[Ipe-discuss] Re: Perspective

[Sherif Ghali]

> From: Tim Hutt <tdhutt at gmail.com>
> To: ipe-discuss at cs.uu.nl
> Sent: Wednesday, March 11, 2009 1:36:51 AM
> Subject: Re: [Ipe-discuss] Re: Perspective
> 
> > But just to make sure: so there is no general perspective
> > transformation available in ipe?
> 
> Not as far as I know. That would be a good addition but I'm not sure
> how you could do it usefully. In the past I've made a 3D model in
> blender and then traced over it in IPE.

You can still draw perspective with Ipe by specifying the vanishing
line for each set of parallel planes, the vanishing point for each set
of parallel lines, and ensuring that incidence relationships hold.

Ipe's layers are then very handy: you can leave behind a "drafting
layer" that does get saved in the xml and pdf files, so you can edit
the file later, but that does not show up in the diagram (by
unchecking the layer).

If your figure has curves, this can quickly become very tedious.
You can use Blender, but I'm curious whether the image you then
traced using Ipe was a raster image (the final diagram is then only
approximate).

Another problem with hand drawing is that you cannot change your mind
about the point of view; you have to start from scratch.

One way to obtain accurate perspective is to draw the main lines
programmatically and then trace the rest of the diagram/annotations
in Ipe. For an example, download
    http://www.cs.ualberta.ca/~ghali/IGC/code/igc-code.tgz
and run pdftoipe on
    igc/15/t3-perspective-with-clipping/psout/t3-cylinder_12-clip-00--+2p5.pdf
itself generated from the source code in
    igc/15/t3-perspective-with-clipping/t3-perspective.C

The full recipe is:
> cd geometry_lib
> make
> cd ../15/t3-perspective-with-clipping
> make
> ./t3-perspective
> cd psout
> make         (this requires epstopdf)
> pdftoipe t3-cylinder_12-clip-00--+2p5.pdf t3-cylinder_12-clip-00--+2p5.ipe
> ipe t3-cylinder_12-clip-00--+2p5.ipe

If you're interested in what is happening, see Figure 18 in 
    http://www.cs.ualberta.ca/~ghali/papers/ogp.pdf

Cheers,
Sherif