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My daughter is at an age (9) when cute girly things are slowly being pushed away in exchange for more grown-up attractions. She wasn't interested in any of the "cute" cake designs we were suggesting for her upcoming birthday party, and she came up with the idea of applying projection mapping to a plain cake (I've been experimenting with projection projects for some time). With the use of a small projector hidden in a light fixture we were able to construct some clever illusions to animate the cake.

"Projection Mapping" can be defined as the use of a video projector to make inanimate shapes come to life; often deforming the video to match the angles of object or buildings to fool the viewer into believing that the object is actually emitting the light, or it is actually deforming before your eyes. Sometimes in mind-bending ways. There are tonnes of examples you can find on YouTube. I am not the first to project video on a cake, there are many professional examples on the web.

This is a very simple approach using free software that is relatively easy to learn. As with all free software, don't expect a smooth ride and don't wait till birthday's eve to learn the software. See the third step to see a compilation of what we accomplished and step by step of the software part.

You will need the following items:

  1. Cake with white icing, many tiers will be more impressive, but also more work
  2. A projector: even the smallest projector (50 lumens) should be good enough, as your subject is close to the source. I used my Touch Pico Pond (from the Indiegogo campaign), which is handy because it has an internal battery and video player built in. I was also able to use the stylus as a magic wand!
  3. Mounting solution: I used my magnetic mount hanging from the light fixture above the dining room table.

Step 1: Baking the Cake

My wife is the expert here, she was a little disappointing that she didn't put the final touches on this year's cake, but she did a fine job of making a delicious cake that was as square and smooth as possible.

We didn't try to have a recipe that gave an extra smooth or white frosting (basically a pound of butter, 8 cups of icing sugar, and 4 tablespoons of milk). Fondant would probably be ideal, you can find an example here.

One thing my wife did, which helps to make a nicer finish is to apply two layers of icing. The first layer is called a "crumb coat" that gets all messy with cake bits. Stick the cake in the fridge for an hour or more to harden the icing. Then apply the second, final, layer.

Step 2: Heavy M

This software was developped by three VJ's in France who wanted to have a simple solution for their shows. They are currently working a big release which was funded on Kickstarter (I put in some of my own dough to help them out). The Beta version is available for free download here. There are several tutorial videos here. I strongly recommend to practice using this software before the big date. I am quite certain you will be going back to the tutorials to troubleshoot.

Please watch the video to see how I made the different images, and play around, there are more options than I have time.

Step 3: Cake on Fire

One of the illusions that I came up with was the cake on fire and it turned out better than expected. One of the kids asked if the cake was really on fire, and touched it to see how hot it was.

The original goal of the paper cake was to plot out all the animations ahead of time. I realized however, that the cake geometry was not going to be as "rectilinear", so I ended up retracing the cake from scratch. It was a good practice run however, and helped me figure out all the workaround to all the bugs that were going to cause me headaches: mainly video capture so I could transfer the movie to the projector and cut all the wires for the big moment.

Using some card paper, I measured out the cake dimensions with the cake pans (1.5 time the depth to account for the thickness of two layers, loss and icing). I scored the folds with a ball point pen (underside) and taped the corners together.

WARNING: Fire can easily get out of control, make sure you have what you need to put it out quickly. The CANDLES will melt and provide a little puddle of fuel. I smothered the fire with a pyrex bowl, but I had a fire extinguisher at arm's reach.

I setup in the garage on some sheet metal on the floor. The sheet metal gave a nice reflective effect that was quite convincing on the table. The tripod was setup as close as possible to the angle that the projector had with respect to the real cake. Also make sure the rotation angle of the cake is the same: the cake is rotated 40 degrees from perpendicular so you have 3 faces available for projection. Make sure to zoom in as closely as you can to the projection limits, and you have the camera set to the same aspect ratio (16:9 or 4:3) as your projector output. Resizing the image is a real pain in the neck, so a little planning ahead of time is strongly suggested.

To get the candles to stand on the paper model, I melted spots of wax and quickly stuck the candles onto them. As the fire blazed, nearby the candles fell, quickly melting into a fiery puddle (HAVE A PLAN TO PUT OUT THE FIRE). I snuffed out the fire with a pyrex bowl; my first attempt with a garbage can lid was a fail...

After filming it, you just need to project it onto your cake. You may have a little leeway by turning your cake a little, raising it or changing the position of the projector. At the end of the day, the spectators are mostly looking at the lit parts of the cake, and wont notice edges that are not getting projected onto.

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<p>Wow! Pretty cool!</p>
<p>Thanks; the kids certainly thought so. </p>

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Bio: A lowly geologist who likes to build stuff.
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