We’re used to mobile applications supporting various types of interactions such as sliding gestures to select, or to drag and drop. What we tend to forget is that there is a growing trend toward unifying the user experience across platforms.

In the early days, iOS and Android each had their own unique feel, but recently they have been growing closer together in terms of the way applications are designed and interactions happen. With bottom navigation and split screen features now available in Android Nougat, Android has a lot in common with iOS these days.

For designers, this coalescing of design languages means that we can often adjust popular features that were once associated with one platform for apps designed for the other.

Recently, to keep up with the trend of merging design styles across platforms, we worked on an Android animation that is inspired by the popular bubble animation in Apple music. Our task was to develop an interface that was easy enough for novice users but that still felt interesting for more experienced users.

Our vibrant BubblePicker is a great way to make an app more content-focused, original, and fun. Google is rolling out their “Material Design” language across all their products, but nevertheless, we decided to experiment with bold colors and gradients this time around to add more depth and volume to the image. Gradients might be the major visual in the display and might attract the attention of new visitors.

Our component has a white background with lots of bright colors and graphics against it.

This high contrast is very helpful for apps rich in content, where users have to choose from a list of options. For example, in our concept we used bubbles to hold the names of potential destinations within a travel app. Bubbles float freely, and when a user taps on one of them, the tapped bubble grows in size.

Moreover, we provide developers with the opportunity to customize the elements of the screen to make the animation suit any app.

While working on this animation we had to deal with the following five challenges:

1. Choosing the optimal development tools

It became clear to us that rendering such a fast animation on Canvas wouldn’t be efficient enough, so we decided to use OpenGL (Open Graphics Library). OpenGL is a cross-platform application programming interface for 2D and 3D graphics rendering. Fortunately, Android supports some versions of OpenGL.

We needed to make our circles move naturally, just as gas bubbles do in a fizzy drink. There are plenty of physics engines available for Android, but we had specific requirements that made it significantly more difficult to make a choice: the engine needed to be lightweight and easy to embed in the Android library. Most engines are developed for games and require you to adapt the project structure to them. After some research we found JBox2D (a Java port of the Box2D engine written in C++); and since our animation isn’t supposed to be used with a great number of physical bodies (in other words it’s not designed for 200 or more objects) we could get away with using a Java port instead of the original engine.

Also, later in this article we’ll explain our choice of programming language (Kotlin) and talk about the advantages of this new language.To find out more about the difference between Java and Kotlin you can read our review in our previous article.

2. Creating shaders

To begin with, it’s important to understand that the building block in OpenGL is a triangle since it’s the simplest shape that can approximate other shapes. Any shape that you create in OpenGL will consist of one or more triangles. To implement our animation we used two combined triangles for every body, so it looks like a square, where I can draw the circle.

To render a shape you need to write at least two shaders – a vertex shader and a fragment shader. The difference between these two is evident by their names. A vertex shader is executed for each vertex of each triangle, while a fragment shader is executed for every pixel-sized part of the triangles.

Vertex shaders are used to control transformations of the shape (e.g scaling, position, rotation), while fragment shaders are responsible for the color of the sample.

// language=GLSL val vertexShader = """ uniform mat4 u_Matrix; attribute vec4 a_Position; attribute vec2 a_UV; varying vec2 v_UV; void main() { gl_Position = u_Matrix * a_Position; v_UV = a_UV; } """ // language=GLSL val fragmentShader = """ precision mediump float; uniform vec4 u_Background; uniform sampler2D u_Texture; varying vec2 v_UV; void main() { float distance = distance(vec2(0.5, 0.5), v_UV); gl_FragColor = mix(texture2D(u_Texture, v_UV), u_Background, smoothstep(0.49, 0.5, distance)); } """

Shaders are written in GLSL (OpenGL Shading Language) and must be compiled at runtime. If you code in Java, the most convenient way to do that is to write your shaders in a separate file and retrieve them using an input stream. As you can see, Kotlin lets developers create shaders in classes more conveniently by putting any multiline code in triple quotes (""").

In GLSL there are several types of variables:

Uniform variables hold the same value for all vertices and fragments

Attribute variables are different for each vertex

Varying variables are used to pass data from a vertex shader to a fragment shader, and their values are linearly interpolated for each fragment

The u_Move variable contains x and y values that should be added to the current position of the vertex. Obviously, these values should be equal for all vertices of the shape and the type of this variable is uniform, while the position of the vertices will differ. So the a_Position variable is an attribute variable. The a_UV variable is needed for two purposes:

1. To find out the distance between the current fragment and the center of the square; depending on this distance, we can change the color of the fragment to draw a circle.

2. To properly place the texture (the photo and the name of the country) in the center of a shape.



The a_UV variable contains x and y values that are different for each vertex and which lie between 0 and 1. In the vertex shader, we just pass the value of the a_UV variable to the v_UV variable, so the v_UV variable can be interpolated for every fragment. As a result, the v_UV variable for a fragment in the center of a shape will contain the value [0.5, 0.5]. To figure out the distance between the picked fragment and the center we use the distance() method. This method uses two points as a parameter.

3. Using smoothstep to draw antialiased circles

Initially my fragment shader looked a bit different:

gl_FragColor = distance < 0.5 ? texture2D(u_Text, v_UV) : u_BgColor;

I changed the fragment color depending on the distance from the center without any antialiasing. And the result was not so impressive — the edges of the circles were notched.

So the smoothstep function was the solution. It smoothly interpolates from 0 to 1 based on distance compared to the start and end point of the transition between the texture and the background. Thus the alpha of the texture on the distance from 0 to 0.49 is 1, on the 0.5 and above it is 0, and between the 0.49 and 0.5 it is interpolated, so the edges of the circles would be antialiased.

4. Using textures to display images and text in OpenGL

Every circle in this animation has two states – normal and selected. In the normal state, the texture of a circle contains text and color; in the selected state, the texture also contains an image. So for every circle we needed to create two different textures.

To create the texture we use a Bitmap instance where we draw all the elements and bind the texture:

fun bindTextures(textureIds: IntArray, index: Int) { texture = bindTexture(textureIds, index * 2, false) imageTexture = bindTexture(textureIds, index * 2 + 1, true) } private fun bindTexture(textureIds: IntArray, index: Int, withImage: Boolean): Int { glGenTextures(1, textureIds, index) createBitmap(withImage).toTexture(textureIds[index]) return textureIds[index] } private fun createBitmap(withImage: Boolean): Bitmap { var bitmap = Bitmap.createBitmap(bitmapSize.toInt(), bitmapSize.toInt(), Bitmap.Config.ARGB_4444) val bitmapConfig: Bitmap.Config = bitmap.config ?: Bitmap.Config.ARGB_8888 bitmap = bitmap.copy(bitmapConfig, true) val canvas = Canvas(bitmap) if (withImage) drawImage(canvas) drawBackground(canvas, withImage) drawText(canvas) return bitmap } private fun drawBackground(canvas: Canvas, withImage: Boolean) { ... } private fun drawText(canvas: Canvas) { ... } private fun drawImage(canvas: Canvas) { ... }

After doing this, we pass the texture unit to the u_Text variable. To get the actual color of a fragment we use the texture2D() method, which receives the texture unit and the position of the fragment respective to its vertices.

5. Using JBox2D to make the bubbles move

The animation is pretty simple in terms of the physics. The main object is a World instance, and all the bodies must be created using this world:

class CircleBody(world: World, var position: Vec2, var radius: Float, var increasedRadius: Float) { val decreasedRadius: Float = radius val increasedDensity = 0.035f val decreasedDensity = 0.045f var isIncreasing = false var isDecreasing = false var physicalBody: Body var increased = false private val shape: CircleShape get() = CircleShape().apply { m_radius = radius + 0.01f m_p.set(Vec2(0f, 0f)) } private val fixture: FixtureDef get() = FixtureDef().apply { this.shape = this@CircleBody.shape density = if (radius > decreasedRadius) decreasedDensity else increasedDensity } private val bodyDef: BodyDef get() = BodyDef().apply { type = BodyType.DYNAMIC this.position = this@CircleBody.position } init { physicalBody = world.createBody(bodyDef) physicalBody.createFixture(fixture) } }

As we can see, it’s easy to create the body: we simply need to specify the body type (e.g dynamic, static, kinematic), and its position, radius, shape, density, and fixture.

Every time the surface is being drawn, it’s necessary to call the step() method of the World instance to move all the bodies. After that we can draw all shapes at their new positions.

The issue we faced is that JBox2D doesn’t support orbital gravity. As a result, we couldn’t move the circles to the center of the screen. So we had to implement this feature ourselves:

private val currentGravity: Float get() = if (touch) increasedGravity else gravity private fun move(body: CircleBody) { body.physicalBody.apply { val direction = gravityCenter.sub(position) val distance = direction.length() val gravity = if (body.increased) 1.3f * currentGravity else currentGravity if (distance > step * 200) { applyForce(direction.mul(gravity / distance.sqr()), position) } } }





Every time the world moves, we calculate the appropriate force and apply it to each body, making it look like the circles are affected by a gravitation force.

6. Detecting user touch in GlSurfaceView

GLSurfaceView , like any other Android view, can react to a user’s touch:

override fun onTouchEvent(event: MotionEvent): Boolean { when (event.action) { MotionEvent.ACTION_DOWN -> { startX = event.x startY = event.y previousX = event.x previousY = event.y } MotionEvent.ACTION_UP -> { if (isClick(event)) renderer.resize(event.x, event.y) renderer.release() } MotionEvent.ACTION_MOVE -> { if (isSwipe(event)) { renderer.swipe(event.x, event.y) previousX = event.x previousY = event.y } else { release() } } else -> release() } return true } private fun release() = postDelayed({ renderer.release() }, 1000) private fun isClick(event: MotionEvent) = Math.abs(event.x - startX) < 20 && Math.abs(event.y - startY) < 20 private fun isSwipe(event: MotionEvent) = Math.abs(event.x - previousX) > 20 && Math.abs(event.y - previousY) > 20

The GLSurfaceView intercepts all touches and its renderer handles all of them:

//Renderer fun swipe(x: Float, y: Float) = Engine.swipe(x.convert(glView.width, scaleX), y.convert(glView.height, scaleY)) fun release() = Engine.release() fun Float.convert(size: Int, scale: Float) = (2f * (this / size.toFloat()) - 1f) / scale //Engine fun swipe(x: Float, y: Float) { gravityCenter.set(x * 2, -y * 2) touch = true } fun release() { gravityCenter.setZero() touch = false }

When a user swipes the screen, we increase the gravity and change its center, so for the user it looks like they are controlling the movements of the bubbles. When the user stops swiping, we return the bubbles to their initial state.

7. Finding a bubble by the coordinates of a user’s touches

When a user clicks on a circle, we receive the touch position on the screen in the onTouchEvent() method. But we also need to find the clicked circle in OpenGL’s coordinate system. By default, the center of the GLSurfaceView has the position [0, 0], and the x and y values lie between -1 and 1. So we also have to consider the ratio of the screen:

private fun getItem(position: Vec2) = position.let { val x = it.x.convert(glView.width, scaleX) val y = it.y.convert(glView.height, scaleY) circles.find { Math.sqrt(((x - it.x).sqr() + (y - it.y).sqr()).toDouble()) <= it.radius } }

When we find the selected circle, we change its radius, density, and texture.

This is the first version of our Bubble Picker, and we surely plan to develop it further. We'd like to give other developers the possibility to customize the physical behavior of bubbles and specify urls to add images to the animation. We also want to add some new features such as the ability to remove bubbles.

Don’t hesitate to send us your experiments, we are curious to see how you use our Bubble Picker. And do let us know if you have any questions or suggestion regarding the animation.

We are going to publish more awesome things soon. Stay tuned!

Check out our BubblePicker animation on GitHub and BubblePicker on Dribbble.