use Automatic Preview.
Now compress untill artifacts are just not appearing (zoom in)
or Compress to size.
If still too big you have to reduce the pixel count
(use 800x600 iso 1024x768 for example).
Noise and sharpness also increment file size.

I like to keep the original anaglyphs in full size and in a lossless format (PNG) because today they have to fit on a PC screen but maybe in a few years time I will project them with a HD beamer...
(my original 3000x2000pixel anaglyph PNGs are between 5 and 9 Mbyte).

Regards.

Refs:
http://en.wikipedia.org/wiki/Dots_per_inch
http://en.wikipedia.org/wiki/Pixels_per_inch
http://en.wikipedia.org/wiki/Image_resolution
http://en.wikipedia.org/wiki/A4_paper#A_series

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