JYR <jyr### [at] hotmailcom> wrote:
> right     x*image_width/image_height

  Using that is not a good idea. There are several reasons:

  If the aspect ratio of the image is adjusted automatically like this,
it means that if someone renders the image with a resolution having a
different aspect ratio than what the author intended, the resulting image
will be either cropped or parts of the scene not seen in the original will
come into view. In many cases this is a bad thing because it affects (usually
negatively) the composition of the scene: Important parts of the scene may
get cropped out of view, or parts of the scene not intended to be seen
(eg. because of lack of modelling) suddenly come into view. If someone
does this he might often not even notice that there's something wrong with
the image.

  If the viewing angle is adjusted with the 'angle' keyword
(as is rather usual), it introduces a rather unwanted effect when
used in addition to the automatic aspect ratio setting with 'right':
The aspect ratio of the image will in this case always be done
in the vertical direction, either by cropping or extending the image in
the upper and lower sides. This means that if you try to eg. render a
"widescreen" version of the scene, you will only end up cropping the
image from the upper and lower sides (instead of adding new scenery
to the left and right sides). More often than not, scenes are designed
to be rather "panoramic" in nature, ie. the image can be expanded
horizontally. However using the automatic aspect ratio correction along
with a fixed 'angle' setting will not achieve this.

  The automatic aspect ratio correction given above assumes that pixels
are square. There are cases where one *wants* to render an image
for a resolution with non-square pixels (examples include the Windows98
startup screen, which uses a resolution of 320x400, and a DVD movie,
where the video has always an aspect ratio of 1.78:1 although it can then
be stretched to the intended aspect ratio, eg. 2.35:1 by the player). If
the automatic aspect ratio correction is used, it would not be possible
to render an image with non-square pixels without modifying the scene.

  The usage of the keywords image_width and image_height are due to change
in future versions of POV-Ray because they break the frontend-backend
boundary, which can cause problems when more complex ways of rendering
scenes are introduced to the program.

-- 
                                                          - Warp

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