This app takes 3D models and projects them on the screen for use in a hologram pyramid. A hologram pyramid (really a frustum) is a pyramid-shaped piece of glass or mirror that can be placed on the screen with the point upside-down like so:
\ / \ / \ / <-- pyramid goes here \ / -------------- <-- phone flat on table
such pyramids are cheap and easy to make
However, the pyramid needs content to use. As a programmer, I thought it would be cool to write an Android app that uses OpenGL ES to project a model with the four views needed for the hologram pyramid.
I also wanted to make the app a little more interactive, so I used my Bluetooth controller to add manual control of the model.
- Create content for my hologram pyramid
- Try out the Rajawali wrapper library for OpenGL
- Try out the Kotlin, now an official language for Android
- Learn the basics of 3D modeling
Obviously, this app requires a hologram pyramid. This can be bought or made by hand.
This app also uses a Bluetooth game controller. This is not required but is highly recommended.
Use Android Studio to compile and install the app to an Android phone.
Model Select Activity
The app opens with a list of 3D models to choose from. Select any of them to go to the hologram projection mode.
Hologram Projection Activity
This activity is where the 3D projection actually happens. Place the pyramid on the center of the phone. There is a grey square in the center of the screen to help align the pyramid correctly.
By default, the model rotates automatically.
With a Bluetooth controller connected, the following buttons let one interact with the model:
|D-pad left/right||In manual rotation mode, change the yaw of the model|
|D-pad up/down||In manual rotation mode, change the pitch of the model|
|A||Toggle automatic/manual model rotation|
|B||Go back to the model select page|
|Start||Reset the manual rotation|
|L1/R1||Adjust the height of the model (to compensate for different screen sizes)|
How it works
The hologram pyramid projection involves 2 different scenes. The 3D scene holds thee object to be projected, and the 2D scene is where the four views of the object are displayed.
This is the actual scene that contains the selected 3D model. It consists of the model at the origin and 4 cameras surrounding it at 90° intervals:
Top down view: back +----- + x cam 2 | | | v +z cam 3 -> obj <- cam 1 ^ | cam 0 front
Hologram Pyramid Cameras
The cameras used are similar to regular 3D cameras, except for the following two changes to the projection matrix:
- The scene is flipped upside down. This is because the views will be seen through a physical mirror.
- The scene is rotated some multiple of 90° counterclockwise. Camera 0 is rotated 0°, Camera 1 is rotated 90° counterclockwise, and so on. This is so the object will appear right-side-up in the hologram pyramid.
The 2D scene consists of 4
ScreenQuads. Each one represents a single view
of the 3D object. They are textured from
RenderTargets (Rajawalli's way of
making off-screen buffers) as explained in the
Here is the layout of the 2D scene:
Top of screen quad 2 (cam 2) quad 3 (cam 3) quad 1 (cam 1) quad 0 (cam 0) Bottom of screen +y | | | +------- +x
HoloPyramidRenderer is a special renderer that renders 4 views of the object.
It does so as follows:
- First the 3D scene is selected.
- For each camera in the 3D scene:
- Select the camera
- Render the view to the corresponding
RenderTargetis used as a texture in the 2D scene.
- Switch to the 2D scene
- Render the four quads to the screen. The quads are textured with the
views from the four