I would like to create two-dimensional projections of three-dimensional geometrical objects. The drawings should be in a vector format, SVG or PDF, which can be imported by Inkscape or Adobe Illustrator. On Wikipedia, I found the following examples (original SVG, CC-SA):
Those are isometric projections. A wireframe version thereof (black lines only) can be constructed with Inkscape's Axonometric Grid feature. However, this method does not help with shading. The sphere would look flat if the gradient was missing. Is there a program similar to Inkscape, which let's you choose the position of a light source and produces a planar projection with gradients?
An other shortcoming of 2D vector applications such as Illustrator or Inkscape is the difficulty to draw an object that is not aligned with the grid axis. Suppose, you'd like to rotate the cube in the figure by 20° without changing the perspective. How would you do this?
Note: With help from the answers found here, I found the following tools promising: