Develop Reference

Canvas fill color between two lines

I have two lines where one line is drawn at the same point every time and the other line is rotated by a certain degree every time the view is invalidated.
The code is as shown below :
protected void onDraw(Canvas canvas) {
int width = getWidth();
int height = getHeight();
int centerx = width/2;
int centery = height/2;
canvas.drawLine(centerx, 0, centerx, height, paint);
canvas.drawLine(0, centery, width, centery, paint);
if (angle_in_degrees!= null)
canvas.rotate(angle_in_degrees, centerx, centery);
paint.setColor(0xff0000ff);
canvas.drawLine(centerx, -1000, centerx, +1000, paint);
canvas.drawLine(-1000, centery, 1000, centery, paint);
paint.setColor(0xff00ff00);
}
Everything works fine, but I want to fill the space between the two lines by a different color. How do I do that? Thanks.

The accepted answer to this question sounds as if it would apply just as well to your situation. Essentially, the author uses a Path object to connect the dots between all points that need to be filled, before finally called canvas.drawPath() to fill the area inside.
In your case, this would probably look something like:
Path path = new Path();
path.lineTo(centerX, 0);
path.lineTo(centerX, height);
path.lineTo(0, centerY);
//etc. Essentially mirroring what your line does, but in one dimension (x,y) points.
canvas.drawPath(path, paint);
Edit: You would also need to make sure the Paint object you use has a FILL style:
paint.setStyle(Paint.Style.FILL);

Draw a hollow circle on an Android Canvas

I am working on creating a custom view where a user can select an angle. Below is an example of what the end result should look like:
I am achieving this with the following code:
mPaint.setColor(Color.BLACK);
canvas.drawCircle((int) (mSize.right / 2), (int) (mSize.bottom / 2),
mWidthOutside, mPaint);
mPaint.setColor(Color.LTGRAY);
canvas.drawCircle((int) (mSize.right / 2), (int) (mSize.bottom / 2),
mWidthInside, mPaint);
The problem with doing it this way, is the background is a static LTGRAY, which I hope to make Transparent.
How would I go about leaving the center of the circle transparent?
I have tried the following hoping the the drawArc function would only create a line the width of the paint, and not fill the center. It does in face fill the center.
RectF rectF = new RectF(centerX - mRadius, centerY - mRadius, centerX
+ mRadius, centerY + mRadius);
canvas.drawArc(rectF, 0, 360, false, mPaint);
Suggestions on how to keep the center of the circle transparent?

As asked :)
The solution is to set the style to be Paint.Style.STROKE

Hollow Circle in canvas.
Bitmap bitmap=Bitmap.createBitmap(500,500, Bitmap.Config.ARGB_8888);
Canvas canvas=new Canvas(bitmap);
//canvas.clipPath(,Region.Op.DIFFERENCE);
Paint paint=new Paint();
paint.setColor(Color.RED);
paint.setStrokeWidth(140);
paint.setStyle(Paint.Style.STROKE);
canvas.drawCircle(250,250,150,paint);
imageView.setImageBitmap(bitmap);

Number inside circle

What is the best way for draw a number inside circle drawed on canvas by drawCircle?
Then this circle can be dragged by user, when circle is empty I don´t have problems.

This will draw text centered at centerX, centerY
Rect rect = new Rect();
paint.getTextBounds(text, 0, text.length(), rect);
float x = centerX - rect.width() / 2;
FontMetrics fm = paint.getFontMetrics();
float y= centerY - (fm.descent + fm.ascent) / 2;
canvas.drawText(text, x, y, paint);

Converting a Circle To a Ring using Canvas?

I'm using a Custom View where I created a Circle:
canvas.drawCircle(width/2, width/2, radius, paint1);
Now, I want to empty the circle from inside, something like a ring.
Drawing another smaller white circle will raise another problem. I'm planning to make my view listen to user clicks. I want to listen to the clicks on the ring only!
Is there a way to exclude some part of the canvas?

In order to draw a ring (i.e., a circle that is NOT filled in but just has a border, with the area inside transparent) do this:
Paint myPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
myPaint.setStyle(Paint.Style.STROKE);
int strokeWidth=4; // or whatever
myPaint.setStrokeWidth(strokeWidth);
myPaint.setColor(0xffff0000); //color.RED
float radius=(float) (0.5*(w+h)*0.5);
canvas.drawCircle((float)0.5*w, (float)0.5*h, radius, myPaint);
The call to drawCircle can also be replaced by drawArc like the code below. Either approach will work. The secret sauce is to set the paint style to Paint.Style.STROKE.
RectF oval = new RectF(w/2 - radius, w/2 - radius, w/2 + radius, w/2 + radius);
canvas.drawArc(oval, 0, 360, false, myPaint);

To draw a ring, use drawArc :
RectF oval = new RectF(width/2 - radius, width/2 - radius, width/2 + radius, width/2 + radius);
canvas.drawArc(oval, 0, 360, false, paint1);

I worked around the problem by drawing an inner white circle and filtering the clicks based on the two radius lengths I had.
So, you can't draw an empty circle but you can make a circle to behave like an empty one.
Thanks.

this solution works:
Paint paintPath;
int circleRadius = (int)
TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 10, context.getResources()
.getDisplayMetrics());
paintPath = new Paint();
paintPath.setStrokeWidth(circleRadius);
paintPath.setColor(Color.WHITE);
paintPath.setStyle(Paint.Style.FILL_AND_STROKE);
paintPath.setAntiAlias(true);
paintPath.setPathEffect(null);
paintPath.setRasterizer(null);
inside on draw
RectF rectF = new RectF(centerx - radius, centery - radius,
centerx + radius, centery + radius);
Path myPath = new Path();
for (int i = 0; i <= 360; i += 1)
{
myPath.addArc(rectF, i, 1);
}
canvas.drawPath(myPath, paintPath);

More short way:
draw = function() {
ctx.globalCompositeOperation = "lighter";
ctx.beginPath();
// ctx.fillStyle = "green";
ctx.arc(this.x, this.y, this.radius, 0, Math.PI*2, false);
// ctx.fill();
ctx.strokeStyle = "yellow";
ctx.stroke();
ctx.closePath();
}

Android & cut (remove) shape from bitmap

How do you cut (remove) a section from a bitmap???
I want that section/shape to be removed.. leave transparent in place of section..
Say shape is cercle or square..

You should be able do this with a Porter-Duff color filter and a Canvas:
public void punchHole(Bitmap bitmap, float cx, float cy, float radius) {
Canvas c = new Canvas(bitmap);
Paint paint = new Paint();
paint.setColorFilter(new PorderDuffColorFilter(0, PorderDuff.Mode.CLEAR));
c.drawCircle(cx, cy, radius, paint);
}
Well, that was wrong. However, using a Porter-Duff transfer mode does work:
public void punchHole(Bitmap bitmap, float cx, float cy, float radius) {
Canvas canvas = new Canvas(bitmap);
Paint paint = new Paint();
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawCircle(cx, cy, radius, paint);
}
(The bitmap passed as an arg needs to be modifiable, of course.)

Use Bitmap.setPixel(x,y,Color) function to set the desired pixels to transparent
for example:
Bitmap bmp = ...;
bmp.setPixel (100,100,Color.TRANSPARENT);
for the pixel at x/y offset 100,100. Though you'll find this potentially slow to do this on many pixels...

Did you try drawing a circle with a transparent color,
ARGB = 0,0,0,0 ?