Develop Reference

Which is the best approach for dynamic drawing in Android?

I want to make a waveform drawing for an audio recorder in Android. The usual one with lines/bars, like this one:
More importantly, I want it live, while the song is being recorded. My app already computes the RMS through AudioRecord. But I am not sure which is the best approach for the actual drawing in terms of processing, resources, battery, etc.
The Visualizer does not show anything meaningful, IMO (are those graphs more or less random stuff??).
I've seen the canvas approach and the layout approach (there are probably more?). In the layout approach you add thin vertical layouts in a horizontal layout. The advantage is that you don't need to redraw the whole thing each 1/n secs, you just add one layout each 1/n secs... but you need hundreds of layouts (depending on n). In the canvas layout, you need to redraw the whole thing (right??) n times per second. Some even create bitmaps for each drawing...
So, which is cheaper, and why? Is there anything better nowadays? How much frequency update (i.e., n) is too much for generic low end devices?
EDIT1
Thanks to the beautiful trick #cactustictacs taught me in his answer, I was able to implement this with ease. Yet, the image is strangely rendered kind of "blurry by movement":
The waveform runs from right to left. You can easily see the blur movement, and the left-most and right-most pixels get "contaminated" by the other end. I guess I can just cut both extremes...
This renders better if I make my Bitmap bigger (i.e., making widthBitmap bigger), but then the onDraw will be heavier...
This is my full code:
package com.floritfoto.apps.ave;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.util.AttributeSet;
import java.util.Arrays;
public class Waveform extends androidx.appcompat.widget.AppCompatImageView {
//private float lastPosition = 0.5f; // 0.5 for drawLine method, 0 for the others
private int lastPosition = 0;
private final int widthBitmap = 50;
private final int heightBitmap = 80;
private final int[] transpixels = new int[heightBitmap];
private final int[] whitepixels = new int[heightBitmap];
//private float top, bot; // float for drawLine method, int for the others
private int aux, top;
//private float lpf;
private int width = widthBitmap;
private float proportionW = (float) (width/widthBitmap);
Boolean firstLoopIsFinished = false;
Bitmap MyBitmap = Bitmap.createBitmap(widthBitmap, heightBitmap, Bitmap.Config.ARGB_8888);
//Canvas canvasB = new Canvas(MyBitmap);
Paint MyPaint = new Paint();
Paint MyPaintTrans = new Paint();
Rect rectLbit, rectRbit, rectLdest, rectRdest;
public Waveform(Context context, AttributeSet attrs) {
super(context, attrs);
MyPaint.setColor(0xffFFFFFF);
MyPaint.setStrokeWidth(1);
MyPaintTrans.setColor(0xFF202020);
MyPaintTrans.setStrokeWidth(1);
Arrays.fill(transpixels, 0xFF202020);
Arrays.fill(whitepixels, 0xFFFFFFFF);
}
public void drawNewBar() {
// For drawRect or drawLine
/*
top = ((1.0f - Register.tone) * heightBitmap / 2.0f);
bot = ((1.0f + Register.tone) * heightBitmap / 2.0f);
// Using drawRect
//if (firstLoopIsFinished) canvasB.drawRect(lastPosition, 0, lastPosition+1, heightBitmap, MyPaintTrans); // Delete last stuff
//canvasB.drawRect(lastPosition, top, lastPosition+1, bot, MyPaint);
// Using drawLine
if (firstLoopIsFinished) canvasB.drawLine(lastPosition, 0, lastPosition, heightBitmap, MyPaintTrans); // Delete previous stuff
canvasB.drawLine(lastPosition ,top, lastPosition, bot, MyPaint);
*/
// Using setPixel (no tiene sentido, mucho mejor setPixels.
/*
int top = (int) ((1.0f - Register.tone) * heightBitmap / 2.0f);
int bot = (int) ((1.0f + Register.tone) * heightBitmap / 2.0f);
if (firstLoopIsFinished) {
for (int i = 0; i < top; ++i) {
MyBitmap.setPixel(lastPosition, i, 0xFF202020);
MyBitmap.setPixel(lastPosition, heightBitmap - i-1, 0xFF202020);
}
}
for (int i = top ; i < bot ; ++i) {
MyBitmap.setPixel(lastPosition,i,0xffFFFFFF);
}
//System.out.println("############## "+top+" "+bot);
*/
// Using setPixels. Works!!
top = (int) ((1.0f - Register.tone) * heightBitmap / 2.0f);
if (firstLoopIsFinished)
MyBitmap.setPixels(transpixels,0,1,lastPosition,0,1,heightBitmap);
MyBitmap.setPixels(whitepixels, top,1, lastPosition, top,1,heightBitmap-2*top);
lastPosition++;
aux = (int) (width - proportionW * (lastPosition));
rectLbit.right = lastPosition;
rectRbit.left = lastPosition;
rectLdest.right = aux;
rectRdest.left = aux;
if (lastPosition >= widthBitmap) { firstLoopIsFinished = true; lastPosition = 0; }
}
#Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
width = w;
proportionW = (float) width/widthBitmap;
rectLbit = new Rect(0, 0, widthBitmap, heightBitmap);
rectRbit = new Rect(0, 0, widthBitmap, heightBitmap);
rectLdest = new Rect(0, 0, width, h);
rectRdest = new Rect(0, 0, width, h);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
drawNewBar();
canvas.drawBitmap(MyBitmap, rectLbit, rectRdest, MyPaint);
canvas.drawBitmap(MyBitmap, rectRbit, rectLdest, MyPaint);
}
}
EDIT2
I was able to prevent the blurring just using null as Paint in the canvas.drawBitmap:
canvas.drawBitmap(MyBitmap, rectLbit, rectRdest, null);
canvas.drawBitmap(MyBitmap, rectRbit, rectLdest, null);
No Paints needed.

Your basic custom view approach would be to implement onDraw and redraw your current data each frame. You'd probably keep some kind of circular Buffer holding your most recent n amplitude values, so each frame you'd iterate over those, and use drawRect to draw the bars (you'd calculate things like width, height scaling, start and end positions etc in onSizeChanged, and use those values when defining the coordinates for the Rects).
That in itself might be fine! The only way you can really tell how expensive draw calls are is to benchmark them, so you could try this approach out and see how it goes. Profile it to see how much time it takes, how much the CPU spikes etc.
There are a few things you can do to make onDraw as efficient as possible, mostly things like avoiding object allocations - so watch out for loop functions that create Iterators, and in the same way you're supposed to create a Paint once instead of creating them over and over in onDraw, you could reuse a single Rect object by setting its coordinates for each bar you need to draw.
Another approach you could try is creating a working Bitmap in your custom view, which you control, and calling drawBitmap inside onDraw to paint it onto the Canvas. That should be a pretty inexpensive call, and it can easily be stretched as required to fit the view.
The idea there, is that very time you get new data, you paint it onto the bitmap. Because of how your waveform looks (like blocks), and the fact you can scale it up, really all you need is a single vertical line of pixels for each value, right? So as the data comes in, you paint an extra line onto your already-existing bitmap, adding to the image. Instead of painting the entire waveform block by block every frame, you're just adding the new blocks.
The complication there is when you "fill" the bitmap - now you have to "shift" all the pixels to the left, dropping the oldest ones on the left side, so you can draw the new ones on the right. So you'll need a way to do that!
Another approach would be something similar to the circular buffer idea. If you don't know what that is, the idea is you take a normal buffer with a start and an end, but you treat one of the indices as your data's start point, wrap around to 0 when you hit the last index of the buffer, and stop when you hit the index you're calling your end point:
Partially filled buffer:
|start
123400
|end
Data: 1234
Full buffer:
|start
123456
|end
Data: 123456
After adding one more item:
|start
723456
|end
Data: 234567
See how once it's full, you shift the start and end one step "right", wrapping around if necessary? So you always have the most recent 6 values added. You just have to handle reading from the correct index ranges, from start -> lastIndex and then firstIndex -> end
You could do the same thing with a bitmap - start "filling" it from the left, increasing end so you can draw the next vertical line. Once it's full, start filling from the left by moving end there. When you actually draw the bitmap, instead of drawing the whole thing as-is (723456) you draw it in two parts (23456 then 7). Make sense? When you draw a bitmap to the canvas, there's a call that takes a source Rect and a destination one, so you can draw it in two chunks.
You could always redraw the bitmap from scratch each frame (clear it and draw the vertical lines), so you're basically redrawing your whole data buffer each time. Probably still faster than the drawRect approach for each value, but honestly not much easier than the "treat the bitmap as another circular buffer" method. If you're already managing one circular buffer, it's not much more work - since the buffer and the bitmap will have the same number of values (horizontal pixels in the bitmap's case) you can use the same start and end values for both

You would never do this with layouts. Layouts are for premade components. They're high level combinations of components and you don't want to dynamically add or remove views from it frequently. For this, you use a custom view with a canvas. Layouts aren't even an option for something like this.

Android GDX game detect the right shape?

I am developing a Gdx game but I have got stuck on some part and I will explain briefly about it:
I have 9 balls organized on 3*3 and I need to detect the ball that I'm touching as shown in the image below:
enter image description here
and I typed this code:
for (int i : listBalls) {
touchPoint = new Vector3(Gdx.input.getX(), Gdx.input.getY(), 0);
rectangle = new Rectangle(sprite[i].getX(), sprite[i].getY(), spriteSize, spriteSize);
if (rectangle.contains(touchPoint.x, touchPoint.y)) {
Gdx.app.log("Test", "Touched dragged " + String.valueOf(i));
pointer = i;
}
}
In case of touching any ball of the above row, it detects the opposite ball in the bottom row. For example in the above image, if I touch ball no 2 on the top it will point to ball no 8, and same if touching any of the bottom balls.
In case of touching any ball of the middle ball, it gives the right on.
I hope I could explain clearly my issue. Please help.

As explained here: LibGDX input y-axis flipped the coordinate system of the input is inverted on the y-axis, try substracting the screen height of your device or camera
int y = screenHeight - Gdx.input.getY();
Keep in mind that using a Camera and unprojecting the input coordinates is the most recommended way to go about detecting input in LibGDX. Example:
#Override
public boolean touchDown(int screenX, int screenY, int pointer, int button) {
Vector3 unprojected = camera.unproject(new Vector3(screenX, screenY,0));
}
You don't even have to invert the y-coordinate, unproject() does this automatically for you. To use the correct x and y coordinates you then could use:
float x = unprojected.x;
float y = unprojected.y;

Draw a segmented circle in Android: OpenGL vs Cavans?

I need to draw something like this:
I was hoping that this guy posted some code of how he drew his segmented circle to begin with, but alas he didn't.
I also need to know which segment is where after interaction with the wheel - for instance if the wheel is rotated, I need to know where the original segments are after the rotation action.
Two questions:
Do I draw this segmented circle (with varying colours and content placed on the segment) with OpenGL or using Android Canvas?
Using either of the options, how do I register which segment is where?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
EDIT:
Ok, so I've figured out how to draw the segmented circle using Canvas (I'll post the code as an answer). And I'm sure I'll figure out how to rotate the circle soon. But I'm still unsure how I'll recognize a separate segment of the drawn wheel after the rotation action.
Because, what I'm thinking of doing is drawing the segmented circle with these wedges, and the sort of handling the entire Canvas as an ImageView when I want to rotate it as if it's spinning. But when the spinning stops, how do I differentiate between the original segments drawn on the Canvas?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I've read about how to draw a segment on its own (here also), OpenGL, Canvas and even drawing shapes and layering them, but I've yet to see someone explaining how to recognize the separate segments.
Can drawBitmap() or createBitmap() perhaps be used?
If I go with OpenGL, I'll probably be able to rotate the segmented wheel using OpenGL's rotation, right?
I've also read that OpenGL might be too powerful for what I'd like to do, so should I rather consider "the graphic components of a game library built on top of OpenGL"?

This kind of answers my first question above - how to draw the segmented circle using Android Canvas:
Using the code found here, I do this in the onDraw function:
// Starting values
private int startAngle = 0;
private int numberOfSegments = 11;
private int sweepAngle = 360 / numberOfSegments;
#Override
protected void onDraw(Canvas canvas) {
setUpPaint();
setUpDrawingArea();
colours = getColours();
Log.d(TAG, "Draw the segmented circle");
for (int i = 0; i < numberOfSegments; i++) {
// pick a colour that is not the previous colour
paint.setColor(colours.get(pickRandomColour()));
// Draw arc
canvas.drawArc(rectF, startAngle, sweepAngle, true, paint);
// Set variable values
startAngle -= sweepAngle;
}
}
This is how I set up the drawing area based on the device's screen size:
private void setUpDrawingArea() {
Log.d(TAG, "Set up drawing area.");
// First get the screen dimensions
Point size = new Point();
Display display = DrawArcActivity.this.getWindowManager().getDefaultDisplay();
display.getSize(size);
int width = size.x;
int height = size.y;
Log.d(TAG, "Screen size = "+width+" x "+height);
// Set up the padding
int paddingLeft = (int) DrawArcActivity.this.getResources().getDimension(R.dimen.padding_large);
int paddingTop = (int) DrawArcActivity.this.getResources().getDimension(R.dimen.padding_large);
int paddingRight = (int) DrawArcActivity.this.getResources().getDimension(R.dimen.padding_large);
int paddingBottom = (int) DrawArcActivity.this.getResources().getDimension(R.dimen.padding_large);
// Then get the left, top, right and bottom Xs and Ys for the rectangle we're going to draw in
int left = 0 + paddingLeft;
int top = 0 + paddingTop;
int right = width - paddingRight;
int bottom = width - paddingBottom;
Log.d(TAG, "Rectangle placement -> left = "+left+", top = "+top+", right = "+right+", bottom = "+bottom);
rectF = new RectF(left, top, right, bottom);
}
That (and the other functions which are pretty straight forward, so I'm not going to paste the code here) draws this:
The segments are different colours with every run.

Imprecise Box2d coordinates using LibGDX

I am using LibGDX and Box2d to build my first Android game. Yay!
But I am having some serious problems with Box2d.
I have a simple stage with a rectangular Box2d body at the bottom representing the ground, and two other rectangular Box2d bodies both at the left and right representing the walls.
A Screenshot
Another Screenshot
I also have a box. This box can be touched and it moves using applyLinearImpulse, like if it was kicked. It is a DynamicBody.
What happens is that in my draw() code of the Box object, the Box2d body of the Box object is giving me a wrong value for the X axis. The value for the Y axis is fine.
Those blue "dots" on the screenshots are small textures that I printed on the box edges that body.getPosition() give me. Note how in one screenshot the dots are aligned with the actual DebugRenderer rectangle and in the other they are not.
This is what is happening: when the box moves, the alignment is lost in the movement.
The collision between the box, the ground and the walls occur precisely considering the area that the DebugRenderer renders. But body.getPosition() and fixture.testPoint() considers that area inside those blue dots.
So, somehow, Box2d is "maintaining" these two areas for the same body.
I thought that this could be some kind of "loss of precision" between my conversions of pixels and meters (I am scaling by 100 times) but the Y axis uses the same technique and it's fine.
So, I thought that I might be missing something.
Edit 1
I am converting from Box coordinates to World coordinates. If you see the blue debug sprites in the screenshots, they form the box almost perfectly.
public static final float WORLD_TO_BOX = 0.01f;
public static final float BOX_TO_WORLD = 100f;
The box render code:
public void draw(Batch batch, float alpha) {
x = (body.getPosition().x - width/2) * TheBox.BOX_TO_WORLD;
y = (body.getPosition().y - height/2) * TheBox.BOX_TO_WORLD;
float xend = (body.getPosition().x + width/2) * TheBox.BOX_TO_WORLD;
float yend = (body.getPosition().y + height/2) * TheBox.BOX_TO_WORLD;
batch.draw(texture, x, y);
batch.draw(texture, x, yend);
batch.draw(texture, xend, yend);
batch.draw(texture, xend, y);
}
Edit 2
I am starting to suspect the camera. I got the DebugRenderer and a scene2d Stage. Here is the code:
My screen resolution (Nexus 5, and it's portrait):
public static final int SCREEN_WIDTH = 1080;
public static final int SCREEN_HEIGHT = 1920;
At the startup:
// ...
stage = new Stage(SCREEN_WIDTH, SCREEN_HEIGHT, true);
camera = new OrthographicCamera();
camera.setToOrtho(false, SCREEN_WIDTH, SCREEN_HEIGHT);
debugMatrix = camera.combined.cpy();
debugMatrix.scale(BOX_TO_WORLD, BOX_TO_WORLD, 1.0f);
debugRenderer = new Box2DDebugRenderer();
// ...
Now, the render() code:
public void render() {
Gdx.gl.glClearColor(0, 0, 0, 1);
Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
camera.update();
world.step(1/45f, 6, 6);
world.clearForces();
stage.act(Gdx.graphics.getDeltaTime());
stage.draw();
debugRenderer.render(world, debugMatrix);
}

Looks like the answer to that one was fairly simple:
stage.setCamera(camera);
I was not setting the OrthographicCamera to the stage, so the stage was using some kind of default camera that wasn't aligned with my stuff.
It had nothing to do with Box2d in the end. Box2d was returning healthy values, but theses values were corresponding to wrong places in my screen because of the wrong stage resolution.

Bitmap Matrix postRotate changes X and Y axis direction

I'm trying to rotate my Bitmap using a readymade solution I found somewhere. The code is below:
public void onDraw(Canvas canvas) {
float x = ship.Position.left;
float y = ship.Position.top;
canvas.drawBitmap(ship.ship, x,y,null);
invalidate();
}
However, when I do it, the X and Y axii change their direction - if I increase the Y the image goes towards the top of the screen, not towards the bottom. Same happens to X if I rotate by 90 degrees.
I need to rotate it but without changing the Y and X axii directions.
Even rotated, I still want the Bitmap to go towards the bottom if I increase Y and to the right if I increase the X.
public void update()
{
if(!moving)
{
fall();
}
else //moving
{
move();
faceDirection();
}
Position.top += Speed;
}
private void move() {
if(Speed < MAXSPEED)
Speed -= 0.5f;
}
private void fall() {
if(Speed > MAXSPEED*-1)
Speed += 0.2f;
}
private void faceDirection() {
double OldDiretion = Direction;
Direction = DirectionHelper.FaceObject(Position, ClickedDiretion);
if (Direction != OldDiretion)
{
Matrix matrix = new Matrix();
matrix.postRotate((float)Direction);
ship = Bitmap.createBitmap(ship, 0, 0, ship.getWidth(),ship.getHeight(), matrix, false);
}
I tried the code above, but it's still changing the Y direction, It's going to bottom of the BitMap, not bottom of the screen.
Here is the project: https://docs.google.com/file/d/0B8V9oTk0eiOKOUZJMWtsSmUtV3M/edit?usp=sharing

You should first rotate, than translate:
matrix.postTranslate(x, y);
matrix.postRotate(degree);
alternative would be to try to use preRotate() instead of postRotate().
I also strongly recommend to translate/rotate the original while drawing. So your createBitmap() call shouldn't modify the orientation. At least not when you change it dynamically on user interaction. Otherwise you would create a lot of bitmaps to represent rotations over and over again which would impact the performance.

The problem is that you don't actually rotate the bitmap - you just draw it rotated. So, the next time you redraw it, you first push it towards the bottom or right by incrementing x/y and then rotate it.
You have to actually rotate the bitmap itself. You could use the following code:
ship.ship = Bitmap.createBitmap(ship.ship, 0, 0, ship.ship.getWidth(), ship.ship.getHeight(), matrix, false);
Here you create a new rotated bitmap and set your reference to point to it.
Note! You must do this only once! So you can't do it in the onDraw method, since then it will get rotated every time it's redrawn. You have to do it somewhere else and then draw it as usual in onDraw (without the matrix rotations).