Sketchbook Pro has been around for almost 10 years and is popular amongst comic, character and concept artists, but its latest release has a toolset that might be of interest to technical illustrators. While it’s a pixel-based application, as opposed to vector, I think it could still be handy for sketching roughs and even looser-styled finals.
These guides let you quickly and intuitively lay down lines converging on one, two, three or even five vanishing points. The tool gives you constant feedback as to where your cursor is positioned in space, and when you draw, it constrains your line to the appropriate heading. This looks like it would be very useful in the planning stages of an illustration, enabling you to quickly sketch in a variety perspectives.
Ruler & French Curves
The Ruler tool allows you to stroke a straight line in any direction. This works similar to Photoshop’s constrain modifier (Shift key) while using the Brush tool, but works in any direction. The French Curves shows an overlay of a variety of arcs and curves which can also be used to constrain a brush stroke. These tools would help add detail to a sketch once you’ve established the overall perspective.
Photoshop and Illustrator may have ellipse tools, but the one in Sketchbook Pro includes a minor axis guideline and displays the rotation and degree of the ellipse for added precision.
Other notable tools include the Symmetry mode that mirrors your drawing along a vertical and/or horizontal axis, Free Transform which is handy for distorting textures, patterns, shadows or repeating features into perspective, and Flipbook which enables frame-by-frame animation.
Sketchbook Pro is US $65, or via subscription for $25/year or $2.99/mo.