As with most technologies, there is a spectrum of products for virtual reality ranging from the simplest and least expensive to the very advanced.

Old fashioned stereoscopes

Cardboard is at the low end of the VR device spectrum. Well, you could even go lower if you consider the ViewMaster that you may have played with as a child, or even the historic stereoscope viewer from 1876 (B.W. Kilborn & Co, Littleton, New Hampshire), as shown in the following image:

In these old fashioned viewers, a pair of photographs display two separate views for the left and right eyes that are slightly offset to create parallax. This fools the brain into thinking that it's seeing a truly three-dimensional view. The device contains separate lenses for each eye that allow you to easily focus on the photo close up.

Similarly, rendering these side-by-side stereo views is the first job of a Google Cardboard application. (Leveraging their legacy, Mattel recently released a Cardboard-compatible ViewMaster brand VR viewer that uses a smartphone, which can be found at http://www.view-master.com/).

Desktop VR and beyond

At the higher end of consumer virtual reality devices are the Oculus Rift, HTC Vive, and Sony PlayStation VR, among others. These products go beyond what Cardboard can do because they're not limited by the capabilities of a smartphone. Sometimes referred to as "desktop VR," these devices are head-mounted displays (HMD) tethered to an external PC or console.

On desktop VR, the desktop's powerful CPU and GPU do the actual computation and graphics rendering and send the results to the HMD. Furthermore, the HMD has higher quality motion sensors and other features that help reduce the latency when updating the display at, say, 90 frames per second (FPS). We'll learn throughout this book that reducing latency and maintaining high FPS are important concerns for all VR development and the comfort of your users on all VR devices, including Cardboard.

Desktop VR devices also add positional tracking. The Cardboard device can detect the rotational movement on any of the X, Y, and Z axes, but it unfortunately cannot detect the positional movement (for example, sliding along any of these axes). The Rift, Vive, and PSVR can. The Rift, for example, uses an external camera to track the position using infrared lights on the HMD (outside-in tracking). The Vive, on the other hand, uses sensors on the HMD to track a pair of laser emitters placed strategically in the room (inside-out tracking). The Vive also uses this system to track the position and rotation of a pair of hand controllers. Both strategies achieve similar results. The user has a greater freedom to move around within the tracked space while experiencing moving around within the virtual space. Cardboard cannot do this.

Note that innovations are continually being introduced. Very likely, at some point, positional tracking will be included with the Cardboard arsenal. For example, we know that Google's Project Tango implements visual-inertial odometry, or VIO, using sensors, gyroscopes, and awareness of the physical space to provide motion and positional tracking to mobile apps. Refer to https://developers.google.com/project-tango/overview/concepts. Mobile device companies, such as LG and Samsung, are working hard to figure out how to do mobile positional tracking, but (at the time of this writing) a universal, low-latency solution does not yet exist. Google's Project Tango shows some promise but cannot yet achieve the time-to-pixel latency required for a smooth, comfortable VR experience. Too much latency and you get sick!

At the very high end are industrial and military grade systems that cost thousands or millions of dollars, which are not consumer devices, and I'm sure can do really awesome things. I could tell you more about it, but then I'd have to kill you. Solutions such as these have also been around since the 1980s. VR is not new—consumer VR is new.

The spectrum of VR devices is illustrated in the following diagram:

When we develop for Cardboard, it is important to keep in mind the things it can and cannot do relative to other VR devices. Cardboard can display stereoscopic views. Cardboard can track rotational head movement. It cannot do positional tracking. It has limitations of graphics processing power, memory, and battery life.

In the very short time it has been available, this generation of consumer virtual reality has demonstrated itself to be instantly compelling, immersive, entertaining, and "a game changer" for just about everyone who tries it. Google Cardboard is especially easy to access with a very low barrier to use. All you need is a smartphone, a low-cost Cardboard viewer (as low as $5 USD), and free apps downloaded from Google Play (or Apple App Store for iOS).

Google Cardboard has been called a gateway to VR, perhaps in reference to marijuana as a "gateway drug" to more dangerous illicit drug abuse? We can play with this analogy for a moment, however decadent. Perhaps Cardboard will give you a small taste of VR's potential. You'll want more. And then more again. This will help you fulfill your desire for better, faster, more intense, and immersive virtual experiences that can only be found in higher end VR devices. At this point, perhaps there'll be no turning back; you're addicted!

Yet as a Rift user, I still also enjoy Cardboard. It's quick. It's easy. It's fun. And it really does work, provided I run apps that are appropriately designed for the device.

I brought a Cardboard viewer in my backpack when visiting my family for the holidays. Everyone enjoyed it a lot. Many of my relatives didn't even get past the standard Google Cardboard demo app, especially its 360-degree photo viewer. That was engaging enough to entertain them for a while. Others jumped to a game or two or more. They wanted to keep playing and try new experiences. Perhaps it's just the novelty. Or, perhaps it's the nature of this new medium. The point is that Google Cardboard provides an immersive experience that's enjoyable, useful, and very easily accessible. In short, it is amazing.

Then, show them an HTC Vive or Oculus Rift. Holy Cow! That's really, really amazing! Well, for this book, we're not here to talk about the higher end VR devices, except to contrast them with Cardboard and to keep things in perspective.

Once you try desktop VR, is it hard to "go back" to mobile VR? Some folks say so. But that's almost silly. The fact is that they're really separate things.

As discussed earlier, desktop VR comes with much higher processing power and other high-fidelity features, whereas mobile VR is limited by your smartphone. If a developer were to try and directly port a desktop VR app to a mobile device, there's a good chance that you'll be disappointed.

It's best to think of each as a separate medium. Just like a desktop application or a console game is different from, but similar to, a mobile one. The design criteria may be similar but different. The technologies are similar but different. The user expectations are similar but different. Mobile VR may be similar to desktop VR, but it's different.

The implication is that Cardboard apps should be designed for shorter, simpler, and somewhat stationary experiences. Throughout this book, we'll illustrate these and other tips and best practices as you develop for the mobile VR medium.

Let's now consider the other ways that Cardboard is a gateway to VR.

We predict that Android will continue to grow as a primary platform for virtual reality in the future. More and more technologies will get crammed into smartphones. And this technology will include features advantageous to VR:

Mobile VR will not give way to desktop VR; it may even eventually replace it.

Furthermore, we'll soon see dedicated mobile VR headsets that have the guts of a smartphone built-in without the cost of a wireless communications contract. No need to use your own phone. No more getting interrupted while in VR by an incoming call or notification. No more rationing battery life in case you need to receive an important call or otherwise use your phone. All these dedicated VR devices will likely be Android-based.

Meanwhile, Android and Google Cardboard are here today on our phones, in our pockets, in our homes, at the office, and even in our schools.

Google Expeditions, for example, is Google's educational program for Cardboard (https://www.google.com/edu/expeditions/), which allows K-12 school children to take virtual field trips to "places a school bus can't," as they say, "around the globe, on the surface of Mars, on a dive to coral reefs, or back in time." The kits include Cardboard viewers and Android phones for each child in a classroom, plus an Android tablet for the teacher. They're connected with a network. The teacher can then guide students on virtual field trips, provide enhanced content, and create learning experiences that go way beyond a textbook or classroom video, as shown in the following image:

In another creative marketing example, in summer of 2015, Kellogg's began selling Nutri-Grain snack bars in a box that transforms into a Google Cardboard viewer. This links to an app that shows a variety of extreme sport 360-degree videos (http://www.engadget.com/2015/09/09/cereal-box-vr-headset/), as shown in the following image:

The entire Internet can be considered a world-wide publishing and media distribution network. It's a web of hyperlinked pages, text, images, music, video, JSON data, web services, and much more. It's also teeming with 360-degree photos and videos. There's also an ever growing amount of three-dimensional content and virtual worlds. Would you consider writing an Android app today that doesn't display images? Probably not. There's a good chance that your app also needs to support sound files, videos, or other media. So pay attention. Three-dimensional Cardboard-enabled content is coming quickly. You might be interested in reading this book now because VR looks fun. But, soon enough, it may be a customer-driven requirement for your next app.

Some examples of types of popular Cardboard apps include:

And much more; thousands more. The most popular ones have had hundreds of thousands of downloads (the Cardboard demo app itself has millions of downloads).

The projects in this book are examples of different kinds of Cardboard apps that you can build yourself today.

Let's take a look at the different Cardboard devices that are available. There's a lot of variety.

Obviously, the original Google design is actually made from cardboard. And manufacturers have followed suit, offering cardboard Cardboards directly to consumers—brands such as Unofficial Cardboard, DODOCase, and IAmCardboard were among the first.

Google provides the specifications and schematics free of charge (refer to https://www.google.com/get/cardboard/manufacturers/). For example, the Version 2.0 Viewer Body schematic is shown as follows:

The basic viewer design consists of an enclosure body, two lenses, and an input mechanism. The Works with Google Cardboard certification program indicates that a given viewer product meets the Google standards and works well with Cardboard apps.

The viewer enclosure may be constructed from any material: cardboard, plastic, foam, aluminum, and so on. It should be lightweight and do a pretty good job of blocking the ambient light.

The lenses (I/O 2015 Edition) are 34 mm diameter aspherical single lenses with an 80 degree circular FOV (field of view) and other specified parameters.

The input trigger ("clicker") can be one of several alternative mechanisms. The simplest is none, where the user must touch the smartphone screen directly with her finger to trigger a click. This may be inconvenient since the phone is sitting inside the viewer enclosure but it works. Plenty of viewers just include a hole to stick your finger inside. Alternatively, the original Cardboard utilized a small ring magnet attached to the outside of the viewer, held in place by an embedded circular magnet. The user can slide the ring magnet, and the change in magnetic field is sensed by the phone's magnetometer and recognized by the software as a "click". This design is not always reliable because the location of the magnetometer varies among phones. Also, using this method, it is harder to detect a "press and hold" interaction, which means that there is only one type of user input "event" to use within your application.

Cardboard Version 2.0 introduced a button input constructed from a conductive "strip" and "pillow" glued to a Cardboard-based "hammer," like the ones in a grand piano. When the button is pressed, the user's body charge is transferred onto the smartphone screen, as if he'd directly touched the screen with his finger. This clever solution avoids the unreliable magnetometer solution, and instead it uses the phone's native touchscreen input, albeit indirectly.

It is also worth mentioning at this point that, since your smartphone supports Bluetooth, it's possible to use a handheld Bluetooth controller with your Cardboard apps. This is not part of the Cardboard specifications and requires some extra configuration: the use of a third-party input handler or controller support built into the app. A mini Bluetooth controller is shown in the following image:

Cardboard viewers are not necessarily made out of cardboard. Plastic viewers can get relatively costly. While they are more sturdy than cardboard they fundamentally have the same design (assembled). Some devices allow you to adjust the distance from the lenses to the screen, and/or the distance between your eyes (IPD or inter-pupillary distance). The Zeiss VR One, Homido, and Sunnypeak devices were among the first to become popular.

Some manufacturers have gone outside the box (pun intended) with innovations that are not necessarily compliant with Google's specifications but provide capabilities beyond the Cardboard design. A notable example is the Wearality viewer (http://www.wearality.com/), which includes a patented 150-degree field of view (FOV) double Fresnel lens. It's so portable that it folds up like a pair of sunglasses. A pre-release version of the Wearality viewer is shown in the following image:

With such a variety of Cardboard devices and variations in lens distance, field of view, distortion, and so on, Cardboard apps must be configured to a specific device's attributes. Google provides a solution to this as well. Each Cardboard viewer comes with a unique QR code and/or NFC chip which you scan to configure the software for that device. If you're interested in calibrating your own device or customizing your parameters, check out the profile generator tools at https://www.google.com/get/cardboard/viewerprofilegenerator/.

To configure your phone to a specific Cardboard viewer, open the standard Google Cardboard app, and select the Settings icon in the bottom center section of the screen, as shown in the following image:

Then point the camera at the QR code for your particular Cardboard viewer:

Your phone is now configured for the specific Cardboard viewer parameters.

At the time of writing this book, Google provides two SDKs for Cardboard:

Let's consider the Unity option first.

More and more is being discovered and written each day about the dos and don'ts when designing and developing for VR. Google provides a couple of resources to help developers build great VR experiences, including the following:

VR motion sickness is a real symptom and concern for virtual reality caused in part by a lag in screen updates, or latency, when you're moving your head. Your brain expects the world around you to change exactly in sync with your actual motion. Any perceptible delay can make you feel uncomfortable, to say the least, and possibly nauseous. Latency can be reduced by faster rendering of each frame to maintain the recommended frames per second. Desktop VR apps are held to the high standard of 90 FPS, enabled by a custom HMD screen. On mobile devices, the screen hardware often limits refresh rates to 60 FPS, or in the worst case, 30 FPS.

There are additional causes of VR motion sickness and other user discomforts, which can be mitigated by following these design guidelines:

Applications for virtual reality also differ from conventional Android apps in other ways, such as:

So, let's say that you've got your awesome Cardboard app done and it is ready to publish. Now what? There's a line you can put in the AndroidManifest file that marks the app as a Cardboard app. Google's Cardboard app includes a Google Play Store browser used to find a Cardboard app. Then just publish it as you would do for any normal Android application.

In this chapter, we started by defining Google Cardboard and saw how it fits in the spectrum of consumer virtual reality devices. We then contrasted Cardboard with higher end VR devices, such as Oculus Rift, HTC Vive, and PlayStation VR, making the case for low-end VR as a separate medium in its own right. There are a variety of Cardboard viewer devices on the market, and we looked at how to configure your smartphone for your viewer using QR codes. We talked a bit about developing for Cardboard, and considered why and why not to use the Unity 3D game engine versus writing a native Android app in Java with the Cardboard SDK. Lastly we took a quick survey of many design considerations for developing for VR that we'll talk more about throughout the book, including ways to avoid motion sickness and tips for integrating Cardboard with Android apps in general.

In the next chapter we start coding. Yaay! For a common point of reference, we'll spend a little time introducing the Android Studio IDE and reviewing the Cardboard Android classes. Then together we'll build a simple Cardboard app, as we lay the groundwork for the structure and function of other projects throughout the book.