LATEX requires you to specify a unitlength, the picture size, and an offset, which determines the picture coordinates of the lower-left corner. Picture objects' locations are specified to LATEX in picture coordinates. When composing a mathematical figure, however, it is generally easier to use Cartesian coordinates. Letting software handle this conversion not only frees you from worrying about (non-portable) picture coordinates, but also makes it easy to change the size of the figure, and to place the figure exactly where LATEX has left space for it. If you've ever had to change the unit length of a figure (for example, because you are American and your coauthor is not), only to have the picture become horribly ugly, overlap the surrounding text, or otherwise break the document, you'll understand why automatic coordinate conversion is a useful feature.
There is one situation where Cartesian coordinates are not adequate,
namely when you are doing visual editing to place a text label in a
figure. A label's size is independent of both picture coordinates and
Cartesian coordinates, so if you use either to position a label,
changing the scale or the Cartesian bounding box will probably break
the positioning. However, true coordinates (measured on the
page) are no good, either, because the size and/or scale of the
picture may change with further editing. ePiX circumvents this
difficulty by allowing labels to be placed in Cartesian coordinates,
but to be offset in true coordinates. For instance, the labels on a
coordinate axis need to be positioned at the proper location in the
Cartesian plane, but because LATEX uses the basepoint to position a
box, the raw Cartesian coordinates are likely to mis-position the
labels. If you fine-tune the labels' positions in Cartesian
coordinates, however, the picture will break if the scale is
changed. Using both Cartesian and true coordinates allows you to
position labels easily so that they are placed correctly if the
\unitlength
is changed, or even if the Cartesian bounding box
is changed.
Aside from labels, all positions in an ePiX file are specified in
Cartesian coordinates. ePiX asks LATEX to set aside a box of size
h_size
v_size
, then does affine scaling on the
rectangle [x_min, x_max]
[y_min, y_max]
to get an
exact fit. You can still force objects to go outside the bounding box,
by giving them Cartesian coordinates outside the box. Positive offsets
in ePiX shift the picture up and right (the opposite of
LATEX picture offsets). ePiX's output is accurate to
starunitlength.