Get an image to lie flat on an axonometric grid in inkscape

I am new to graphic design and I am working on some figures for a paper. I want to use the isometric perspective and I have been drawing on the default axonometric grid with snapping. I want to put a rectangle down with rounded corners, but I cannot get it to sit right. When I got to document properties my grid parameters are 30 degrees in the x and z axis.

• do you want the bottom or the rectangle flat ? that would make it easier with snapping or turn snapping off Commented Apr 2, 2018 at 8:32
• @Junme I want the rounded rectangle to be in plane with the larger rectangular base. I have all the snaps turned on when I drew this. Commented Apr 2, 2018 at 15:42

1. Draw a rounded rectangle on the grid.

2. Select the rectangle.

3. Set up the Skew Transform in the Transform panel, with the horizontal scew set to -60, and the vertical set to -30.

1. Apply the transform.

2. Reposition the shape to snap to the grid.

Note: after the transform, the height of the rectangle appears to double as it becomes stretched. So if you create your initial rectangle with half the required height, you should get the result you want. Or draw it at normal height, and apply a 50% scale transform on the vertical axis before you skew.

The nice thing about this method is that it works for any complex shape you care to make.

• Why not fill in the matrix instead? Commented Apr 2, 2018 at 11:39
• @joojaa yes, could draw your shape at normal height, then apply a 50% transform on the vertical axis, then apply the skew. Commented Apr 2, 2018 at 11:44
• it maybe be simpler and more accurate just to set the matrix directly, then it can be easilöy reversed Commented Apr 2, 2018 at 11:46
• @joojaa I've added the bit about rescaling the vertical 50% before skewing. Thanks for pointing that out. As for editing the matrix, I'm afraid that's a little beyond me. Perhaps you could add that as an answer. I might learn something! Commented Apr 2, 2018 at 12:12
• Done, sorry for the crappy windowing system on my light as hell linux ;) Commented Apr 2, 2018 at 12:50

You can transform a object from normal view into isometric in the transform palette with the matrix operation in one go. The benefit of this approach is that you can undo it later, to make edits to your object and transform it back.

What you do is:

1. expand the transom palette shift+ctrl+M
2. Go to the matrix subtab and
1. Enable edit current matrix
2. Type in the matrix transform you (need below), Ignore E and F values

The isometric top/bottom (with right pointing towards NE) matrix is:

`````` 0.866  0.866
-0.5    0.5
``````

The Right side matrix is:

`````` 0.866  0
-0.5    1
``````

And the Left side matrix is

`````` 0.866 0
0.5   1
``````

Image 1 Left side matrix transform

You can reset to original by setting the matrix to

`````` 1 0
0 1
``````

Other matrices could be computed. 0.866 is cos 30 degrees as well as cos of 150 , while sin 30|150 = 0.5 signs are taken based on direction i want to go. These are the angles we want our bases to face. Other matrices could be made like what does a 15 degree around up vector rotated left side plane look like

• It would be nice if Inscape would propose default values for this matrix! I never recall these values. I filed a feature request at Inkscape's gitlab issue tracker, see gitlab.com/inkscape/inkscape/-/issues/3110 Commented Jan 14, 2022 at 14:27
• @Diederik its sin 60 and cos 60. You can draw the unit axis object and measure the x and y components of it too. See this image Commented Jan 14, 2022 at 15:09

You obviously drew a rounded rectangle, applied -30 degrees vertical skew and then it didn't fit.

The clou is to have right dimensions before skewing:

1. The black rectangle has width = 3 grid horizontal diagonals. For testing other things in the same image there's also equally wide red rectangle with sharp corners. Those other things:

• removing the rounding from the bottom corners and
2. Transformation Skew 0 degrees horizontal, -30 degrees vertical is applied.The black rectangle fits exactly to the grid. The shorter side is now 6 grid units (=2*3). The red rectangle is skewed, too

3.-4. Red rectangle is subtracted from the black rectangle (Path > Difference) to remove 2 roundings easily. As well you can make an Union. The latter can save the height intact. Here the original black rectangle shortened one grid unit,

1. The shape is duplicated and moved a little to up and to right. You must switch snap to grid and snap to points (you need them, too) on and off in turns. With high zoom 3 sub-grid units shift snapped easily. Then

• draw a diagonal (=the red line)
• Draw 2 lines (=cyan) in parallel with the grid lines. They become the corners. The sharp corner at bottom right is drawn directly to its final place.
2. Place the corner lines. You need point snapping.

• move the beginning of the upper corner line to the crossing of the rectangle and the red diagonal
• rise the further black rectangle on top
• with the node tool insert a node to the further rectangle at the crossing with the cyan line
• select the new node and the old corner node (see the orange arrows) and apply "Break path at selected nodes" in the info panel NOTE1: The rectangle is now a combined path. It must be broken apart before trying to delete the invisible part.
3. Use Path > Break apart and DEL to remove the invisible part of the further rectangle. Delete the diagonal. Shorten the corner lines to fit. Make them black. There's no need to join the pieces if you use round stroke ends.

NOTE2: The following shape was created in a freeware 3D program. It took a minute. The shape is available as editable 2D vector drawing if one prints the scene as PDF and opens it in Illustrator or Inkscape.

The latter needs a trick. Filename extension pdf must be changed to ai. Opening in Illustrator is the better option, if available, because Illustrator can automatically simplify PDF's multilevel groups. Opened in Inkscape this shape had 20 level grouping - like every new element was grouped with the group of already inserted elements.

ADD: Inkscape's extensions menu fortunately has "Arrange > Deep Ungroup" which is useful in this case. It flattens multilevel groupings in few seconds.

Used freeware CAD = DesignSpark Mechanical, a heavily decimated version of SpaceClaim, but still useful.

• The OP is asking about Inkscape Commented Apr 2, 2018 at 8:03
• @user287001 This i an excellent answer, not what I was trying to do on this occasion, but helpful. Commented Apr 2, 2018 at 15:49

you could convert the rectangle to path. Path > Object to Path. that will give you a node on each end of the radius of the corners for snapping