Warp wrote:
> James <hy5### [at] hanmailnet> wrote:
> 
>>Now, our images from POV-RAY are 72
>>pixels/inch.
> 
> 
>   That's in no way POV-Ray's fault. It's just the program you are using
> (photoshop perhaps?) which by default prints things at 72 dpi. Just tell
> that program to print at a higher dpi.
>
Besides that, the dpi (or ppi, same thing) that's stored in an image 
file is nothing more than a data value stored in the file header.  It 
has NO effect on the image data itself.  What's more, this value can be 
changed arbitrarily by most graphic editing software -- sometimes it can 
be obscure to find, but it's generally there someplace.  In Paint Shop 
Pro for example, it's in the resize dialog box.  But in reality this 
stored dpi value IS TOTALLY IRRELEVANT -- forget about it.  The key 
point (as everyone has been telling you) is the image size in pixel 
dimensions.

If you are aiming at a specific size/quality, start from there and work 
backwards, the math is straight-forward.  For an arbitrary example, say 
you want a 20" x 30" poster.  If it's to be printed at 100 dpi it has to 
be 2000 x 3000 pixels.  300 dpi would be 6000 x 9000 pixels.  600 dpi 
would be 12000 x 18000.  And so on...  (Be prepared for loooooong 
renders -- quality takes time.)

Now, the 72 dpi value you see all the time is used simply as an average 
video screen resolution.  Naturally, the real ppi value varies with the 
screen size and resolution, but 72 is used as a typical average.  That's 
why most graphics software uses this as a default dpi value.  But again, 
it has no real significance.  When an image is displayed on a video 
screen, the stored dpi value is completely ignored -- after all, the 
screen has a fixed resolution and can't use anything else.  OTOH, 
printers can, and the printing software can adjust the output for 
different dpi resolutions.  It might start with the stored dpi value, 
but it can adjust it from there, so it's still basically irrelevant.

      -=- Larry -=-

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