Develop Reference

Android Canvas rectangle change color

I have a custom view with a rectangle:
mCanvas.drawRect(0, 0, 50, 50, paint);
I want to change the fill of the rectangle based on touch input.
How do I change the paint after the rectangle has been drawn?

You can't change the paint like that. You need to redraw the rectangle.

Android canvas hidden shape

I am making a small app that requires drawing some rectangle and using its contains() method. My problem is to draw a hidden rectangle. I'm trying to use Paint's setStyle(Paint.Style.STROKE), then setStrokeWidth(0). But the stroke is still visible.

Set your paint color to transparent color. try the below
Paint paint = new Paint();
paint.setColor(0xFF);
OR
paint.setColor(Color.TRANSPARENT);
canvas.drawRect(30, 30, 80, 80, paint);

Canvas keep ignoring Paint in drawBitmap method

I would like to draw bitmap(with specified color) on canvas.
Paint paint = new Paint();
paint.setAntiAlias(true);
paint.setStyle(Paint.Style.FILL);
paint.setColor(Color.RED);
// create bitmap
canvas.drawBitmap(bitmap, 0, 0, paint);
Well, the bitmap is visible on the canvas, but color of drawable didn't change.
Where is the problem?

I would like to draw bitmap(with specified color) on canvas.
A bitmap contains an image and drawing an image in single color doesn't make any sense. What do you expect it to do? Draw a red rectangle? Shapes can be drawn with color, not images...
The Color attribute of your Paint will be ignored. That Paint parameter is used to pass other settings such as anti-aliasing.
I hope this clarifies.

paint.setColor(Color.RED) is irrelevant. If your image comes with an alpha channel and you want it to be drawn in a single color, use ColorFilter instead:
paint.setColorFilter(new PorterDuffColorFilter(Color.RED, PorterDuff.Mode.SRC_IN);

Circular sector shaped clipping mask with Path.addArc?

I need to create a clipping mask with a shape of a circular sector.
I am able to draw one using the following:
paint.setColor(0x88FF0000);
paint.setStyle(Style.FILL);
canvas.drawArc(oval, 0, 30, true, paint);
I want to use it as a clipping path, so I've tried:
Path path = new Path();
path.addArc(oval, 0, 30);
canvas.clipPath(path, Op.REPLACE);
However addArc doesn't have the useCenter parameter so what I get is not a sector but a segment.

Ok, it seems there is no proper way doing this using a clipping mask.
However there is an alternate way using PorterDuffXfermode. See Xfermodes in ApiDemos.
I simply draw a sector using drawArc over my image with the DST_OUT operator. This makes the part of the image covered by the sector invisible (not drawn).
paint = new Paint(Paint.ANTI_ALIAS_FLAG);
paint.setColor(0xFFFFFFFF);
paint.setStyle(Style.FILL);
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
drawable.draw(canvas);
canvas.drawArc(oval, 30, 90, true, paint);

I needed to tweak the AChartEngine library for Android to turn the pie chart into a donut chart. I needed to replace the following:
canvas.drawArc(oval, startAngle, sweepAngle, useCenter, paint);
by using the Path class in order to be able to use clipping. The following code did the job for me:
Path path = new Path();
path.moveTo(oval.centerX(), oval.centerY());
path.addArc(oval, startAngle, sweepAngle);
path.lineTo(oval.centerX(), oval.centerY());
canvas.drawPath(path, paint);
I hope this saves some headaches to people not familiar with the Canvas API as myself.

Android: DrawText with background contrast

How to "set up" a paint to accomplish the "second" image above?
paint.setColor(Color.BLACK);
canvas.drawText(strValue, x, y, paint);
First Image: text all in black as result of that code above.
Second Image: better constrat to backgroud colors (edited with graphic editor just to illustrate here)
Note that "31" is partially black and partially white (but it could be any other color with a better contrast to red, as "36" could be to blue).

You could draw with PixelXorXfermode.

the only solution I could think of is that first on your onDraw you have a variable Canvas that you equals to the actual one and then you draw your number,
paint.setColor(Color.BLACK);
canvas.drawText(strValue, x, y, paint);
then you draw the red Rect
canvas.drawRect(myRect, redPaint);
then you draw your line
canvas.drawline(mStartX,mStartY, mFinishX, mFinishY, myLinePaint);
and at the very end outside your onDraw, you call a method like this one:
public void myMethod(){
Paint paint = new Paint();
paint.setColor(Color.BLACK);
this.canvas.drawText(strValue, x, y, paint);
//here you will define the area that you will mark as dirty
//(wich can have the same values as your red Rect)
Rect myRect = new Rect();
myRect.set(x0,y0,x1,y1);
//and finally here you invalidate ONLY the red area
this.canvas.invalidate(myRect);
}
Note: this will require that on your onDraw you verify that the global Canvas is not null
and if so, then you equals your global to the actual.
I'm not sure if this will actually work, however is the only solution I could think of doing so.

PixelXorXfermode is not good method when AntiAlias is set.
if you can get the red rectangle, I think use canvas.clipRect is better. like this
textpaint.setColor(black);
canvas.drawText(str,x,y,textpaint);
Rect oldClipRect = canvas.getClipBounds();
canvas.clipRect(rcRed,Op.REPLACE);
textpaint.setColor(white);
canvas.drawText(str,x,y,textpaint);
canvas.clipRect(oldclipRect,Op.REPLACE);