Projection Tracing Technique for Large Walls in Confined Spaces.

If you've traced a projection onto a wall before, you will know its important to try and get the projector perpendicular with the wall so as not to distort the image. The problem with this is that you have to have the projector a long way away from the surface in order to make the image bigger, and that your body will cast shadows onto the plane your trying to map-out unless you duck squirm and contort your body around the image.

I found this handy technique while working on a very tall mural in a very long & narrow space. Not only does it allow you to set up the projector closer to the plane your working on but also substantially reduces the shadows you will cast in front of yourself.

What you'll Need:
- A projector, preferably with a lens that can adjust direction without moving the whole unit.
- A laptop with image manipulation software that has a 'distort' function that can hook up to the projector.
- Basic  understanding of said software
- A ladder,  if the mural will be taller than you can reach
- Adjustable platform for the projector, to bypass objects between the wall and the projector that may obstruct the image.
- Digital media you intend to project , In my case long exposure photos made with lasers pointers combined with 3d models
- Physical media, In my case chalk.
- Friends to help. 

Step one: Prep surface, darken room, and chose projection location.
- Position the projector in a good location so as to illuminate as much of your picture plane as possible.  Clear obstructions to the image or adjust the angle of the projection to bypass the objects; In my situation, i put the projector on top two stacked tables to avoid a coffee bar from casting a shadow in the bottom right part of the picture plane. Ideally set the projector up on the same side as the hand you will use to trace, so if you are left handed its best to set the projector up to your left.  If you cant illuminate the whole area, start with the top or one side of the image area luminated. 
- Situate a computer- connected the the projector- in a place you can comfortably access it throughout the process.

Step two: Marking up the wall with reference points
- Make lines evenly spaced up and down the picture plane parallel to the ground plane. 4 should do the trick.  These markings need to be bright enough so you can read them in relation to the projected image but faint / erasable enough that they don't ruin your image. Make two perpendicular, verticle lines at the limit of where you think the left and right sides of the image will be.

Step Three: Preparing the digital image.
- Open the digital media in a image manipulation program. Make sure you can locate the image projected on the wall and on the screen: Its necessary to read these two images against each-other. The projected image will be a distortion of what is on the screen.
- Place the image in the middle of another blank file  several times as many pixels in width and hight as the media.
- Find the top bottom left and right limits of the image you want to project and then make a outlined box incasing these limits.
- Divide this box vertically  with lines the same number and way as the parallel lines marked on the wall wall.
- edit the color of the image if the contrast with the wall is not good enough.

Step Two: Scaling the digital canvas with the physical canvas.
- Locate the top left  corner of the intersection between the furthest top and left parallel and vertical lines.
- select a masking or selection box just bigger than the limits of the image.
- Drag the same corner of the digital coordinate to  hover over the physical coordinates.
- select a masking or selection box just bigger than the limits of the image.
- Resize the image maintaining the ratio of width to hight (fixed aspect ratio) so that the the second line down at the intersection of the left hovers over the physical version of the same coordinate. (Note: its important to note the zoom level the image is being sized to)
-find the ratio between the length and width of the digital box
-find the ratio of the scale of the physical & digital hight of the image, you can then use this to locate the rightmost boundary line of the digital model in in the physical space of the wall. This step will only work roughly and the best measure is by eyeballing it, but locating this line well will mean less eyeballing.(O)

Step Three: Distort the digital image on the screen so it looks as if it Isn't distorted on the wall.
- Select the binding box on the digital image with a masking rectangle
- Use the distort function to turn the corners of this box into movable anchor points. By dragging these, you can distort the box into a trapezoidal shape where everything bound by them gets pushed and pulled in the direction the anchor points are moved. 
- While looking at the projection on the wall, drag the anchor points so they hover over and correlate to the corners of the box on the wall. If you haven't found the coordinates closely you can move the lines until they are parallel with the lines of the line marked up on the wall. Making the lines parallel is most important because it ensures the image is parallel with the plane of the wall;  matching the anchor points to the corners  is secondary as it ensures that the hight to width ratio is maintained.
(tip: if your image is too big to distort easily, you can zoom out to a level where you can see the whole image and then go on to moving the lines of the digital box so they are parallel with the physical ones. when you zoom back in to the original scale the width to hight ratio will be off and need further eyeballing and edits )
- At this point you need to asses how acceptable the projecting image is for mapping the physical media to the wall. You maybe have to resize the width hight, distort, zoom out and in several times till you are happy with the image. 

Step Four: Tracing
- When you start tracing its best to work from the top corner opposite to where the projection is coming from, proceeding down and towards the light light source. This ensures no shadows are cast over territory yet to be traced, and the shadows you are making have the hand traced lines to references from already.
- If the image is bigger than you can project in one view, you can  shingle the projected sections to overlap a section of the previously traced  with the area to be traced. Trace one batch from the projector starting from one of the lined up corners then move the projector lens into a different territory that overlaps on one side with the last traced part; This way, you can then move the digital file around on the screen until the overlapping sections hand-drawn lines matches up with the shapes and lines in the digital file and continue on this way.  If you have to shingle it more than 6 times the project may be beyond the scope of this techniques usefulness.

I made it at techshop : www.techshop.ws