Jim Charter wrote:
> Don't you want to render your image at a higher resolution?  An 
> Photoshop will allow you to change the ppi under
> Image->ImageSize | Resolution
> but all that it is doing is resizing the image to a larger resolution, 
> using, I guess, some mode of interpolation.  But it really is not 
> increasing the amount of information beyond what was in the original 
> image.  I suspect the new resolution it chooses relates to the 133 lines 
> per inch standard of commercial printing?

Sorry to say, but your guessing is not only wrong but also superfluous: 
Photoshop has two modes of operation for changing the resolution.  This is 
easily controlled in the dialog you mentioned with the "Resample Image" 
option.  Disable it, and Photoshop just changes the resolution, which 
effectively changes the image size in real-world units (i.e. centimeters, 
inch, etc) without changing any pixel data.  On the other hand, if you 
enable "Resample Image", Photoshop will interpolate (using the specified 
method in the popup menu next to the resampling checkbox) the pixel data 
maintaining the same image size in real-world units.

About pixels:

A pixel is nothing more than an atomic unit of color information.  It has no 
resolution whatsoever, it does not have a unit it is measured in.

The resolution is implicitly defined by taking a (usually rectangular) block 
of many pixels, commonly called an image, which is a specified number of 
pixels wide and high.  You then can define that the width of the image, lets 
say it is 1000 pixels, shall be interpreted as being 2 real-world units in 
width.

A real world unit could for example be an inch.  In this case, there would 
be 1000 pixels  to be fitted such that they are two inches in total.  Thus, 
each pixel is interpreted to be 2 inch divided by 1000 pixel, which equals 
500 pixels per inch.  You can just as well replace the two by 200, and then 
  you get 5 pixels per inch.  Or you use some other unit to map the pixel 
to.  The essence is, a pixel has no dimensions, and thus no unit can measure 
its size.

In summary: A pixel is just a color information you can interpret to have 
any width and any height you like.  And that is what is referred to as 
resolution.

	Thorsten

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