Oreopoulos is right - the two programs are completely different.
Piranesi takes a project created and rendered in another programme (ie ArchiCAD or ArchiCAD+Rendering Program) and allows you to manipulated the image to produce artistic effects (watercolour, sketch, etc.)
Sketchup is a front-end tool which allows you to build the project and has an built-in "sketch" rendering engine.
In reality the workflow would be:
1. Concept design in SketchUp or ArchiCAD
2. Detailed design in ArchiCAD with 3D output by ArchiCAD to Artlantis (or some other renderer) - then output a Piranesi friendly file.
3. Manipulation of that output by Piranesi.
Sketchup and Piranesi are both great programs but if you already have ArchiCAD, you'd probably find Piranesi more useful for creating presentations.
Cheers, Cameron
Cameron Hestler, Architect
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