Here’s how to use Autocad to model mirror use and beam folding in designing your box. If you can accurately
build what you draw, this method will create absolutely NO keystoning of your projected image.
Autocad and Google Sketchup (free!) both contain a mirror fuction that will allow you to create the pyramid of
your projector’s beam and the fold it at your mirror points to create and then re-measure your actual beam and
box.
My example is a touch more invovled than most as I am using 4 mirrors to spin the original beam more than 360
degrees and reduce a 72” beam path to a 22” depth including the projector. The same method described here will
work for fewer mirrors (and more if you have truly lost your mind).
For start, you will need a few existing and defined dimensions: 1)the desired display size 2)the projection
distance for your projector to create this display size 3) your projector’s offset angle (or offset distance at
the projection distance). Try this handy calculator:
http://www.projectorcentral.com/Optoma-EP1690-projection-calculator-pro.htm
It shows display distances for a variety of projectors, but you will have to find out your projector’s offset
(try your manufacturer’s website).
Now you will make a drawing similar to this:
The projector is red, projection distance is white, Offset is yellow, and screen vertical size is magenta.
those dimensions allow you to draw and measure the two blue lines which are your actual projection beam’s side
view. This blue/blue/magenta triangle is the actual triangle we will be folding. You will never need the offset
and projection distance again in your model, so erase them 
Throughout the rest of these 2D images, yellow will represent mirrors, light blue will be the top beam line,
dark blue will be the bottom beam line, new lines generated will be green until their next drawing.
At this stage I found it handy to make a paper scale model of the beam triangle so I could play with folding it
in different ways. As long as the paper can fold that way, and the entire line of a fold is outside of the next
section of the beam (can’t have a mirror blockin our view can we?), it is a viable fold point.
When I find a fold pattern that makes me happy, I simply hold the paper model up against the monitor to draw in
my mirrors at the appropriate points on the beam triangle. Save this, then copy the whole thing to somewhere
else on your drawing. You can now create a side view by folding your beam with the mirror function at the
mirror points, and drawing in the rest of your box materials as you like.
I turned mine around so the projection surface was on the bottom of the drawing so it would display larger on
your monitor...yielding something that looks like a ‘Doomsday Button” when viewed as a thumbnail
This third picture shows how to create the top and bottom horizontal beam dimensions, and get the rest of your
mirror sizes from your first drawing, instructions follow.
Make an extension line (line1) from your projection surface straight up some convenient small distance. Now
extend that line by half the distance of your horizontal screen size, and then draw the other half of the
screen size. Now delete line1...and you have a horizontal screen dimension with a handy centerpoint neatly in
line with your vertical screen from the first steps.
Copy your top beam line with a base point where it joins vertical screen, paste it to the centerpoint of your
horizontal screen line, rotate it so that it is perpindicular to the screen line.
(My camera shares the bounce path of the projector, so I made a line of at the other end that allows the
camera’s view angle to start beside the projector by effectively widening the ‘origin point’ into a line and redid everything that follows correctly, the rest of this post is the projector only version).
Now close up the sides of the triangle (or trapezoid if you are sharing the bounce path), it is your top
horizontal beam, so bisect it with lines that are parallel to the screen where you have mirrors hitting the top
beam on your side view.
Repeat using the bottom beam line for your bottom horizontal beam view.
You can now get dimensions for all mirrors & distances, and if you are after the classic rectangular box, just
frame this in & you’re ready.
Still here? OK then...Here’s where I get less helpful.
This link is an XViD .avi view of my first model that shows the beam patterns and mirror placements.
http://rapidshare.com/files/114250743/mirrorz2agk.avi
Someone who actually UNDERSTANDS Autocad or sketchup is encouraged to rewrite this 3d part. This was my first
3D modeling project & I honestly don’t remember much other than a lot of cursing & the conceptual steps & order
I used. Here is what I did...as far as exactly how....? (Autocad’s ‘3Dalign’ function is your friend here,
Sketchup doubtless has a similar funtion & it’s probably easier)
Copy your whole bottom horizontal beam view...open a new 3D file, and paste it in.
Go back to your 2D, copy the whole side view beam drawing with a base point where the bottom beam and the
display line intersect.
Paste your side view onto the horizontal one in your 3D, with the base point at the centerline of the
horizontal one. Now align the vertical one so the whole centerline lines up correctly, and figure out how to
make it ‘stand-up’…
Now copy your top horizontal view with the center of the screen line as a base point, paste it onto the top of
your vertical line set, and again, align it.
Now use the 3D mirror funtion to fold the pyramid where you have mirrors.
I next skinned over the entire exterior with lovely triangles & trapezoids, and by selecting them , copying
them and pasting them into a 2D drawing, was able to create cut sheets for my exterior shell pieces...it worked
great when I made a cardboard mockup...which I destroyed before making pictures lol...but the final one is
being built slowly right now...pix, vids, and the cut sheets will follow this build.
