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+## Intro to Reverse-Engineering
+
+Q. What is reverse-engineering?
+
+A. Stepping backwards from a finished design, analyzing how it can be re-created, usually for the purpose of modification, enhancement, or re-production.
+
+Q. Why would we want to reverse-engineer?
+
+Reverse-engineering is used for far more than simply re-creating commercial products (making a "knock off"). Throughout history, many of mankind's best scientific advances have been the result of reverse-engineering designs from nature.
+
+In modern times, reverse-engineering has become ever more important. Today many products are intentionally made to break or to become obsolete. For such objects to continue to be useful instead of heading for a land-fill, reverse-engineering plays a vital role, allowing makers to extend the life of their tools and every-day items.
+
+Reverse-engineering is also an outstanding outlet for creativity and personal expression, allowing creative people to customize and personalize the items we carry with us every day.
+
+In this third lesson of Practical Printing we will walk through a reverse-engineering exercise based on a common device - a tablet.
+
+### Reference Images
+
+Reference images are vital for reverse engineering and can be obtained any number of ways. We will focus on two methods - a 2D scanner, and a camera.
+
+#### 2D Scanner
+
+The big advantage of using a scanner is that items scanned will be preserved at accurate scale when we attempt to use them in a CAD software.
+
+Of course the down-side of a 2D scanner is that many objects will not fit properly in the scanner so it's use may be limited.
+
+#### Camera
+
+Using a camera allows us to take a wide variety of reference images. But there are a significant number of disadvantages as well:
+
+* Lens distortion - ALL photographic images suffer from lens distortion, and the smaller the lens the more they tend to distort. This can play havoc with the accuracy of reverse-engineering.
+* Orientation - it is difficult to orient objects relative to a camera in a way that provides good reference surfaces.
+* Shadows, tricks of light, etc. - These can all make it difficult to cleanly re-create designs.
+
+### Exercise 1: Reference Images
+
+We will use a combination of techniques involving both the scanner and a camera.
+
+* Scan the two faces of the ipad
+* Using a level, set up a shoot for the other surfaces of the ipad including the bottom, top, and sides
+
+### Exercise 2: Photographic Editing
+
+We will prep our images before bringing them into modelling software. This is important, as it is virtually certain that the images will have a certain amount of rotation in them, along with the lens distortion. Use Inkscape to remove rotation in the scanned images, and clip them.
+
+The images taken by camera will not know their scale, and so they will need to be fixed in scale along with rotation. (They should not have a perspective problem, provided the images were taken at perfect level)
+
+### Exercise 3: Reference Images in CAD
+
+Learn to bring images into FreeCAD and orient them on their correct planes. In Inkscape, a common graphic/vector editing software we will learn to:
+
+* Rotate Images
+* Create a clipping mask
+* Scale a clipped image
+* Export a corrected image
+
+### Exercise 4: Modelling
+
+Learn to model using the photographic reference material. Topics covered include:
+
+* Multi-body modelling
+* Use of the "additive pipe" (also known as a sweep)
+* Using boolean operations (for cutting out ports)
+* Cloning bodies
+* Slicing bodies as prep for printing
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