Problem with external graphics and 3D pgfplots

Question

I want to include an external graphics with pgfplots. I tried to follow the instructions of the manual (p45 et seq.) as carefully as possible. But the result is hoorible as hell.

Here is the code I am using:

\documentclass[12pt,twoside,a4paper]{report}
\usepackage{tikz} 
\usepackage{pgfplots}
\pgfplotsset{compat=1.6}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
axis on top=true,
grid=both,minor tick num=1,
xlabel=$x$,ylabel=$y$,zlabel=$z$]
\addplot3 graphics[
points={%difference of canvas y coord. because pgfplots counts from the lower left.
(353,754.7,196) => (2,1192-944)
(356,757,198) => (476,1192-153)
(353,758.5,198) => (188,1192-1)
(356,753.3,196) => (242,1192-1192)
 }] {Example.png};
\end{axis}
\end{tikzpicture}
\end{document}

And here is the graphics I try to include:

Here is the same graphics in which I marked in orange the 3D-coordinates and below in black the canvas coordinates (which I read out via paint).

And here is the same graphics in a box (made with Mathematica) to show that I read out the coordinates correctly and to give an impression of the dimensions and the orientation:

And here is the screenshot of the odd result:

I guess I am doing something wrong. But I cannot find my mistake. Can somebody of you give me a hint?

Answer 1

PGFPlots requires four linearly independent points: they must not share the same hyperplane.

I think that Luigi found the clue why it produced a result at all: your image has been generated with a perspective projection. Consequently, the four provided vectors are linearly independent - but they are still from a hyperplane.

I would bet that if you have a parallel projection (aka orthogonal projection), PGFPlots would refuse to accept the points because the linear system will be underdetermined.

If only three of your points are on the hyperplane and one is outside of it, you satisfy the requirements and the linear system is well-defined.