Je pense que la solution la plus simple est de calculer la hauteur de l'intersection : \documentclass[border = 8 mm]{standalone} \usepackage{tikz} \usetikzlibrary{positioning} \usetikzlibrary{3d} \usetikzlibrary{calc} \begin{document} \begin{tikzpicture} % ground \begin{scope}[canvas is xz plane at y=0] \draw (0, 0) rectangle (15, -10); \end{scope} % ceiling \begin{scope}[canvas is xz plane at y=10] \draw (0, 0) rectangle (15, -10); \end{scope} % vertical edges \begin{scope}[canvas is xy plane at z=0] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} \begin{scope}[canvas is xy plane at z=-10] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} % blue diagonal \begin{scope}[canvas is xz plane at y=10] \draw[blue] (0, 0) -- (15, -10); \end{scope} \draw[red] (0,10,0) -- ($(15,10,-10)+(0,{-1*sqrt(15*15+10*10)*tan(10)},0)$); \end{tikzpicture} \end{document} ![alt text][1] [1]: https://texnique.fr/osqa/upfiles/document_LVRrIar.png

Je pense que la solution la plus simple est de calculer la hauteur de l'intersection : \documentclass[border = 8 mm]{standalone} \usepackage{tikz} \usetikzlibrary{positioning} \usetikzlibrary{3d} \usetikzlibrary{calc} \begin{document} \begin{tikzpicture} % ground \begin{scope}[canvas is xz plane at y=0] \draw (0, 0) rectangle (15, -10); \end{scope} % ceiling \begin{scope}[canvas is xz plane at y=10] \draw (0, 0) rectangle (15, -10); \end{scope} % vertical edges \begin{scope}[canvas is xy plane at z=0] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} \begin{scope}[canvas is xy plane at z=-10] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} % blue diagonal \begin{scope}[canvas is xz plane at y=10] \draw[blue] (0, 0) -- (15, -10); \end{scope} \draw[red] (0,10,0) -- ($(15,10,-10)+(0,{-1*sqrt(15*15+10*10)*tan(10)},0)$); \end{tikzpicture} \end{document} ![alt text][1] [1]: https://texnique.fr/osqa/upfiles/document_hlvWWxR.pnghttps://texnique.fr/osqa/upfiles/document_LVRrIar.png

Je pense que la solution la plus simple est de calculer la hauteur de l'intersection : \documentclass[border = 8 mm]{standalone} \usepackage{tikz} \usetikzlibrary{positioning} \usetikzlibrary{3d} \usetikzlibrary{calc} \begin{document} \begin{tikzpicture} % ground \begin{scope}[canvas is xz plane at y=0] \draw (0, 0) rectangle (15, -10); \end{scope} % ceiling \begin{scope}[canvas is xz plane at y=10] \draw (0, 0) rectangle (15, -10); \end{scope} % vertical edges \begin{scope}[canvas is xy plane at z=0] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} \begin{scope}[canvas is xy plane at z=-10] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} % blue diagonal \begin{scope}[canvas is xz plane at y=10] \draw[blue] (0, 0) -- (15, -10); \end{scope} \draw[red] (0,10,0) -- ($(15,10,-10)+(0,{-1*sqrt(15*15+10*10)*tan(10)},0)$); \end{tikzpicture} \end{document} ![alt text][1] [1]: https://texnique.fr/osqa/upfiles/document_hlvWWxR.png

\documentclass[border = 8 mm]{standalone} \usepackage{tikz} \usetikzlibrary{positioning} \usetikzlibrary{3d} \usetikzlibrary{calc} \begin{document} \begin{tikzpicture} % ground \begin{scope}[canvas is xz plane at y=0] \draw (0, 0) rectangle (15, -10); \end{scope} % ceiling \begin{scope}[canvas is xz plane at y=10] \draw (0, 0) rectangle (15, -10); \end{scope} % vertical edges \begin{scope}[canvas is xy plane at z=0] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} \begin{scope}[canvas is xy plane at z=-10] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} % blue diagonal \begin{scope}[canvas is xz plane at y=10] \draw[blue] (0, 0) -- (15, -10); \end{scope} \draw[red] (0,10,0) -- ($(15,10,-10)+(0,{-1*sqrt(15*15+10*10)*tan(10)},0)$); \end{tikzpicture} \end{document}\end{document} ![alt text][1] [1]: https://texnique.fr/osqa/upfiles/document_hlvWWxR.png

\documentclass[border = 8 mm]{standalone} \usepackage{tikz} \usetikzlibrary{positioning} \usetikzlibrary{3d} \usetikzlibrary{calc} \begin{document} \begin{tikzpicture} % ground \begin{scope}[canvas is xz plane at y=0] \draw (0, 0) rectangle (15, -10); \end{scope} % ceiling \begin{scope}[canvas is xz plane at y=10] \draw (0, 0) rectangle (15, -10); \end{scope} % vertical edges \begin{scope}[canvas is xy plane at z=0] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} \begin{scope}[canvas is xy plane at z=-10] \draw[blue, dashed] (0, 0) -- (0, 10) ; \draw[blue, dashed] (15, 0) -- (15, 10) ; \end{scope} % blue diagonal \begin{scope}[canvas is xz plane at y=10] \draw[blue] (0, 0) -- (15, -10); \end{scope} \draw[red] (0,10,0) -- ($(15,10,-10)+(0,{-1*sqrt(15*15+10*10)*tan(10)},0)$); \end{tikzpicture} \end{document}