The document presents a comprehensive discussion on creating custom views in Android, with a focus on drawing and animating graphics on the canvas. It provides practical advice on initialization, size calculation, and the animation loop, including easing functions and dynamic frame calculations for realistic animations. Additionally, it addresses how to incorporate external data inputs, such as audio visualizations, and emphasizes the importance of interpolation for ensuring smooth animation performance at high frame rates.

1. BREATHING LIFE INTO THE
CANVAS
#droidconzgTomislav Homan
Tomislav Homan, Five

2. INTRO

3. Intro
● Why custom views?
● Not everything can be solved with standard views
● We want to draw directly onto the Canvas
● Graphs and diagrams
● External data from sensors and mic (equalizer)
● ….

5. Couple of advices 1 / 3 - Initialize paints early
public final class StaticGraph extends View {
public StaticGraph(final Context context) {
super(context);
init();
}
public StaticGraph(final Context context, final AttributeSet attrs) {
super(context, attrs);
init();
}
public StaticGraph(final Context context, final AttributeSet attrs, final
int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
…..

6. Couple of advices 1 / 3 - Initialize paints early
private void init() {
axisPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
axisPaint.setColor(Color.BLACK);
axisPaint.setStyle(Paint.Style.STROKE);
axisPaint.setStrokeWidth(4.0f);
…..
}

7. Couple of advices 2 / 3 - Memorize all the measures necessary to draw early
private PointF xAxisStart;
private PointF xAxisEnd;
private PointF yAxisStart;
private PointF yAxisEnd;
……..
@Override
protected void onSizeChanged(final int width, final int height,
final int oldWidth, final int oldHeight) {
super.onSizeChanged(width, height, oldWidth, oldHeight);
calculateAxis(width, height);
calculateDataPoints(width, height);
}

8. Couple of advices 3 / 3 - Use onDraw only to draw
@Override
protected void onDraw(final Canvas canvas) {
super.onDraw(canvas);
canvas.drawLine(xAxisStart.x, xAxisStart.y, xAxisEnd.x, xAxisEnd.y, axisPaint);
canvas.drawLine(yAxisStart.x, yAxisStart.y, yAxisEnd.x, yAxisEnd.y, axisPaint);
….
canvas.drawPath(graphPath, graphPaint);
}

9. ANIMATING CUSTOM VIEWS

10. ● Every view is a set of states
● State can be represented as a point in a state space
● Animation is a change of state through time
A bit of philosophy
Animating custom views

11. Animating custom views
Let’s start with simple example - just a dot
● State contains only two pieces of information, X and Y position
● We change X and Y position through time

13. Animating custom views
The recipe
● Determine important constants
● Initialize paints and other expensive objects
● (Re)calculate size dependent stuff on size changed
● Implement main loop
● Calculate state
● Draw

14. Determine important constants
private static final long UI_REFRESH_RATE = 60L; // fps
private static final long ANIMATION_REFRESHING_INTERVAL = TimeUnit.SECONDS.toMillis(1L) / UI_REFRESH_RATE; //
millis
private static final long ANIMATION_DURATION_IN_MILLIS = 1500L; // millis
private static final long NUMBER_OF_FRAMES = ANIMATION_DURATION_IN_MILLIS /
ANIMATION_REFRESHING_INTERVAL;

15. Animating custom views
Determine important constants
● For animation that lasts 1500 milliseconds in framerate of 60 fps...
● We should refresh the screen every cca 16 milliseconds
● And we have cca 94 frames

16. Initialize paints and other expensive objects - business as usual
private void init() {
dotPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
dotPaint.setColor(Color.RED);
dotPaint.setStyle(Paint.Style.FILL);
dotPaint.setStrokeWidth(1.0f);
endPointPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
endPointPaint.setColor(Color.GREEN);
endPointPaint.setStyle(Paint.Style.FILL);
endPointPaint.setStrokeWidth(1.0f);
}

17. (Re)calculate size dependent stuff on size changed
@Override
protected void onSizeChanged(final int width, final int height, final int oldWidth, final int oldHeight) {
super.onSizeChanged(width, height, oldWidth, oldHeight);
startPoint = new PointF(width / 4.0f, height * 3.0f / 4.0f);
endPoint = new PointF(width * 3.0f / 4.0f, height / 4.0f);
….
}

18. Implement main loop
private final Handler uiHandler = new Handler(Looper.getMainLooper());
private void startAnimating() {
calculateFrames();
uiHandler.post(invalidateUI);
}
private void stopAnimating() {
uiHandler.removeCallbacks(invalidateUI);
}

19. Implement main loop
private Runnable invalidateUI = new Runnable() {
@Override
public void run() {
if (hasFrameToDraw()) {
invalidate();
uiHandler.postDelayed(this, ANIMATION_REFRESHING_INTERVAL);
} else {
isAnimating = false;
}
}
};

20. Calculate state
● Create frames array
● Determine step by which state changes
● Increase positions by step
Animating custom views

21. Calculate state
private void calculateFrames() {
frames = new PointF[NUMBER_OF_FRAMES + 1];
….
float x = animationStartPoint.x;
float y = animationStartPoint.y;
for (int i = 0; i < NUMBER_OF_FRAMES; i++) {
frames[i] = new PointF(x, y);
x += xStep;
y += yStep;
}
frames[frames.length - 1] = new PointF(animationEndPoint.x, animationEndPoint.y);
currentFrame = 0;
}

22. Animating custom views
Draw
● Now piece of cake
● Draw static stuff
● Take and draw current frame
● Increase the counter

23. Draw
@Override
protected void onDraw(final Canvas canvas) {
super.onDraw(canvas);
drawDot(canvas, startPoint, endPointPaint);
drawDot(canvas, endPoint, endPointPaint);
final PointF currentPoint = frames[currentFrame];
drawDot(canvas, currentPoint, dotPaint);
currentFrame++;
}

24. Now we want to animate the graph from one state to another

25. Animating custom views
Now we to animate the graph from one state to another
Recipe is the same, state more complicated
Dot state:
private PointF[] frames;
Graph state:
private PointF[][] framesDataPoints;
private float[] framesAxisZoom;
private int[] framesColor;

26. EASING IN AND OUT

27. Easing in and out
● Easing in - accelerating from the origin
● Easing out - decelerating to the destination
● Accelerate, hit the inflection point, decelerate to the destination
● Again - dot as an example

28. Easing in and out
Easing out (deceleration)
● Differences while calculating frames
● Replace linear trajectory with quadratic
● The step that we used in first animation isn’t valid anymore
float x = animationStartPoint.x;
float y = animationStartPoint.y;
for (int i = 0; i < NUMBER_OF_FRAMES; i++) {
frames[i] = new PointF(x, y);
x += xStep;
y += yStep;
}

29. A bit of high school math….

30. ….gives us the following formula:
Xi = (- L / N^2) * i^2 + (2 * L / N) * i
● Xi - position (state) for the ith frame
● L - length of the dot trajectory
● N - number of frames
● i - order of the frame

31. The rest of the recipe is same:
● Calculation phase modified to use previous formula
final float aX = -pathLengthX / (NUMBER_OF_FRAMES * NUMBER_OF_FRAMES);
final float bX = 2 * pathLengthX / NUMBER_OF_FRAMES;
final float aY = -pathLengthY / (NUMBER_OF_FRAMES * NUMBER_OF_FRAMES);
final float bY = 2 * pathLengthY / NUMBER_OF_FRAMES;
for (int i = 0; i < NUMBER_OF_FRAMES; i++) {
final float x = calculateFunction(aX, bX, i, animationStartPoint.x);
final float y = calculateFunction(aY, bY, i, animationStartPoint.y);
frames[i] = new PointF(x, y);
}
private float calculateFunction(final float a, final float b, final int i, final float origin) {
return a * i * i + b * i + origin;
}

32. Easing in (acceleration)
● Same approach
● Different starting conditions - initial velocity zero
● Renders a bit different formula

33. Acceleration and deceleration in the same time
● Things more complicated (but not a lot)
● Use cubic formula instead of quadratic
● Again some high school math - sorry :(

34. The magic formula:
Xi = (- 2 * L / N^3) * i^3 + (3 * L) / N^2 * i^2
● Xi - position (state) for the ith frame
● L - length of the dot trajectory
● N - number of frames
● i - order of the frame
● Same as quadratic interpolation, slightly different constants and powers

35. Easing in and out
Graph example
● Again: Use same formulas, but on a more complicated state

36. DYNAMIC FRAME CALCULATION

37. Dynamic frame calculation
Actually 2 approaches for calculating state
● Pre-calculate all the frames (states) - we did this
● Calculate the next frame from the current one dynamically

38. Dynamic frame calculation
Pre-calculate all the frames (states)
● All the processing done at the beginning of the animation
● Everything is deterministic and known in advance
● Easy to determine when to stop the animation
● Con: takes more space - 94 positions in our example

39. Dynamic frame calculation
Dynamic state calculation
● Calculate the new state from the previous one every loop iteration
● Something like a mini game engine / physics simulator
● Wastes far less space
● Behaviour more realistic
● Con: if calculation is heavy frames could drop
● Respect sacred window of 16 (or less) milliseconds

40. Dynamic frame calculation
● First example - a dot that bounces off the walls
● Never-ending animation - duration isn’t determined
● Consequently we don’t know number of frames up-
front
● Perfect for using dynamic frame calculation
● Twist in our recipe

41. Dynamic frame calculation
The recipe
● Determine important constants - the same
● Initialize paints and other expensive objects - the same
● (Re)calculate size dependent stuff on size changed - the same
● Implement main loop - move frame calculation to drawing phase
● Calculate state - different
● Draw - almost the same

42. Implement the main loop
private final Handler uiHandler = new Handler(Looper.getMainLooper());
private void startAnimating() {
calculateFrames();
uiHandler.post(invalidateUI);
}
private void stopAnimating() {
uiHandler.removeCallbacks(invalidateUI);
}

43. Implement the main loop
private Runnable invalidateUI = new Runnable() {
@Override
public void run() {
if (hasFrameToDraw()) {
invalidate();
uiHandler.postDelayed(this, ANIMATION_REFRESHING_INTERVAL);
} else {
isAnimating = false;
}
}
};

44. Implement the main loop
private void startAnimating() {
uiHandler.post(invalidateUI);
}
private void stopAnimating() {
uiHandler.removeCallbacks(invalidateUI);
}
private Runnable invalidateUI = new Runnable() {
@Override
public void run() {
invalidate();
uiHandler.postDelayed(this, ANIMATION_REFRESHING_INTERVAL);
}
};

45. Draw
@Override
protected void onDraw(final Canvas canvas) {
super.onDraw(canvas);
drawDot(canvas, startPoint, endPointPaint);
drawDot(canvas, endPoint, endPointPaint);
final PointF currentPoint = frames[currentFrame];
drawDot(canvas, currentPoint, dotPaint);
currentFrame++;
}

46. Draw
private PointF currentPosition;
@Override
protected void onDraw(final Canvas canvas) {
super.onDraw(canvas);
canvas.drawRect(topLeft.x, topLeft.y, bottomRight.x, bottomRight.y, wallsPaint);
drawDot(canvas, currentPosition, dotPaint);
if (isAnimating) {
updateWorld();
}
}

47. Calculate state
private void updateWorld() {
final float dx = currentVelocity.x; // * dt
final float dy = currentVelocity.y; // * dt
currentPosition.set(currentPosition.x + dx, currentPosition.y + dy);
if (hitRightWall()) {
currentVelocity.x = -currentVelocity.x;
currentPosition.set(topRight.x - WALL_THICKNESS, currentPosition.y);
}
//Same for every wall
}
private boolean hitRightWall() {
return currentPosition.x >= topRight.x - WALL_THICKNESS;
}

48. Dynamic frame calculation
Add gravity to previous example
● Just a couple of lines more
private void updateWorld() {
final float dvx = GRAVITY.x;
final float dvy = GRAVITY.y;
currentVelocity.set(currentVelocity.x + dvx, currentVelocity.y + dvy);
final float dx = currentVelocity.x; // * dt
final float dy = currentVelocity.y; // * dt
currentPosition.set(currentPosition.x + dx, currentPosition.y + dy);
…..
}

49. Springs
● Define a circle of given radius
● Define couple of control points with random
distance from the circle
● Let control points spring around the circle

50. private void updateWorld() {
final int angleStep = 360 / NUMBER_OD_CONTROL_POINTS;
for (int i = 0; i < controlPoints.length; i++) {
final PointF point = controlPoints[i];
final PointF velocity = controlPointsVelocities[i];
final PointF springCenter = CoordinateUtils.fromPolar((int) radius, i * angleStep, centerPosition);
final float forceX = -SPRING_CONSTANT * (point.x - springCenter.x);
final float forceY = -SPRING_CONSTANT * (point.y - springCenter.y);
final float dvx = forceX;
final float dvy = forceY;
velocity.set(velocity.x + dvx, velocity.y + dvy);
final float dx = velocity.x;
final float dy = velocity.y;
point.set(point.x + dx, point.y + dy);
}
}

51. Dynamic frame calculation
Usefulness of those animations
● Not very useful per se
● Use springs to snap the objects from one position to another
● Use gravity to collapse the scene
● You can simulate other scene properties instead of position such as color,
scale, etc...

52. ANIMATING EXTERNAL INPUT

53. Animating external input
● It sometimes happens that your state doesn’t depend only on internal
factors, but also on external
● For example equalizer
● Input is sound in fft (fast Fourier transform) data form
● Run data through the “pipeline” of transformations to get something that
you can draw
● The recipe is similar to the precalculation style, but animation isn’t
triggered by button push, but with new sound data arrival

54. Main loop - just invalidating in 60 fps
private void startAnimating() {
uiHandler.post(invalidateUI);
}
private void stopAnimating() {
uiHandler.removeCallbacks(invalidateUI);
}
private Runnable invalidateUI = new Runnable() {
@Override
public void run() {
invalidate();
uiHandler.postDelayed(this, ANIMATION_REFRESHING_INTERVAL);
}
};

55. Data input
private static final int SOUND_CAPTURE_RATE = 20; // Hz
private void startCapturingAudioSamples(int audioSessionId) {
visualizer = new Visualizer(audioSessionId);
visualizer.setCaptureSize(Visualizer.getCaptureSizeRange()[1]);
visualizer.setDataCaptureListener(new Visualizer.OnDataCaptureListener() {
@Override
public void onWaveFormDataCapture(Visualizer visualizer, byte[] waveform, int samplingRate) {
}
@Override
public void onFftDataCapture(Visualizer visualizer, byte[] fft, int samplingRate) {
calculateData(fft);
}
}, SOUND_CAPTURE_RATE * 1000, false, true);
visualizer.setEnabled(true);
}
Triggered 20 times in a second

56. Transforming data
private void calculateData(byte[] bytes) {
final int[] truncatedData = truncateData(bytes);
final int[] magnitudes = calculateMagnitudes(truncatedData);
final int[] outerScaledData = scaleData(magnitudes, OUTER_SCALE_TARGET);
final int[] innerScaledData = scaleData(magnitudes, INNER_SCALE_TARGET);
final int[] outerAveragedData = averageData(outerScaledData);
final int[] innerAveragedData = averageData(innerScaledData);
this.outerPoints = calculateContours(outerPoints, outerAveragedData, OUTER_OFFSET, true);
this.innerPoints = calculateContours(innerPoints, innerAveragedData, INNER_OFFSET, false);
currentFrame = 0;
}
This is now drawable

57. Animating external input
Important!!! - interpolation
● Data arrives 20 times a second
● We want to draw 60 times a second
● We have to “make up” - interpolate 3 frames

58. Interpolation
private PointF[][] calculateContours(final PointF[][] currentData, final int[] averagedData, final int offset, final boolean goOutwards) {
…….
fillWithLinearyInterpolatedFrames(newFrames);
…….
return newFrames;
}
private void fillWithLinearyInterpolatedFrames(final PointF[][] data) {
for (int j = 0; j < NUMBER_OF_SAMPLES; j++) {
final PointF targetPoint = data[NUMBER_OF_INTERPOLATED_FRAMES - 1][j];
final PointF originPoint = data[0][j];
final double deltaX = (targetPoint.x - originPoint.x) / NUMBER_OF_INTERPOLATED_FRAMES;
final double deltaY = (targetPoint.y - originPoint.y) / NUMBER_OF_INTERPOLATED_FRAMES;
for (int i = 1; i < NUMBER_OF_INTERPOLATED_FRAMES - 1; i++) {
data[i][j] = new PointF((float) (originPoint.x + i * deltaX), (float) (originPoint.y + i * deltaY));
}
}
for (int i = 1; i < NUMBER_OF_INTERPOLATED_FRAMES - 1; i++) {
data[i][NUMBER_OF_SAMPLES] = data[i][0];
}
}

59. Drawing - nothing unusual
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
drawContour(canvas, outerPoints, currentFrame, outerPaint);
canvas.drawCircle(center.x, center.y, radius, middlePaint);
drawContour(canvas, innerPoints, currentFrame, innerPaint);
currentFrame++;
if (currentFrame >= NUMBER_OF_INTERPOLATED_FRAMES) {
currentFrame = NUMBER_OF_INTERPOLATED_FRAMES - 1;
}
}

60. All together
20 Hz
Visualizer
Average Scale Filter Interpolate 60 Hz
onDraw

61. CONCLUSION

62. Conclusion
● Animation is change of state through time
● State can be anything from color to position
● Target 60 (or higher) fps main loop, beware of frame drops
● Pre-calculate whole frameset or calculate frame by frame
● Take it slow, make demo app, increment step by step
● Use code examples as a starting point and inform me where are memory
leaks :)

63. QA

64. QA
To save myself from having to answer embarrassing questions face to face
● Have you measured how much does it suck life out of battery? - No, but
we’ve noticed it does
● Why don’t you use OpenGL or something. - It’s next level, this is first
approximation
● What about object animators - Same amount of code, took me same
amount of time, more expensive, less flexible if you know what are you
doing. Can’t apply to the equalizer. It is more OO approach though.

65. Presentation: SpeakerDeck
Assets
Source code: https://bitbucket.org/fiveminutes/homan-demo/

66. http://five.agency/about/careers/