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RadialGradientPaint Equivalent in Android

I am drawing a radial gradient circle on an image like this
I have java code for this
private void drawRadialGradientCircleJava(String imagePath, double posX, double posY, float radius, String outputPath) throws IOException{
BufferedImage city = ImageIO.read(new File(imagePath));
BufferedImage mask = new BufferedImage(city.getWidth(), city.getHeight(), BufferedImage.TYPE_INT_ARGB);
Graphics2D g2d = mask.createGraphics();
Color transparent = new Color(255, 0, 0, 0);
Color fill = Color.RED;
RadialGradientPaint rgp = new RadialGradientPaint(
new Point2D.Double(posX, posY),
radius,
new float[]{0f, 0.75f, 1f},
new Color[]{transparent, transparent, fill});
g2d.setPaint(rgp);
g2d.fill(new Rectangle(0, 0, mask.getWidth(), mask.getHeight()));
g2d.dispose();
BufferedImage masked = new BufferedImage(city.getWidth(), city.getHeight(), BufferedImage.TYPE_INT_ARGB);
g2d = masked.createGraphics();
g2d.setColor(Color.RED);
g2d.fillRect(0, 0, masked.getWidth(), masked.getHeight());
g2d.drawImage(city, 0, 0, null);
g2d.setComposite(AlphaComposite.DstAtop);
g2d.drawImage(mask, 0, 0, null);
g2d.dispose();
ImageIO.write(masked,"png", new File(outputPath));
}
I want to do same thing in Android, I have an image view in which I have an image, now I want to touch a point in image and draw this transparent circle around that point
I have following Android code as well but id doesn't draw anything on the image
private void drawRadialGradientCircleAndroid(ImageView imageView, float posX,
float posY, float radius) throws IOException {
RadialGradient gradient = new RadialGradient(posX, posY, radius, Color.TRANSPARENT,
Color.TRANSPARENT, android.graphics.Shader.TileMode.CLAMP);
Paint p = new Paint();
p.setDither(true);
p.setShader(gradient);
Bitmap bm = ((BitmapDrawable) imageView.getDrawable()).getBitmap();
Bitmap bmOverlay = Bitmap.createBitmap(bm.getWidth(), bm.getHeight(), bm.getConfig());
Canvas canvas = new Canvas(bmOverlay);
canvas.drawBitmap(bm, new Matrix(), null);
canvas.drawCircle(posY, posX, radius, p);
imageView.setImageBitmap(bmOverlay);
}
Please help how can I achieve this in Android.

We should migrate this to the answer boxes.
OP has basically got it here- and in fact the OP's revised gist is brilliant.
Some general tips regarding the first attempt in the question:
1) In protected void onSizeChanged(int w, int h, int oldw, int oldh):
width = w; there is no reason why you can't call getWidth() when you require this. The reason it's advisable is because the View's internal width is set quite late after onMeasure. Consequently, onDraw may be the next time you want a most up to date version, so use the getter there.
mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);. Creating a bitmap is an expensive and memory intensive operation. Unless you want to write a bitmap to a file, or send it to a BitmapDrawable for an ImageView or something, you don't need to do this. Especially with effects drawn onto the UI with android's graphics library.
mCanvas = new Canvas(mBitmap); followed by a draw operation onto the new canvas. This is never needed. And yet I've seen it (not work) in many code bases and attempts. I think it's the fault of an old stack overflow post that got people doing this so that they could transform a canvas on a custom view without effecting the drawing onto the rest of the canvas. Incidentally, if you need this, use .restore() and .save() instead. If you see new Canvas, be suspicious.
2) onDraw(...):
Yes, you need to avoid doing things in onDraw, like, creating objects, or any heavy processing. But you still need to do the things in onDraw you need to do in onDraw!
So here you simply need to call : canvas.drawCircle(float cx, float cy, float radius, Paint paint) with arguments as per the docs.
This really isn't that sinful for onDraw. If you're worried about calling this too much, as might be the case if your entire button is animating across the screen, you need to use hardware acceleration available in later API versions, as will be detailed in an article called Optimizing the View; very helpful reading if you're using lots of custom drawn views.
3) That pesky radial gradient. The next issue you had is that you quite rightly created your paint in an initmethod so that the object creation was off the draw. But then quite rightly it will have IllegalArgumentExceptioned (I think) on you because at that stage the getHeight() of the view was 0. You tried passing in small pixel values- that won't work unless you know some magic about screen sizes.
This isn't your issue as much as the annoying view cycle at the heart of Android's design patterns. The fix though is easy enough: simply use a later part of the view's drawing process after the onMeasure call to set the paint filter.
But there are some issues with getting this right, namely that sometimes, annoyingly, onDraw gets called before the point at which you'd expect it. The result would be your paint is null and you wouldn't get the desired behavior.
I have found a more robust solution is simply to do a cheeky and naughty little null check in the onDraw and then once only construct the paint object there. It's not strictly speaking optimal, but given the complex way in which the Paint objects hook up with Android's graphics native layer better than trying to straddle the paint configuration and construction in many frequently called places. And it makes for darn clearer code.
This would look like (amending your gist):
#Override
protected void onDraw(final Canvas canvas) {
super.onDraw(canvas);
if (mPaint == null) {
mPaint = new Paint();
mPaint.setColor(Color.BLACK);
mPaint.setStrokeWidth(1);
mPaint.setStyle(Paint.Style.FILL_AND_STROKE);
mPaint.setShader(new RadialGradient(getWidth() / 2, getHeight() / 2,
getHeight() / 3, Color.TRANSPARENT, Color.BLACK, TileMode.MIRROR));
}
width = getWidth();
height = getHeight();
canvas.drawCircle(width / 2, height / 2, height / 3, mPaint);
}
So note a few changes- I think from your description you want the two colours swapped round in the arguments, also don't forget to center the center of your gradient in your view: width/2 and height/2 arguments.
Best of luck!

How to draw a circle on a bitmap without filling it?

I am ddrawing a circle on the bitmap and setting this bitmap to ImageView. the circle is drawn correctly but i dont want it to be a filled circle is there any way to make the filled area transparent?
i am using the following code
Bitmap bmp = RasterImageConverter.convertToBitmap(_loadedImage, ConvertToImageOptions.NONE.getValue());
Canvas c = new Canvas(bmp);
c = new Canvas(bmp);
myimgview.draw(c);
Paint p = new Paint();
p.setColor(Color.RED);
float x=(float) circleX;
float y=(float) circleY;
float Tx=(float) textX;
float Ty=(float)textY;
// c.drawLine(x, y, xend, yend, p);
c.drawCircle(300, 300, 200, p);
c.drawText(myText, Tx, Ty, p);
myimgview.setImageBitmap(bmp);

You need to inform the paint that you dont' use a fill style, but a stroke style.
The default is FILL
The Style specifies if the primitive being drawn is filled, stroked, or both (in the same color). The default is FILL.
So your code must be:
Paint p = new Paint();
p.setStyle(Paint.Style.STROKE);
Here are explained the differences between the different styles.
STROKE
Geometry and text drawn with this style will be stroked, respecting the stroke-related fields on the paint.
FILL
Geometry and text drawn with this style will be filled, ignoring all stroke-related settings in the paint.
FILL_AND_STROKE
Geometry and text drawn with this style will be both filled and stroked at the same time, respecting the stroke-related fields on the paint.

You need to change the Paint style to stroke:
Paint p = new Paint();
p.setStyle(Paint.Style.STROKE);

Use the setStyle(Paint.Style.STROKE) method on your paint p instance.

You are almost there... you need to set the style of the Paint object...
by invoking the method
p.setStyle(Paint.Style.STROKE);
The options are:
Example:
Bitmap bmp = RasterImageConverter.convertToBitmap(_loadedImage, ConvertToImageOptions.NONE.getValue());
Canvas c = new Canvas(bmp);
c = new Canvas(bmp);
myimgview.draw(c);
Paint p = new Paint();
p.setColor(Color.RED);
p.setStyle(Paint.Style.STROKE);

LibGDX how to use different batches in one Stage

I'm using LibGDX with Scene2D for my java game. I know my issue is connected to Scene2D, because I used the EXACT same class passing it normally to SpriteBatch (not through Stage instance) and it worked as expected.
I let Stage manage all of my drawables entities which are actors. It draws everything using implementor of Batch; SpriteBatch is the default. And it was working until I wanted to draw a polygon, which has to be drawn by PolygonSpriteBatch, not SpriteBatch.. So during one Stage.draw() call I need to use them both.
I made a CheckedPolygon class which is basically two PolygonSprites drawn on top of each other (one is semi-transparent). SpriteBatch passed in draw() method is temporarily ended to enable PolygonSpriteBatch for a moment, draw the polygon and disable it.
And the output is empty screen, I get nothing. Again, it worked when I wasn't using Stage class.
Here's the class, so you get a better understanding.
NOTE: I know this is bad in terms of performance, because I don't dispose of Texture and keep Batch for one object but it's for the sake of simplicity.
NOTE2: Yes, I passed it properly to stage, the code is executed, I checked through debugging.
public class CheckedPolygon extends Actor {
private final PolygonSprite mBackground, mCross;
private final PolygonSpriteBatch mPolygonSpriteBatch;
private final Camera mCamera;
public CheckedPolygon(Camera camera, float[] vertices, short[] triangles) {
super();
mCamera = camera;
mPolygonSpriteBatch = new PolygonSpriteBatch();
Texture textureBack = new Texture(Gdx.files.internal("source/back.png"));
textureBack.setWrap(Texture.TextureWrap.Repeat, Texture.TextureWrap.Repeat);
PolygonRegion regionBack = new PolygonRegion(new TextureRegion(textureBack), vertices, triangles);
mBackground = new PolygonSprite(regionBack);
Texture textureCross = new Texture(Gdx.files.internal("source/cross.png"));
textureCross.setWrap(Texture.TextureWrap.Repeat, Texture.TextureWrap.Repeat);
PolygonRegion regionCross = new PolygonRegion(new TextureRegion(textureCross), vertices, triangles);
mCross = new PolygonSprite(regionCross);
}
#Override
public void draw(Batch batch, float parentAlpha) {
batch.end();
mPolygonSpriteBatch.setProjectionMatrix(mCamera.combined);
mPolygonSpriteBatch.begin();
mBackground.draw(mPolygonSpriteBatch);
mCross.draw(mPolygonSpriteBatch);
mPolygonSpriteBatch.end();
batch.begin();
}
}
And I use it like this:
CheckedPolygon polygon = new CheckedPolygon(mStage.getCamera(), mTextureAtlas, new float[]{0, 500, 0, 0, 0, 500}, new short[]{0, 1, 2});
mStage.addActor(polygon);
I checked values from methods PolygonSprite.getVertices() and PolygonSprite.getBoundingRectangle() and got some weird outputs...

You create an invalid polygon with 0,500 being the first and last point.
So you need to chose valid vertices, for example:
new float[] {0, 0, 0, 500, 500, 500}
I created a small image to visualize it. The line on top is your "polygon", the triangle below is a polygon with the vertices from above:
The vertices you get back from the polygon sprite are otherwise ok, since they contain x, y, color, u, v.

Why this code work only once [Android-canvas]

I want to draw circle in canvas. I use function to do id:
public static void add()
{
float a = 20 + (new Random()).nextInt(width-40);
float b = 20 + (new Random()).nextInt(height-40);
paint.setColor(Color.rgb(13, 13, 13));
c.drawCircle(a, b, r, paint);
paint.setColor(Color.rgb(119, 119, 119));
c.drawCircle(a, b, r-3, paint);
}
It works only once, when it called from "onDraw".
p.s.
paint, width, height, c - public varibles.
UPD.:
protected void onDraw(Canvas canv)
{
super.onDraw(canv);
c = canv;
paint = new Paint();
paint.setStyle(Paint.Style.FILL);
paint.setAntiAlias(true);
paint.setColor(Color.WHITE);
c.drawPaint(paint);
add();
}

onDraw() will be called whenever the view needs to re-draw itself. This can be due to many reasons, like layout changing, scrolling etc.
You can also call invalidate() on a View to cause a re-draw.
If you are going to draw at a very high rate, like touch painting or some game etc, consider using a TextureView instead.

I did it as follows.
1) Define a custom View.
2) on its onDraw method, do this creating/showing circle.
3) call invalidate() method in the last line of onDraw method of custom View.
let me know if it works

PorterDuffXfermode DST_IN not working as expected

So I'm trying to speed up some drawing we're doing (drawing a portion of an arc with alpha transparency) and was attempting to cache the entire arc into a separate bitmap, and show it selectively with an alpha mask.
From the research I've done (the Xfermodes API demo for Android, this example, and this tool), if I have for example the following two graphics:
and draw using the following:
Xfermode DST_IN = new PorterDuffXfermode(PorterDuff.Mode.DST_IN);
Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
canvas.drawBitmap(circle, 0, 0, paint);
paint.setXfermode(DST_IN);
canvas.drawBitmap(arc, 0, 0, paint);
paint.setXfermode(null);
I should get this result:
Where the destination image (the circle) is clipped to the area where the source image (the arc) is drawn. Instead, I get the full circle. If I just draw the arc, it appears in the correct location, and if I use DST_OUT instead, I get the inverse of the expected result (the other three quadrants of the circle).
I've also made sure to disable hardware rendering for this view, in case there was an issue with this Xfermode, but it doesn't make a difference.
I broke it out into a separate project at the simplest level trying to get it to work, and using the following code, I still have the same problem:
public class ClippedView extends View {
private Xfermode DST_IN, DST_OUT;
private Paint paint;
public ClippedView(Context context) {
super(context);
init();
}
private void init() {
setLayoutParams(new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
this.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
paint = new Paint(Paint.ANTI_ALIAS_FLAG);
DST_IN = new PorterDuffXfermode(PorterDuff.Mode.DST_IN);
DST_OUT = new PorterDuffXfermode(PorterDuff.Mode.DST_OUT);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
paint.setColor(Color.GREEN);
canvas.drawRect(0, 0, getWidth() / 2, getHeight() / 2, paint);
paint.setColor(Color.BLACK);
paint.setXfermode(DST_IN);
canvas.drawCircle(getWidth() / 2, getHeight() / 2, getWidth() / 2, paint);
paint.setXfermode(null);
}
}
Am I using it wrong? Am I just missing something? Have I found a bug? :)

There's a much cheaper and easier way to achieve this: use clipping. Canvas.clipRect() is enough. Your solution is burning a lot of fillrate. You can get the effect you want by using SRC_IN instead of DST_IN. Be careful though: it will work only in a transparent Bitmap or in layer. When you draw directly on screen, the destination is already filled by the window background.