Isometric graphics in Inkscape

Sometimes I find myself in need of making a schematic using an isometric projection. When the schematic is complicated the best shot is to use some CAD like FreeCAD, but sometimes it's just needed in simple diagrams. Another situation where this is a common needed is in video games, where "isometric art" and pixel art are pretty common.

What we want is depicted in the following figure.

That is, we want to start with some information that is drawn, or written in the case of the example, and we want to obtain how would it been seen on one of the faces of an isometric box.

Following, I will describe briefly the transformations involved in this process. If you are just interested in the recipe for doing this in Inkscape, skip to the end of this post.

Transformations involved

Since we are working on a computer screen, we are talking of 2 dimensions. Hence, all transformations are represented by 2×2 matrices. In the particular example of interest in this post we need the following transformations:

1. Vertical scaling
2. Horizontal skew
3. Rotation

Following are the transformation matrices.

Scaling in the vertical direction

The matrix is given by

\begin{equation*} M_\text{scaling} = \begin{bmatrix} 1 &0\\ 0 &a\end{bmatrix}\, , \end{equation*}

where $a$ is the scaling factor.

Horizontal skew

The matrix is given by

\begin{equation*} M_\text{skew} = \begin{bmatrix} 1 &\tan a\\ 0 &1\end{bmatrix}\, , \end{equation*}

where $a$ is the skewing angle.

Rotation

The matrix is given by

\begin{equation*} M_\text{rotation} = \begin{bmatrix} \cos a &-\sin a\\ \sin a &\cos a\end{bmatrix}\, , \end{equation*}

where $a$ is the rotation angle.

Complete transformation

The complete transformation is given by

\begin{equation*} M_\text{complete} = M_\text{rotation} M_\text{skew} M_\text{scaling}\, , \end{equation*}

particularly,

\begin{align*} &M_\text{side} = \frac{1}{2}\begin{bmatrix} \sqrt{3} & 0\\ -1 &2\end{bmatrix}\approx \begin{bmatrix} 0.866 & 0.000\\ -0.500 &1.000\end{bmatrix}\, , \\ &M_\text{front} = \frac{1}{2}\begin{bmatrix} \sqrt{3} & 0\\ 1 &2\end{bmatrix}\approx \begin{bmatrix} 0.866 & 0.000\\ 0.500 &1.000\end{bmatrix}\, , \\ &M_\text{plant} = \frac{1}{2}\begin{bmatrix} \sqrt{3} & -\sqrt{3}\\ -1 &1\end{bmatrix}\approx \begin{bmatrix} 0.866 & -0.866\\ 0.500 &0.500\end{bmatrix}\, . \end{align*}

The values seem a bit arbitrary, but they can be obtained from the isometric projection itself. But that explanation would be a bit too long for this post.

Tranformation in Inkscape

We already have the transformation matrices and we can use them in Inkscape. But first, we need to understand how it works. Inkscape, uses SVG, the web standard for vector graphics. Transformations in SVG are done using the following matrix

\begin{equation*} \begin{bmatrix} a &c &e\\ b &d &f\\ 0 &0 &1\end{bmatrix}\, , \end{equation*}

that uses homogeneous coordinates. So, one can just press Shift + Ctrl + M and type the components of the matrices obtaines above for $a$, $b$, $c$, and $d$; leaving $e$ and $f$ as zero.

My preferred method, though, is to apply each transformation after the other in the Transform dialog (Shift + Ctrl + M). And, this is the method presented in the cheatsheet at the bottom of this post.

TL;DR

If you are just interested in the transformations needed in Inkscape you can check the cheatsheet below. It uses third-angle as presented below.

Cheatsheet

Keep in mind that Inkscape treats clockwise rotation as positive. Opposite to common notation in mathematics.