Hi!

I was surprised to find that povray doesn't support an image map type
that does a projection from a single point through a planar (possibly
tiled) image. The effect would be very much like a slide projector where
a point light source sends rays of light through a slide some distance
away from it. I would like to see this added in a future version. Is
this group the correct place to make such a suggestion?

To be more precise: I'd embed the user's image at z=0 in a square with
edges of 2 unit lengths and centered on the z-axis. In other words, the
lower left corner of the input image would be at <-1, -1, 0> and the
upper right corner at <1, 1, 0>. This planar image would be turned into
a 3D "block of clay" by adding lines through the point z = <0, 0, 1>.

In other words, for every pixel of the object to be textured, compute
the distance to <0, 0, 1> and scale it so that the z coordinate of the
distance becomes 1. Then the x and y coordinates will give the
corresponding image position. Depending on the use of the "once"
keyword, coordinates would be mapped to or cut at the [-1, 1] range.
If the positoon to be mapped already has z = 1, then the image map
cannot provide any data, and default texturing should be used instead,
i.e. transparent pigment, no normale modification, or whatever.

The choice of coordinates is such that a matrix transformation can be
easily used to adjust the position and direction of the slide as well as
distance of the projection point. Two columns of the transformation
matrix would describe the orientation and size of the slide in space,
the third would give the distance vector between its center and the
projection point. The translation column would translate this whole
setup, thereby determining the location of the center of the slide. It
might make sense to provide a macro which computes this matrix from four
suitable vectors, e.g. three slide corners and the projection point.
Having the image start at <0, 0, 0> instead of <-1, -1, 0> would make
the transformation even easier, but would require a shear transformation
in the case of a centered image, which I would expect to be the most
common use case.

Possible applications include conformal (i.e. angle-preserving) textures
on parts of a sphere; textures on infinite parametric objects with
limited angle between the asymptotes (e.g. hyperboloid); projection of a
previously ray-traced scene (with a perspective camera) onto a (probably
simpler) object; and probably many more.

I've had a look at the code in imageutil.h. Adjusting map_pos to provide
one more map type should be simple enough. Writing the code for the
mapping described above is easy as well. The undefined case z==1 can be
easily handled by returning 0 from the new slide_image_map function,
causing map_pos to return 1 as it does for the other map types. I'm not
sure about the role of the image->Gradient vector. It seems that this
vector is initialized to  <1, -1, 0> but never modified afterwards.
Could this simply be removed from the code, or should new map types
honour its value in any way?

Similar questions arise in imageutil.h, where other map types are
listed, like PARABOLIC_MAP and so on. Are they listed for historic
reasons? Should new mapping types follow those already listed? Or should
OLD_MAP always be last? Having a gap between the existing
implementations for type 0 through 2 and a new implementation for type 8
or so might confuse readers of the documentation.

If developers are interested in having this feature, I'd be willing to
write a patch. Answers to the above questions would increase its quality.

If I fail to reply to postings in this group, please send me a personal
email instead.

Greetings,
 Martin von Gagern

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